A33D-1531
Nocturnal Vertical Profiles of Nitrogen Oxides, Ozone, Aerosol and VOC From the NOAA P-3 During TexAQS 2006
Vertical stratification and reduced mixing in the lower troposphere at night has implications for chemical transformations of ozone, nitrogen oxides, VOC and aerosol. Consequently, vertical profiling from the NOAA P-3 aircraft was an important focus of the nighttime flights during the TexAQS 2006 campaign in Houston, Texas. Profiles obtained from ascents and descents upon takeoff, landing and missed approaches to airfields penetrated the shallow nocturnal boundary layer and showed sharp contrasts between this layer and the residual daytime boundary layer above it. Discrete, shallow plumes with a depth less than 100 m were observed within the residual layer. Analysis of lifetimes of NO3 and N2O5 and partitioning between these compounds and NO2 indicated that chemistry adjacent plumes within the residual layer differed substantially in terms of loss rates for NO3 or N2O5. Profiles extending to higher altitude showed that NO3 and N2O5 much longer lived in the free troposphere above the residual daytime boundary. This contribution examines nocturnal transport and chemical transformation within each of these layers - the nocturnal boundary layer, residual layer and free troposphere.
A33D-1532
Influence of Nocturnal Vertical Stability on Daytime Chemistry: A One-dimensional Model Study
Nocturnal chemistry can play an important role in determining the initial conditions for photochemistry of the next day, through the chemical removal and conversion of air pollutants such as nitrogen oxides (NOx) and volatile organic compounds (VOC). The quantification of these processes is challenging because radiation cooling leads to the suppression of turbulence and vertical mixing. Consequently, emissions at the ground are only slowly transported vertically and vertical concentration gradients develop for many trace gases, making nocturnal chemistry dependent on altitude and vertical stability. The stable nocturnal boundary layer which is often capped by a neutrally stratified residual layer breaks up after sunrise and a vertically well-mixed boundary layer forms. The transition from a vertically non-uniform chemical regime to a well-mixed boundary layer makes the assessment of the influence of nocturnal vertical stability on daytime chemistry challenging. Here we present one-dimensional (1-D) chemical transport model calculations for different nighttime vertical stability (stable, weakly stable and neutral) and different O3 formation regimes, i.e. NOx- vs VOC-sensitive. We investigate the influence of nocturnal vertical stability on O3 formation during the next day by analyzing the vertically integrated nocturnal loss of NOx, and the concentrations of NOx and O3 during the next day. Our results show that the impact of nocturnal chemistry depends on whether O3 formation occurs under NOx- or VOC-sensitive conditions.
A33D-1533
Nocturnal Vertical Profiles of Nitrogen Oxides and Related Species from a 300m Tower at Erie, Colorado
The 300-meter tower in Erie, Colorado, is a platform that provides a unique opportunity to study near-surface vertical distributions of trace gases. The tower has an external, movable carriage with a payload sufficient for multiple instruments and a one-way transit time of 9 minutes. An air quality study took place in July and August 2007. One focus of the study was on nighttime nitrogen oxide chemistry. Concentrations of NO3, N2O5 and NO2 were measured by pulsed cavity ringdown spectroscopy. In addition, measurements of ozone and NO concentrations (chemiluminescence detectors), volatile organic compounds (PTRMS), aerosol size distributions and surface areas were performed. Preliminary analysis of the nighttime data shows distinctive layered structures of nitrogen oxides and VOCs with typical layer thicknesses of 40 to 100m. Trace gas concentrations were highly variable between different layers with peak values of e.g. NO3 mixing ratios of a few hundred pptv and N2O5 mixing ratios of a few ppbv within the altitude range of the tower. In general, a depletion of nitrogen oxide radicals could be observed within the surface layer which was typically less than 20m deep along with strongly enhanced mixing ratios of aromatic VOCs. The results suggest that a representative overview of nighttime nitrogen oxide concentrations requires measurements of the vertical distributions of trace gases throughout and above the shallow nocturnal boundary layer.
A33D-1534
Intercomparison Campaign of NO3 and N2O5 Detection Techniques at the Atmosphere Simulation Chamber SAPHIR
The nitrate radical and its equilibrium partner N2O5 are of central importance for the chemistry and the loss of nitrogen oxides in the nighttime atmosphere. Since the first atmospheric detection of NO3 by Differential Optical Absorption Spectroscopy (DOAS) in the early eighties no other technique was available for about twenty years. Within the last decade new complementary techniques have been developed which allow sub-ppt in-situ measurements of NO3 with high time resolution on mobile platforms. A striking advantage of most of these instruments is their capability to measure concurrently the concentration of N2O5 via thermal conversion to NO3 in an additional heated inlet. Some instruments have already been deployed to field campaigns, however, there has been no systematic comparison between them. In order to strengthen the community's confidence in the ability to measure atmospheric NO3 and N2O5 accurately an intercomparison campaign was conducted in June 2007 at Forschungszentrum Jülich. Research groups from England, Ireland, Japan, the United States, and Germany gathered at the atmosphere simulation chamber SAPHIR in Jülich. SAPHIR is ideally suited for instrument validation, i.e., it allows the controlled production and destruction of NO3 and N2O5 and it provides a well mixed gas volume of 270 m3 for multiple instruments to sample from. All participating instruments applied optical spectroscopy for NO3 detection: four techniques based on Cavity Ring-Down Spectroscopy, three using Cavity Enhanced Absorption Spectroscopy, two Laser- Induced Fluorescence instruments, and one folded long-path DOAS took part. All instruments have measured simultaneously during ten chamber experiments covering different chemical scenarios. The experiments were selected to test the specific instrumental performance as a function of varying trace gas concentrations (O3, NO2, reactive hydrocarbons), humidity, and aerosols. In this contribution we will present an overview and selected results of the campaign discussing accuracy, precision, and time response of the different instruments. http://www.fz- juelich.de/icg/icg-2/accent/no3_comp
A33D-1535
Detection of NO3 and N2O5 by thermal dissociation with LIF detection of NO2 and NO3
The nitrate radical, NO3, is believed to be an important sink of anthropogenic alkenes and many unsaturated biogenic volatile organic compounds. The chemistry of NO3 and its reservoir N2O5 are also effective NOx sinks due to these same oxidation reactions which produce organic nitrates and HNO3 and to heterogeneous chemistry on aerosol surfaces producing HNO3. Measurements of NO3 and N2O5 by 10 different instruments were obtained simultaneously during the 2007 NO3/N2O5 intercomparison at the SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber) chamber in Juelich, Germany. Here we compare our measurements of NO3 and N2O5 by two independent Thermal Dissociation - Laser Induced Fluorescence (TD-LIF) techniques that employ detection of NO3 and NO2. In one instrument NO3 is detected by electronic excitation at 662nm with an inexpensive, low-power (30mW) diode laser followed by collection of red shifted fluorescence (700-750 nm). N2O5 is detected in this instrument by thermal dissociation to NO3 and NO2 in a second heated inlet at 200°C followed by LIF detection of NO3. N2O5 is calculated by subtracting the two signals. In this instrument detection limits of 74 pptv and 38 pptv have been achieved (signal/noise = 2) for NO3 and the sum of NO3 + N2O5 respectively with 5 minutes of signal averaging. In a second instrument NO2 is excited with a diode laser at 408nm and fluorescence longer than 650nm is collected. N2O5 is detected in this instrument by thermal dissociation to NO2 using a heated inlet held at 180C. NO3 has also been observed to be detected in this instrument using a heated inlet at 600°C, with decreased efficiency relative to the NO2 fragment from N2O5. In this paper, details of the instrument design and calibration, discussion of the inlet transmission efficiency and the mechanism for NO3 detection in the NO2 LIF system are discussed.
A33D-1536
Modeled Effects of Observed Nitryl Chloride Concentrations in the Houston Area
The recent TexAQS II field study involved intensive air pollutant measurements over Texas during the summer and fall of 2006. During TexAQS II, NOAA researchers measured concentrations of nitryl chloride of up to 1 ppb in the Houston urban area. Nitryl chloride is potentially important to atmospheric chemistry in urban environments because its photolysis products include both NO2 and chlorine radicals. Chlorine radicals have previously been shown to significantly increase ozone formation in urban Houston. If the values of nitryl chloride measured in Galveston Bay are widespread, this compound has the potential to significantly affect the local reactive chlorine budget and ultimately ozone mixing ratios. Photochemical modeling was performed using the comprehensive air quality model with extensions (CAMx) to investigate the possible effects of measured nitryl chloride concentrations on local chemistry. CAMx was modified to include nitryl chloride and its photolysis reaction (the dominant loss mechanism of this compound). The rate constant of nitryl chloride photolysis was calculated within the model using a scaling factor to the photolysis rate constant of formaldehyde. This scaling was done to simplify the execution of the model. It was found that under standard conditions (ozone column of 300 Dobson units, albedo of 0.06, and a measurement height of 640 m) this scaling factor varied very little with shifting zenith angles. Calculations at zenith angles of 0, 10, 20, 30, 40, 50, 60, 70, 78, and 86 degrees, showed that scaling factor values had a standard deviation of less than five percent of the mean value. The photochemical modeling runs were performed for a series of days during the TexAQS II study. After an initial 3 day ramp-up period, model runs were stopped at 7am every morning (the approximate time of sunrise). At that time, measured concentrations of nitryl chloride were inserted into the modeling domain in the three by three grid cell region around the measurement point (a 12 square km region). The parcel of air which contained the initial nitryl chloride concentrations was followed throughout the day. Chlorine radical and ozone concentrations in this model run were compared to those predicted in a basecase model run which did not include any nitryl chloride. The authors would like to acknowledge the NOAA research team led by James Roberts and thank them for allowing access to their nitryl chloride measurement data.
A33D-1537
Chemical Processing in a Polluted Forest Canopy: a Model Comparison of the Regional Atmospheric Chemistry Mechanism, Version 2 and SAPRC07
In this study, we present a comparison of modeled chemical processing in polluted forest canopies using the Regional Atmospheric Chemistry Mechanism, Version 2 and SAPRC07. The comparison involves a number of field studies, one, for example, was conducted at a forested site near Oak Ridge, TN during the summer of 1999. The sites were under the influence of nitrogen oxide and hydrocarbon emissions from suburban automobile traffic. Air chemistry measurements (e.g., ozone, NOx and VOC) and meteorological measurements were collected within and above the forest canopies. A comparison of the degree of chemical processing by HO and NO3 radicals and O3 for biogenic compounds, and the amount of HOx formation between the two mechanisms is presented.
A33D-1538
An Examination of Organic Nitrates Using a 0-D Model and Experimental Data From an Isoprene Irradiation Chamber Experiment
Isoprene (2-methyl-1,3-butadiene) is the most prevalent hydrocarbon that is emitted into the atmosphere by various plant species, at a global rate of approximately 500 Tg/year. Isoprene is highly reactive with hydroxyl radicals, and through this process contributes significantly to tropospheric ozone production in a sufficiently NOx- rich environment. Additionally, methyl vinyl ketone and methacrolein, both of which are major isoprene oxidation products, can lead to additional ozone formation due to their high reactivity with hydroxyl radicals. Formation of organic nitrates from isoprene, methyl vinyl ketone and methacrolein is one of the termination steps in the cycle that produces ozone, and that converts NOx into a form with an uncertain lifetime, depending on subsequent chemistry. Here we discuss results of simulations of isoprene photochemistry using a comprehensive 0-D model, to thoroughly evaluate the distribution of nitrogen in the isoprene/NOx photochemical system. The model includes detailed chemistry of the formation and removal of a wide variety of nitrates, produced from both OH and NO3 reaction with isoprene and its major oxidation products. The model simulations will be compared with data that was collected from a series of isoprene/NOx irradiation experiments that were performed in June of 2006. In particular, the sum of all nitrates have been measured, in order to further understand the important role that these nitrates play in the chemistry of the troposphere.
A33D-1539
Determination of Isomer-Specific Isoprene Nitrate Production Yields
Isoprene nitrates are produced from the reaction of isoprene, the dominant biogenic volatile organic compound emission, with NO in the atmosphere. Determining the identity and yields for the 8 isomers is important, as the reactivity and thus fate of the individual nitrates is expected to vary widely. Here we report on smog chamber studies of the yields and fate of each of the isoprene nitrate isomers.
A33D-1540
Measurement of OH, HO2, and OH reactivity in Houston 2006
OH, HO2 and OH reactivity were measured from August 16 to Sep 27, 2006 on the top of Moody Tower at the University of Houston in Houston Texas. The site is located a few kilometers south of downtown Houston and a few kilometers west of the Houston Ship Channel, one of the busiest sea ports in United States. Diurnal variations of measured OH, HO2, and OH reactivity are presented. To further understand the photochemistry in the Houston city, measured OH and HO2 are compared to a chemical box model calculation. Measured OH reactivity is also compared to the calculated OH reactivity based on all available VOC measurements and other inorganic species.
A33D-1541
Photochemical Activity in Mexico City during MILAGRO 2006: results from the T1 site
A large suite instruments were deployed at a ground based site in Tecámac, México (near the northern boundary of Mexico City) to measure SO2, NO, CO, O3, H2SO4, OH and HO2 +RO2 during MIRAGE-Mex field campaign (March 2006). These and other measurements are used to characterize atmospheric oxidation and predict sulfuric acid and OH concentrations at the site. The observations in conjunction with the NASA LARc Photochemical box model are used to explore ozone production, nitrate and sulfate formation, and radical levels and radical production rates during the day. The 1-minute predicted OH values agreed well with observations (R2 = 0.88). Maximum ozone production was calculated to be 25 ppb/hr and 30 ppb/hr from the observations and model predictions, respectively. Most of the photochemical activity is dominated by the radical fluxes in the morning hours, nitrate formation peaks around 10 am LST; this early morning peak is also seen in ozone production. Although the diurnal model predictions agree well with measured species (HO2 + RO2 radical Model/observed (M/O) ~0.85 and OH M/O ~ 0.98), there is disagreement between the model and observations within the early morning.
A33D-1542
CMAQ Analysis of PAN Data at the Moody Tower and the Aldine Site During TexAQS II
During the TexAQS II campaign in summer 2006 (08/07/06 - 09/30/06) continuous online measurements of speciated PANs were carried out at the Moody Tower supersite and within the scope of the Houston Triangle experiment (09/13 – 09/29/06) at the Aldine site north of downtown Houston. Observations have shown that some days reveal distinct differences between both sites, whereas a few days reflect meteorological conditions when both sites were impacted by the same air mass. Using CMAQ modeling this paper compares both sites in terms of the different VOC precursor matrices including data of C2 - C10 Volatile Organic Compounds (VOCs) obtained at the Moody Tower and VOC data from the TCEQ and Enhanced Industry Sponsored Monitoring auto-GC networks near the Ship Channel area and their impact on the partitioning of PAN species.
A33D-1543
PANs measurements on board theNOAA P-3 during TexAQS-II
Measurements of peroxycarboxylic nitric anhydrides (PANs, i.e. PAN, PPN, PiBN, APAN, MPAN, and MoPAN) were made using the NCAR PAN-CIGARette chemical ionization mass spectrometer on board the NOAA P-3 aircraft during the 2006 Texas Air Quality Study II (TexAQS-II). In this poster, we present the PANs measurements made during the flight on September 27th as a case study. Two separate plumes from Downtown Houston and the Houston Ship Channel were transported in parallel to the north on that day. The flight track crossed these plumes 8 times at increasing distances downwind, and according to the CO distribution, dilution with surrounding air masses was very slow. These conditions make this very nice case for a pollutant transport and chemistry study. The PAN/PPN ratio increased about 18% as the air mass moved away from the pollution source to the furthest leg which is about 130 km north of downtown Houston. As the photolysis rates for PAN and PPN are similar to each other and the thermal decomposition of PAN is faster than PPN, this ratio change is most likely owing to the difference in the chemistry of the source hydrocarbons for these two PAN species, and indicates a faster depletion of PPN precursors (mainly propanal and 1-butene) as the air masses get older. Also, the relative production of ozone and PANs for the Houston city plume and the ship channel plume are analyzed and compared for this flight, demonstrating the difference in the photochemical processes for urban pollution vs. petroleum industry emissions.
A33D-1544
Interactions of Gas-Phase Nitric/Nitrous Acids and Primary Organic Aerosol in the Atmosphere of Houston, TX
Concentrations of aerosol and gas-phase pollutants were measured on the roof of an 18-story building during the Texas Air Quality Study II Radical and Aerosol Measurement Project (TRAMP) from August 15 through September 28, 2006. Aerosol measurements included size-resolved, non-refractory mass concentrations of ammonium, nitrate, sulfate, chloride, and organic aerosol in submicron particles using an Aerodyne quadrupole aerosol mass spectrometer (Q-AMS). Particulate water-soluble organic carbon (PWSOC) was quantified using a mist chamber/total organic carbon analysis system. Concentration data for gas-phase pollutants included those for nitric acid (HNO3), nitrous acid (HONO), and hydrochloric acid (HCl) collected using a mist chamber/ion chromatographic technique, oxides of nitrogen (NOx) collected using a chemiluminescent method, and carbon monoxide (CO) collected using an infrared gas correlation wheel instrument. Coincident increases in nitrate and organic aerosol mass concentrations were observed on many occasions throughout the measurement campaign, most frequently during the morning rush hour. Based on the lack of organic aerosol processing (defined by the ratio of m/z = 44/57 in the Q-AMS spectra), strong correlation with NOx and CO, and a lack of significant increase in PWSOC concentration, the spikes in organic aerosol were likely associated with primary organic aerosol (POA). During these events, gas-phase HNO3 concentration decreases were observed simultaneously with increases in gas-phase HONO concentrations. These data likely indicate uptake of HNO3 and subsequent heterogeneous conversion to HONO involving POA. Preliminary calculations show that HNO3 partitioning could account for the majority of the observed HONO and aerosol nitrate concentrations during these events. Q-AMS chloride and HCl data also indicate uptake of chloride by particles during these events. This phenomenon was also observed during the night, but these nocturnal events were less frequent and less distinct.
A33D-1545
Nitric, Nitrous, and Pernitric Acids in and around Mexico City
Nitric, nitrous, and pernitric acids were measured from the NCAR C130 aircraft platform during the MILAGRO intensive in Mexico during the spring of 2005. Measurements were conducted in-situ using chemical ionization mass spectrometry. These observations provide a sensitive test of the coupled HOx / NOx chemistry in an urban environment. Measurements are presented along with comparison to box model calculations.
A33D-1546
Characteristics of particulate nitrite and HONO formation in Korea
Continuous and simultaneous measurements of gaseous species (NH3, HONO, NO2, HNO3, and HCl) and particulate inorganic components (NH4+, SO42-, NO3-, and NO2-) were made in Seoul (urban site; from 6 to 20 May, 2005 and from 23 June to 7 July, 2005) and Gosan (background site; from 17 March to 4 April, 2005), using a Particle-into-Liquid Sampler coupled to a dual channel Ion-Chromatography (PILS-IC) and a denuder-filter pack system (URG Corporation). In both the sites, high levels of nitrous acid (HONO) were observed in the gas-phase, indicating possible active heterogeneous HONO productions at the surface of ambient aerosols. However, particulate nitrite (NO2-) concentrations ranging from 0.33 μ g/m3 to 4.66 μ g/m3 were only measured in the Seoul site. In this study, we therefore investigate likely reason of the particulate nitrite formation in Seoul. In Seoul, sufficient alkalinity can be provided into the atmospheric aerosols from NH3 excessively present in the gas-phase. For example, equivalence ratios of total ammonium to total acidic particulate components in Seoul are by far larger than 1 during the measurement periods, suggesting that particles measured in Seoul are likely in an alkaline condition. Due to the alkaline condition, the HONO molecules produced at the surface of atmospheric aerosols appear to be captured by ammonium (NH4+) inside atmospheric particles, thereby not being able to further participate in the atmospheric O3/NOy/HOx chemical cycles.
A33D-1547
In-Situ (MC/IC) and Remote (Long-Path DOAS) Measurements of HONO During TRAMP.
Nitrous acid is an important component of night-time N-oxide chemistry, and provides a significant source of both OH and NO in polluted urban airmasses shortly after sunrise. Several recent studies have called for new sources of HONO to account for daytime levels much higher than are consistent with current understanding. However, measurement of HONO is problematic, with most in-situ techniques reporting higher values than simultaneous optical measurements by long-path DOAS, especially during daytime. The discrepancy has been attributed to: positive interference in the in-situ techniques, negative interference in DOAS retrievals, the difficulty of comparing the different airmasses sampled by the methods, or combinations of these. During August and September, 2006, HONO mixing ratios from collocated long-path DOAS and automated mistchamber-ion chromatograph (MC/IC) systems ranged from several ppbv during morning rush hour to daytime minima near 100 pptv. Agreement between the two techniques was excellent across this entire range during many days, giving us confidence that both instruments accurately measured HONO during this campaign. Discrepancies that were observed during some nights can be explained by vertical mixing of HONO. Daytime discrepancies during several days in late August/early September are more difficult to explain, but perhaps informative. It should be noted that in previous campaigns at South Pole (summertime) and central Greenland (springtime) HONO mixing ratios in the 10-30 pptv range from the MC/IC were suggested to be too high (compared to NOx and OH) due to unknown interferants. The results from Houston suggest that the polar interferants either do not scale with NOx in urban air, or are thermally labile, hence too-short lived in Houston to be significant.
A33D-1548
Particulate and Gaseous Species in fog and Clear air in Highly Polluted Urban Region of South Asia
An extensive study of PM2.5 composition was conducted in Lahore, Pakistan during winter of 2005-2006 that included both clear air and periods of fog. We deployed a low-volume sampler connected with an annular denuder system, which consisted of two diffusion denuders and a filter pack consisting of Teflon and nylon filters in series, to sample acidic gases, ammonia, and PM2.5. Teflon filter samples were used to determine PM2.5 mass, anions (F-, BrO3-, Cl-, NO2-, Br-, NO3-, SO42- and C2O42-), cations (Na+, NH4+, K+, Mg2+ and Ca2+) and elements (Be, Na, Mg, Al, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, As, Se, Sr, Mo, Ag, Cd, Sn, Sb, Ba, Tl and Pb). Denuder samples were used to measure selected gaseous species; HCl, HONO, HNO3, SO2 and NH3. Exceedingly high concentrations of all species, relative to major urban areas of US and Europe, were observed. Mean concentrations of the PM2.5 mass, Pb, HONO and NH3 were 191, 96, 19.6 and 50 μg m-3, respectively, which are exceptionally high even at the polluted atmospheric context. Concentrations of most species showed a distinct diurnal variation. Mixing heights, sun index and wind speed played a major role in defining the diurnal pattern. Our data showed a distinct enhancement in the oxidation of SO2 with duration of fog. We use air parcel back trajectories, intercomponent relationships and meteorological observations to explain the sources and the impacts of fog chemistry and mixing heights on atmospheric processing of the chemical constituents. Aerosols were found to carry the signatures of emissions from coal and oil combustion, industrial processes, construction activities and biomass burning in North and Central Pakistan, North India and West Afghanistan, in addition to the local pollution sources. Source apportionment based on positive matrix factorization is in progress. Findings of our study will improve the understanding of the critical roles and interactions between chemical composition and size of atmospheric particles, atmospheric boundary layer and meteorological phenomena that manipulate the chemistry of an urban atmosphere.
A33D-1549
TexAQS Revisited - A Comparison of VOC Measurements in the LaPorte, Texas Vicinity From 2000 and 2006
Gas phase volatile organic compounds (VOCs) were measured by gas chromatography as part of the TexAQS2K campaign during August-September 2000 in LaPorte, Texas then again aboard the NOAA ship Ronald H. Brown while sailing in the Houston vicinity in August-September 2006 supporting the TexAQS06 campaign. Data acquired in 2006 at Barbours Cut, about 4 miles east of the TexAQS2K sampling site, will be compared with results from the earlier campaign. Generally, VOC mixing ratios appear to have decreased over the 6-year interval from 0-40% depending on species. Whether this is due to meteorology, location or actual emission reduction will be discussed at the meeting.
A33D-1550
VOC Source Apportionment in Houston, TX, during TexAQS II
During the TexAQS II campaign in summer 2006 (08/07/06 - 09/30/06) continuous online measurements of C2 - C10 Volatile Organic Compounds (VOCs) were performed with an online gas chromatograph (GC/FID) at the Moody Tower supersite on the University of Houston campus. In addition, VOC data were obtained by the TCEQ and Enhanced Industry Sponsored Monitoring auto-GC networks near the Houston Ship Channel area. A comparative analysis of source contributions at the Moody Tower receptor site and at one Ship Channel site representing major industrial emissions, with the goal of assessing the impact of these emissions onto the 10 km distant receptor site, were performed. The analysis of VOC results indicates complex mixtures from traffic, industrial, and biogenic sources. In order to identify and apportion these source contributions of the VOCs to the receptor site during the campaign, multivariate receptor models were applied. The results of a principal component analysis (PCA) and a positive matrix factorization (PMF) reveal a set of factors and source profiles that are associated with several anthropogenic (industrial, traffic) as well as biogenic sources.
A33D-1551
VOC Measurements In Egbert, Ontario. Characterization Of Sources And Chemical Transformations Of Gas-Phase Species
A number of volatile organic compounds (VOC) were measured at the Environment Canada Centre for Atmospheric Research Experiments in Southern Ontario as a part of the Egbert 2007 study. The sampling location is situated in a rural area 70 km north of Toronto. Measurements were performed in May and June 2007. During this time period the site experienced influence by polluted urban/industrial/traffic air masses and relatively clean continental air. VOC measurements were conducted using various techniques (Gas chromatography/flame ionization detection, Hantsch monitor and Proton transfer reaction mass spectrometry) and some species were detected by more than one method. The results of the intercomparison will be presented. Preliminary data analyses showed that VOC concentrations exhibited a significant time variation associated with air origin and photochemical air aging. The toluene to benzene ratio was used to identify traffic-related VOC sources. Under certain meteorological conditions during night time this ratio reached high values (>4) indicating non processed emissions transported to the sampling site. Apart from anthropogenic influence, biogenic VOC emissions were observed, and will be contrasted to the more polluted periods. During these episodes higher concentrations of isoprene and monoterpenes were detected.
A33D-1552
VOCs measurement at Tomakomai research forest, Japan
Intensive atmospheric field measurement was held on at Tomakomai research forest during summer in 2006. The purpose of the measurement was to observe the atmospheric chemistry at enhanced biogenic emission area influenced by urban polluted air, especially in the view point of OH reactivity. Tomakomai research forest is located by city area. Several VOCs including both anthropogenic and biogenic VOC were observed by different methods, canister grab sampling (GC-FID, and GC-MS), on site GC-FID measurement, and PTR-MS measurement. Also general atmospheric species (O3, CO, NOx, SO2) were observed. From the results of anthropogenic species, influence of urban air nearby and of long-range transport was both expected. Also influence of volcano was expected by SO2. The comparison of biogenic VOCs by different methods showed good agreement. The problem during the canister storage was not observed. Observed terpenes (alfa-pinene, camphene, limonene, and beta-pinene) showed higher concentration during night and lower during daytime. The maximum concentration was about 900 ppt as the sum of terpenes. On the other hand, isoprene showed higher concentration during daytime and low during night. The maximum concentration was about 1.5 ppb. There concentrations were not high as expected as a forest atmosphere. Since the temperature during the campaign was 27 C at maximum, the emission from the plants was not very enhanced. The comparison with directly observed OH reactivity and calculated OH reactivity from various species showed some difference during daytime. The difference has good correlation between isoprene. The difference would be caused by unmeasured species, which should have similar concentration variation as isoprene. The signal of m/z=71 observed by PTR-MS, corresponding to VMK and MACR, showed quite similar variation of isoprene. They ware produced by the oxidation of isoprene. Even if their very high concentrations were assumed, the difference of the OH reactivity can not be explained.
A33D-1553
Ozone Photochemistry in Houston urban and industrial plumes as reflected by ambient measurements of carbonyls and organic nitrates
Comprehensive airborne measurements of ozone and its precursors, as well as other secondary photochemical products were made during the Texas Air Quality study in the late summer and early fall of 2006 on board the NOAA WP3 aircraft. The oxidation of volatile organic compounds (VOCs) in the presence of nitrogen oxides leads to the formation of ozone. Besides ozone, other secondary species such as carbonyls and organic nitrates are formed that are characteristic of the parent VOC species. The ambient measurements of secondary species reflect the integrated effect of emissions, photochemical production and loss, as well as removal processes. The formation of the secondary trace gases during progressive transects of plumes from the urban and industrial emission regions of Houston will be examined to elucidate the contribution of precursor emissions on the rate and efficiency of the formation of ozone.
A33D-1554
Determination of formaldehyde mixing ratios in polluted air with PTR-MS: Laboratory experiments and field measurements
Formaldehyde (HCHO), the most abundant carbonyl compound in the atmosphere, is generated as an intermediate product in the oxidation of nonmethane hydrocarbons. Proton transfer reaction mass spectrometry (PTR-MS) has the capability to detect HCHO from ion signals at m/z 31 with high time-resolution. However, the detection sensitivity is low compared to other detectable species, and is considerably affected by humidity, due to back reactions between protonated HCHO and water vapor prior to analysis. We performed a laboratory calibration of HCHO by PTR-MS and examined the detection sensitivity and humidity dependence at various field strengths. Subsequently, we deployed the PTR-MS instrument in a field campaign at Mount Tai in China in June 2006 to measure HCHO in various meteorological and photochemical conditions; we also conducted intercomparison measurements by Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS). Correction of interference in the m/z 31 signals by fragments from proton transfer reactions with methyl hydroperoxide, methanol, and ethanol greatly improves agreement between the two methods, giving the correlation [HCHO]MAX-DOAS = (0.99 ± 0.16) [HCHO]PTR-MS + (0.02 ± 0.38), where error limits represent 95% confidence levels.
A33D-1555
Formaldehyde distributions and its relation to ozone formation in Seoul metropolis during the summer of 2004 and 2005
Carbonyl compounds and other reactive gases were measured at the campus of Korea University, Seoul, South Korea in June of 2004 and 2005. Carbonyl measurement was performed by using DNPH-derivative method and automated carbonyl measurement system in 2004 and 2005, respectively and they provided reliable data. Also, meteorological parameters were measured. Daily max concentrations of measured species were examined with HCHO and O3. The correlations in 2004 were relatively clearer that 2005 and this was resulted from the different phase of each wind plots of carbonyl and other gases. In 2004 and 2005, HCHO variations during the whole period were very similar with temperature. In addition, correlation between max HCHO and max photochemical reactivity (= K*VOC concentrations) showed an improved one than when max HCHO and max VOCs. O3 also showed good correlations with other gases. HCHO plays a significant role in O3 production, but O3 production was limited by the NO2 concentrations. In addition, meteorological parameters also influenced the amount of O3 produced during the daytime. This indicates that meteorological parameters should be considered to investigate the photochemical evolution process and that formation of HCHO and O3 in urban area is under influence of photochemical reactivity and NO2 concentrations, respectively.
A33D-1556
Aircraft and Ground-Based Measurements of Hydroperoxides during the 2006 MILAGRO Field Campaign
Mixing ratios of hydrogen peroxide and hydroxymethyl hydroperoxide were determined aboard the U.S. Department of Energy G-1 Research Aircraft during the March, 2006 Max-Mex/MILAGRO field campaign in Mexico. Ground measurements of total hydroperoxide were made at the T1 site at Universidad Technologica de Tecámac, about 35 km NW of Mexico City. Aloft, hydrogen peroxide mixing ratios near the source region generally were near 1 ppbv and hydroxymethyl hydroperoxide concentrations were at or near the detection limit of 0.38 ppbv. At the T1 site, the average hydroperoxide concentrations were typically 1 to 2 ppbv. Such concentrations are much lower than predicted from photochemical models based on the 2003 Mexico City study. Strong southerly flow resulted in transport of pollutants from the T0 to T1 and T2 surface sites on several flight days. On these occasions, we observed that peroxide concentrations aloft progressively decreased as the G-1 flew downwind, consistent with the low or negative net peroxide production rates computed using a photochemical box model. The pervasive low hydrogen peroxide concentrations are a consequence of the lack of availability of its precursor HO2, whose concentration is suppressed through reaction with the extremely high concentrations of NO in the Mexico City plateau. In contrast, higher values of peroxide were observed at takeoff and landing near Veracruz, a site with higher humidity and much lower NOx concentrations.
A33D-1557
Calculations of the Chemical Composition of the Sacramento Urban Plume
Recent measurements within the Sacramento urban plume have provided a detailed benchmark for testing our understanding of tropospheric chemistry. Available measurements include a wide suite of VOC and BVOC, NOy,i, O3, and CO at the source and at a receptor site five hours downwind. Further, the meteorology in the region is extremely regular making it possible to evaluate effects of temperature or day-of-week patterns with a single season of measurements. Here we use a Lagrangian model representing transport from Granite Bay, a suburb at the eastern edge of Sacramento, to the University of California Blodgett Forest Research Station (UC- BFRS). The model represents chemistry based on MCM v3.1 along with mixing and dilution. The model is initiated with concentrations of NOx, peroxynitrates, alkyl and multifunctional nitrates, HNO3, VOCs and O3 based on measurements at the edge of the Sacramento suburban sprawl east of the city. Biogenic VOC emissions throughout the transect are included. The outputs of the model are compared with ozone measurements at Cool three hours downwind, and detailed measurements of VOC, the speciation of the nitrogen oxides and O3 at UC-BFRS, 5 hours downwind of the Sacramento suburbs in the center of the Mountain counties air basin. The comparisons indicate 1) O3 at UC-BFRS and Cool is largely driven by the combination of rural biogenic emissions and urban NOx emissions, 2) that OH is underestimated by standard chemical models, 3) that partitioning of NOy is dominated by peroxy and other multifunctional nitrates that are not represented in standard chemical models and which have a strong impact on how much NO2 is available for ozone production. We also investigate model representation of temperature and weekend/weekday effects.
A33D-1558
Influence of Black Carbon on Photolysis Rates and Ozone Production Rates in an Urban Environment
Tropospheric ozone (O3) is a key compound influencing urban air quality. It is produced by the reaction of oxygen atoms (O) with oxygen molecules (O2). In the troposphere, oxygen atoms are primarily produced by photolysis of nitrogen dioxide (NO2). The presence of aerosols, in particular black carbon (BC), has previously been shown to reduce nitrogen dioxide photolysis rates. It has been suggested that ozone concentrations can be reduced by up to 20% because of the reduction in photolysis rates by BC. In this work, we will present results from the NOAA WP-3 research aircraft participating in the 2006 Texas Air Quality Study (TEXAQS2006). On board the aircraft, spectral actinic fluxes from 280 to 690 nm were measured at 1 Hz using a spectroradiometer. Photolysis rates of 19 compounds, including nitrogen dioxide and ozone, were calculated from the spectral actinic fluxes. Black carbon mass concentrations were measured at 1 Hz using a Single-Particle Soot Photometer (SP2). Photolysis rates were reduced by up to 30% under high BC mass loadings during the measurement campaign. Box model calculations of the tropospheric ozone chemical mechanism, including various precursors, e.g. CO, NO, and NO2, help to describe how these significant reductions in photolysis rates influenced ozone production rates during TEXAQS2006. We will also discuss the impact of black carbon on photolysis rates and ozone production in urban areas in general.
A33D-1559
Gas and Particulate Aircraft Emissions Measurements: Impacts on local air quality.
Air travel and freight shipping by air are becoming increasingly important and are expected to continue to expand. The resulting increases in the local concentrations of pollutants, including particulate matter (PM), volatile organic compounds (VOCs), and nitrogen oxides (NOX), can have negative impacts on regional air quality, human health and can impact climate change. In order to construct valid emission inventories, accurate measurements of aircraft emissions are needed. These measurements must be done both at the engine exit plane (certification) and downwind following the rapid cooling, dilution and initial atmospheric processing of the exhaust plume. We present here results from multiple field experiments which include the Experiment to Characterize Volatile Aerosol and Trace Species Emissions (EXCAVATE) and the four Aircraft Particle Emissions eXperiments (APEX- 1/Atlanta/2/3) which characterized gas and particle emissions from both stationary or in-use aircraft. Emission indices (EIs) for NOx and VOCs and for particle number concentration, refractory PM (black carbon soot) and volatile PM (primarily sulfate and organic) particles are reported. Measurements were made at the engine exit plane and at several downstream locations (10 and 30 meters) for a number of different engine types and engine thrust settings. A significant fraction of organic particle mass is composed of low volatility oil-related compounds and is not combustion related, potentially emitted by vents or heated surfaces within aircraft engines. Advected plumes measurements from in-use aircraft show that the practice of reduced thrust take-offs has a significant effect on total NOx and soot emitted in the vicinity of the airport. The measurements reported here represent a first observation of this effect and new insights have been gained with respect to the chemical processing of gases and particulates important to the urban airshed.
A33D-1560
Comparison of a 13,500-Reaction Near-Explicit Chemical Mechanism With Smog Chamber Data and its Implementation Into a Fast Solver to Study the Ambient Sensitivity of E85 Versus Gasoline Emissions
The study of urban, regional, and global air pollution requires the accurate and fast simulation of atmospheric chemistry, particularly for determining the composition of gases that condense to particulate matter. To date, atmospheric models that have solved chemistry in 3-D have been constrained by computer time, requiring chemical mechanisms used within them to be condensed. When chemicals are grouped within the mechanism, though, individual characteristics and accuracy are often lost. Here, the near-explicit Master Chemical Mechanism (MCM, version 3.1, LEEDS University) is implemented into the SMVGEAR II chemical ordinary differential solver to provide the speed necessary to simulate explicit chemistry in three dimensions. The MCM has over 13,500 organic reactions and 4,600 species. SMVGEAR II is a sparse-matrix vectorized Gear solver that reduces the computation time significantly while maintaining any specified accuracy. The near-explicit mechanism allows species to maintain their individual characteristics and improves the modeling of organic and inorganic chemicals in the atmosphere. This is important for simulating the gas-to-particle conversion of explicit chemicals. The model is used to compare the MCM treatment of alkene and aromatic chemistry with smog chamber data and another chemical mechanism. A test case for the air-pollution impacts of the tailpipe emissions from E85 is also performed. Potential impacts of ethanol are increasingly relevant today because of the increased use of ethanol in transportation fuels. The air pollution impacts of all fuels should be investigated before they are used widely.
A33D-1561
A Study of the Source-Receptor Relationship Using the BTEX Ratios in the Houston Area
The BTEX group includes aromatic species such as benzene, toluene, ethyl benzene and xylene isomers, which are known as hazardous air pollutants due to their impact on human health and the environment. These aromatics present common properties, such as: 1) they are only primary pollutants, which react with the hydroxyl radical mostly during the day and are stable at night; 2) they are frequently emitted together; 3) they are important precursors for ozone and peroxyacetyl nitrate formation. Previous studies used the BTEX ratios to describe the photochemical processes, as well as the physical processes, such as transport and dilution processes. During our presentation we focus on the study of the source-receptor relationship using the BTEX ratios in the Houston area; for selected days of the TexAQS II campaign period the ratios were computed based on the data provided by TCEQ and Enhanced Industry Sponsored Monitoring auto-GC networks near the Ship Channel area and the University of Houston online GC at the Moody Towers. The BTEX compounds were also simulated with the Community Multiscale Air Quality Modeling (CMAQ) system, using an extended version of the Statewide Air Pollution Research Center (SAPRC) mechanism, which explicitly represents more aromatic species than the standard version. The CMAQ results were compared with available observational data.
A33D-1562
Analysis of Nitrogen Dioxide and Sulphur Dioxide in Lima, Peru: Trends and Seasonal Variations
This research was carried out to show a general analysis of the monthly and yearly variation (1996-2002) and the tendency of the nitrogen dioxide (NO2) and sulfur dioxide (SO2) for the 5 stations of the air quality network of Lima. The SO2 and NO2 concentrations were measured by the Dirección General de Salud Ambiental (DIGESA), using the active sampling method and the chemical analysis has been determined by Turbidimetry and Colorimetry for the SO2 and NO2 respectively. The monthly average variation (1996-2001) of SO2 in the Lima Center station has a small annual range (32,4 mikrograms/m3) with maximum values in autumn (April) and minimum in winter (June). The NO2 presents a higher annual range (128,2 mikrograms/m3) and its minimum values occur in the summer and the maximum in spring. The annual averages analysis (2000-2002) of the air quality monitoring network of Lima shows that the SO2 and NO2 values are maximum in the Lima Center station and exceed the Peruvian air quality standard (ECAs) in 30% and 75% respectively. The yearly variation (1996-2001) in the Lima Center station show an increasing tendency in the SO2 (significant) and NO2 (not significant) values, which indicates the critical level of the air quality in Lima, therefore the implementation of the air pollution control programs is urgent.
A33D-1563
Chemical Cycling of Nitrogen Oxides on Urban Films
The films coating urban impervious surfaces have been found to be comprised of about 7 percent inorganic nitrate and about 10 percent organic compounds (by mass). A simple steady-state analysis of the lifetime of the nitrate in the film suggests the existence of a loss process(es) in addition to washout by rainfall. Gas phase nitric acid can be taken up in organic films and lower the film pH. Photolysis of nitrated films using actinic illumination causes loss both of protons and of nitrate anions. This is likely due to a combination of direct and indirect (photosensitized) photochemistry involving nitrate ions, yielding gas phase HONO and / or NO2.
A33D-1564
Ozonolysis of Unsaturated Phosphocholines on NaCl as a Model for Organics on Sea Salt Particles
Sea salt particles, which are a major contributor to the global aerosol burden, may be coated with organic material. A major source of organic material is the decomposition of marine organisms, which have biomembranes that are a mixture of lipids, hydrophobic proteins and carbohydrates. The ozonolysis of 1-oleoyl- 2-palmitoyl-sn-glycero-3-phosphocholine (OPPC) on NaCl as a model for this organic coating on sea salt was followed in real time using diffuse reflectance infrared Fourier transform spectrometry (DRIFTS). Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry and Auger electron spectroscopy were used to confirm the identification of the products. Upon exposure to O3, we observed a stable secondary ozonide (SOZ) as well as carbonyl-containing products. The kinetics of formation of these products and the impact of water vapor and photolysis on them were studied. Water vapor decreased the SOZ yield, and photolysis in the actinic region led to loss of the SOZ and the generation of aldehydes, carboxylic acids and anhydrides. These experiments elucidate the mechanism of formation of secondary ozonides on surfaces in polluted and relatively dry environments, which should be taken into account in the chemistry, photochemistry and toxicity of coastal atmospheres and those in the vicinity of dry, alkaline lakes.
A33D-1565
A New Pathway for Oxidation of Organics through Aqueous Nitrate Ion Photochemistry
Not much is known about the interactions of gas phase organic compounds with nitrate-containing particles in the presence of light. In this work, the photooxidation of alpha-pinene in the presence of deliquesced NaNO3 was studied, and for comparison, with increasing concentrations of NO2 in a 100 L Teflon chamber at 70-88 % R.H. and 296-304 K. The gas phase reaction products tentatively identified by proton transfer reaction mass spectrometry (PTR-MS) include formaldehyde, acetaldehyde, acetone, glyoxal, propanedial, 2,3-dioxobutanal, 3,5,6-trioxoheptanal, pinonaldehyde, pinene oxide, and 4-oxopinonaldehyde. Single particle mass spectrometry (SPLAT II) measured the effective density of newly formed SOA (1.25 g cm-3) and showed that such SOA consisted of a complex mixture of organics and organic nitrates, possibly including pinonic acid and trans- sobrerol. The loss of alpha-pinene per mass of SOA formed was significantly greater in the NaNO3 experiments than in NO2 experiments, indicating enhanced loss of alpha-pinene at the NaNO3 thin film during photolysis. Molecular dynamics simulations predict that alpha-pinene has a significant residence time and contact with nitrate at the surface of the deliquesced nitrate thin film. The combination of experiment and theory indicate that alpha-pinene is oxidized at the interface of the aqueous NaNO3 by O(3P) and OH generated in the nitrate photolysis. This new mechanism of oxidation of organics may be partially responsible for the correlation between nitrate and the organic component of particles observed in some field studies, and it may also contribute to the missing source of SOA needed to reconcile model predictions and field measurements. Some results from a new, large (20 foot, 18 in. dia.) flow system designed to study nitrate aerosol interaction with organics during irradiation are also presented.
A33D-1566
Estimation of Ultraviolet/Visible Absorption by Secondary Organic Aerosols From the Spectra of Condensible Gas Phase Precursors
The production of atmospheric secondary organic aerosols (SOA) is believed to proceed via the condensation of a large number of partly oxygenated intermediates of the gas-phase degradation of hydrocarbons. While the majority of these intermediates has not yet been detected in the atmosphere, their chemical identities and concentrations are predicted by highly detailed gas-phase chemical schemes, such as the Master Mechanisms. Here, we use the Self-Generating Master Mechanism (SGMM) to represent the gas-phase chemistry of hydrocarbons in the atmosphere of Mexico City, and to partition between gas and particle phases using Raoult's law. The SGMM is nearly explicit, with ca. 105-106 intermediate gas-phase species. Because the SGMM identifies the molecular structure of these intermediate species, it is possible to compute some of their fundamental properties including saturation vapor pressures, solubility coefficients, and spectral absorption. Specifically, we use a chromophore additivity approximation to estimate the absorption spectrum of each gaseous intermediate. When these gases are partitioned to the particles, the spectral information is retained and used to estimate the absorption spectrum of the particles - assuming of course that no additional chemical transformations take place in the particle phase, modifying the absorption. The predicted particle absorption spectrum is compared to measurements obtained in Mexico City.
A33D-1567
Formation and Processing of Organic Aerosols Measured by a Time of Flight Aerosol Mass Spectrometer during TexAQS/GoMACCS 2006
Formation of particulate matter is common in areas with high emissions of volatile organic compounds (VOCs), NOx, and SO2. These particles have lifetimes of days to weeks, and thus can have both local and regional effects on visibility, air quality, and human health as well as direct and indirect effects on climate. During TexAQS 2006, mass concentrations of non-refractory inorganic species (sulfate, ammonium, and nitrate) and total organics in submicron aerosols were measured by a Compact Time of Flight Aerosol Mass Spectrometer (C-ToF-AMS) onboard the NOAA WP-3D aircraft. In this presentation, we analyze composition changes of organic aerosols in different air masses. We examine organic mass spectra along with simultaneous measurements of VOCs and their oxidation products in order to determine the contribution of anthropogenic and biogenic sources to the aerosol organic mass. These measurements were performed in plumes intercepted during the daytime north of Houston where large isoprene emissions were observed. Furthermore, the fresh hydrocarbon-like (HOA) and processed oxygenated-like organics (OOA) fractions of the total organic aerosol mass in several plumes transected during daytime and nighttime are presented and compared. We will also discuss differences in correlations between organic aerosol composition markers and primary or secondary gas-phase species in different plumes.
A33D-1568
High Resolution LC/MS Identification of Water Soluble Organic Matter in Atmospheric Fog Water Samples
Polar organic compounds are found ubiquitously in ambient aerosol, cloud, and fog water samples. Many of these compounds are highly water soluble and have been suspected to affect aerosol and water uptake properties. Despite many efforts, the nature, identity, and origin of these compounds are still not well understood. This stems largely from the many analytical challenges these compounds present. To overcome the presented analytical challenges we have used a combination of liquid chromatography and high resolution mass spectrometry with accurate mass measurement. Our interest is to first understand the identity of these compounds. The high resolution mass spectrometer measurements with 3 ppm mass accuracy allow us to calculate the empirical formulas giving strong indications to the identity of unknown organic compounds. Additionally, the high sensitivity of this instrument allows for the analysis of water samples without pre- concentration or chemical derivatization techniques. We found hundreds of individual organic components in our fog water samples collected in Fresno, California. These organic compounds include molecules containing oxides of nitrogen, oxides of sulfur, and sometimes both together in the same molecule. We studied 25 of these compounds across a 9 hour fog event and measured the changes in concentration during the event. These changes were grouped into three trends implying differences in fog processing of polar organic compounds.
A33D-1569
Comparison of Biomass Burning Signatures From Controlled Laboratory Fires With Burning Events Observed in Polluted Urban Environments of Mexicali and Mexico City
A Proton Transfer Reaction Mass Spectrometer (PTR-MS) and an Aerosol Mass Spectrometer (AMS) were used to measure the gas phase VOCs and fine particle compositions, respectively, associated with biomass burning in a series of controlled laboratory biomass fire experiments during project FLAME (Missoula Fire Lab 2007) and three separate field campaigns, Mexico City 2003, 2006 and Mexicali 2005. For the laboratory fire experiments, a quantitative comparison of the biomass burning tracer species such as acetonitrile and hydrogen cyanide in the gas phase to particulate levoglucosan and the biomass burning organic aerosol (BBOA) fraction is presented as a function of fuel type. Using the relationships derived from the laboratory experiments, specific time intervals were identified during the field campaigns when biomass burning plumes were mixed into the polluted urban plumes and the relative contributions of different emission sources are evaluated.
A33D-1570
Modeling Gas-Aerosol Processes during MILAGRO 2006
Significant gas-aerosol interactions are expected in the Mexico City outflow due to formation of various semi- volatile secondary inorganic and organic gases that can partition into the particulate phase and due to various heterogeneous chemical processes. A number of T0-T1-T2 Lagrangian transport episodes during the MILAGRO campaign provide focused modeling opportunities to elucidate the roles of various chemical and physical processes in the evolution of the primary trace gases and aerosol particles emitted in Mexico City over a period of 4-8 hours. Additionally, one long-range Lagrangian transport episode on March 18-19, 2006, as characterized by the Controlled Meteorological (CMET) balloon trajectories, presents an excellent opportunity to model evolution of Mexico City pollutants over 26 hours. The key tools in our analysis of these Lagrangian episodes include a comprehensive Lagrangian box-model and the WRF-chem model based on the new Model for Simulating Aerosol Interactions and Chemistry (MOSAIC), which simulates gas-phase photochemistry, heterogeneous reactions, equilibrium particulate phase-state and water content, and dynamic gas-particle partitioning for size- resolved aerosols. Extensive gas, aerosol, and meteorological measurements onboard the G1 and C130 aircraft and T0, T1, and T2 ground sites will be used to initialize, constrain, and evaluate the models. For the long-range transport event, in-situ vertical profiles of wind vectors from repeated CMET balloon soundings in the Mexico City outflow will be used to nudge the winds in the WRF-chem simulation. Preliminary model results will be presented with the intention to explore further collaborative opportunities to use additional gas and particulate measurements to better constrain and evaluate the models.