A43A-01
MCCM-WEPS: Coupling of Meteorological, Air Quality and Erosion Models for Mexico City
Since natural dust emissions are an important factor in the air quality of Mexico City, a modeling effort to quantify their sources and evaluate their impact on the population is presented. The meteorological and air quality model Multiscale Climate and Chemistry Model (MCCM) provides the meteorological inputs to the erosion model Wind Erosion Prediction System (WEPS) that then provides the natural PM10 emissions to be transported. The system was developed to study the particles dispersion from natural sources (unprotected soils) as agricultural lands and Lake of Texcoco. These sources are located around the Valley of Mexico City. As a result of this research we developed a system with the capability of modeling the phenomenon of air pollution by natural particles emitted by wind erosion and to generate case study scenarios useful to propose control policies. Some of them are presented here. Also an effort to predict with anticipation this phenomenon is under way.
A43A-02
WRF/Chem Analyses and Comparisons with In Situ, Aircraft and Satellite Data during MILAGRO
During March 2006, the Megacity Initiative: Local and Global Research Observations (MILAGRO) field campaign
was conducted over the Mexico City area in collaboration with several national and international institutions and
agencies. Coordinated aircraft-based and ground-based measurements for gaseous pollutants (CO, O3,
NOx, SO2 etc.) and aerosol particles (PM2.5, PM10) were made during MILAGRO supported by extensive
satellite observations and modeling activities at various scales. The goal of this study is to examine and compare
fully coupled WRF/Chem (Weather Research and Forecasting - Chemistry) model simulations at 3-km resolution
with in situ, aircraft and satellite data during MILAGRO. Emissions input data compiled by the Molina Center for
Energy and the Environment (MCE2) based on the official emissions inventory for Mexico City in 2004 are used for
this study. The emissions rates in this inventory are regarded as representative for typical weekdays in Mexico
City. For Saturday and Sunday, the emissions data are obtained by scaling the total emissions rates by 85% and
75%, respectively. For holidays, the emissions data are obtained by scaling the total emissions rates by 90%.
Preliminary analyses suggest that the model simulations for gaseous species on weekdays agree reasonably
well with observations in terms of pollutant concentrations and diurnal cycles. On weekends and holidays,
appreciable discrepancies are noted between the model simulations and the observations, likely reflecting
uncertainties in the emissions rates for weekends and holidays. Work is underway to employ the MCE2's newly
constructed aerosols emissions data to validate the WRF/Chem simulated aerosol particles and to examine the
transport and transformation of aerosols on local and regional scales. In this study, we will also discuss
emissions control strategies for alleviating pollution problems in Mexico City based on model sensitivity results.
http:aerosols.lanl.gov
A43A-03
Characterizing Ozone Production in Mexico City Using a Chemical Transport Model
Understanding the relation between ozone (O3) and its precursors, nitrogen oxides (NOx) and volatile
organic compounds (VOCs), is a prerequisite for formulating an effective O3 control strategy. In this study, a
3-D chemical transport model (CAMx) was used to investigate the characteristics of O3 production and the
response of O3 production to changes in precursor emissions in the urban region of Mexico City
Metropolitan Area (MCMA) under different meteorological conditions ("O3 - South", "O3 - North", and
"Cold Surge"). Simulated concentrations of O3, CO, NOx and various speciated VOCs were compared
with the measurements from MCMA-2003 Campaign. Uncertainties in the emission inventory were evaluated in
the context of model-observation comparison. This study aims to present a comprehensive and coherent view of
the O3 production characteristics in the MCMA.
http:www.atmos-chem-
phys.net/special_issue21.html
A43A-04
A wintertime study of atmospheric aerosols collected in São Paulo, Brazil.
Atmospheric pollution has become a significant problem for urban areas worldwide. In megacities aerosols are released by different emissions sources. These sources can emit particles in a large variety of sizes. Size distributions provide information on the sources and the sources processes of particles or species (Maenhaut et al. 2002). In the wintertime (2005) twenty PM10 samples were collected in São Paulo city using hi-vol sampler and fine particles (from 6 to 18,000 nm) were collected using NanoMoudi impactor. Polycyclic aromatic hydrocarbons (PAH) recognised as mutagen and carcinogen compounds were identified in PM10 samples and gravimetric concentrations were calculated for fine particles. The results showed PAH total concentrations ranged from 0.27 to 43.8 ng m-3. Phenanthrene (7.3 ng m-3) and anthracene (2.2 ng m-3), lighter compounds were the most abundant PAH; benz(a)pyrene (1.4 ng m-3) mutagenic compound was found in higher concentration than previous work. For PM10 concentrations, 60% of the samples (n=12) are higher than WHO standards (50 μg m-3). Three modes can be found in the particles with mean diameter: 44 nm, 440 nm and 14000 nm. It is in agreement with the fact this site is located inside a large green area (campus of the University of São Paulo), which can be source of particles between 25 and 440 nm. We conclude that at this urban site vehicular emissions are indicated as being the most important PAH emitters. For fine particles, soil resuspension and biogenic emissions seem to be the most important sources.
A43A-05
Influence of Anthropogenic Sources on the Cloud Condensation Nuclei (CCN) in zones near to Mexico City
During March 2006, a field experiment was conducted at the Paso de Cortes site, 60 km SE of Mexico City, at an altitude (4000 m, a.s.l.) where the site is sometimes in the free troposphere and otherwise within the boundary layer. During this experiment, cloud condensation nuclei (CCN) concentrations were measured with a thermal gradient diffusion chamber at applied supersaturations that ranged from 0.2 to 0.7 %. In addition, aerosol size distributions were obtained with a differential mobility analyzer (DMA and concentrations of condensation nuclei (CN) with a TSI Model 3010 CPC. The wind trajectories were computed for the altitude of the observation site from NCEP analysis. In this work the methodological approach was to determine the relationships between CN and CCN concentration and associated size distributions for various air masses arriving at the observation site from different directions according to the potential origin of the anthropogenic particles. Air masses are classified into four principal types: 1) Mexico City (NW), 2) Puebla (SE), 3) Cuernavaca (SW) and 4) local biomass burning. The CCN concentrations clearly depend on the source of the air mass, i.e., the highest concentrations are related to the biomass burning and the lowest from Cuernavaca. Moreover, a typical range can be attributed to each air mass type for the CCN/CN ratio. The objective of this study is to evaluate the evolution of CN and CCN as related to the anthropogenic sources.
A43A-06
ISOTOPIC COMPOSITION OF PARTICULATE MATTER PM2.5 DETECTED DURING MILAGRO CAMPAIGN IN THE SUPERSITE T1
During the Campaign MILAGRO, several filters of Teflon were collected in site T1 (TECAMAC), using the PQ200 sampler of 2.5 micrometer fine particles and measured by means of HPGe nuclear detector at the Research Center for Advanced Materials SC (CIMAV). The objective of the present work is to determine by means of the multivariate analysis, if the isotopic contents of the aerosols (238U, 232Th and 40K) are dependant or not form the originating plume of the metropolitan zone of the Mexico Valley (ZMVM). The results of the research indicate that there isn't a high correlation between the specific activities of the natural isotopes in the study with the wind direction or its strength. Therefore, the conclusion is that the natural radioactive contents of the particulate matter in air are a complete local dependency.
A43A-07
Evaluation of Air Pollution Control Policies in Mexico City Using a Markov Observation Based Model
This paper proposes a Markov observation based model where the transition matrix is constructed using air quality monitoring data for specific pollutant. The primary objective of this model is to analyze the corresponding stationary distributions and evaluate sceneries for the air quality impact of pollution control policies. The model is non predictive and could be applied to every pollutant included in air monitoring data. Two cases of study are presented, ozone and sulfur, over central zone of Mexico City for a seven years span from 2000 to 2006. For analysis purposes, each year were divided in two semesters. For the ozone case, the stationary distribution of both semesters shows a probability diminution of the higher ozone concentrate levels. This tendency along with increasing probabilities of medium levels has been previously observed as "piston effect". In the sulfur case, the first semester displays an oscillatory behavior with little tendency to decrease of the higher sulfur concentrate levels. The second semester shows decreasing probabilities of the higher sulfur levels. The results support a small improvement of air quality and then a favorable evaluation of the diverse pollution control strategies that had been implemented in Mexico City over the last several years.
A43A-08
Diurnal Evolution of Aerosol Optical Properties and Morphology at Pico Tres Padres: A Phenomenological Analysis
Aerosol optical properties affect planetary radiative balance and therefore climate. The optical properties are
related to chemical composition, size distribution, and morphology, which also have implications for human
health and environmental degradation. During the MILAGRO field campaign, we measured ensemble aerosol
absorption and angle-integrated scattering in Mexico City. These measurements were performed using the Los
Alamos aerosol photoacoustic instrument with an integrated nephelometer (LAPA) operating at 781 nm. The
LAPA was mounted on-board the Aerodyne Inc. mobile laboratory, which hosted a wide variety of gaseous and
aerosol instruments. During the campaign, the Aerodyne mobile laboratory was moved to different sites,
capturing the influence of spatial and temporal parameters including location, aging, elevation, and sources on
ambient air pollution. The LAPA operated almost continuously between the 3rd and the 28th of March 2006.
During the same period we collected ambient aerosols on more than 100 Nuclepore filters for scanning electron
microscopy (SEM) analysis. Filter samples were collected during specific pollution events and different times of
the day. Subsequently, SEM images of selected filters were taken to study particle morphology. The elemental
composition of a few individual particles was also qualitatively assessed by energy dispersive X-ray
spectroscopy.
Between March 7th and 19th the laboratory was sampling air close to the top of the Pico Tres Padres, a ~3000 m
high mountain on the north side of the Mexico City. Daily changes of aerosol loading and pollutant concentrations
followed the expected diurnal variations of the boundary layer height. Here we report a preliminary analysis of
aerosol absorption, scattering, and morphology at Pico Tres Padres for three specific days (9th, 11th and 12th of
March 2006). The single scattering albedo (ratio of scattering to total extinction) during these three days showed a
characteristic drop in the tens-of-minutes-to-hour time frame immediately following the growth of the boundary
layer above the sampling site. Later in the day the single scattering albedo grew steadily to reach a maximum in
the late afternoon. The SEM images show a wide variety of aerosol shapes including fractal-like chain aggregates
(possibly soot), spherical particles (possibly tar balls), cylinders, and irregular non-fractal shapes. The increased
afternoon single scattering albedo in the hottest part of the day qualitatively correlated with a relative increase in
spherical particles that typically are not strongly light absorbing relative to fractal-like chain aggregates that are
typically strongly light absorbing. These changes in optical properties and/or morphology can be explained by
multiple mechanisms such as the collapse of fractal-like chain aggregates due to thermal effects and/or
condensation of volatile compounds, coating by organic compounds, and photochemical secondary organic
particle formation. Elemental analysis of a few individual particles yielded a relative large carbon abundance
combined with smaller fractions of oxygen, silicon, metals, and other elements.
http:aerosols.lanl.gov
A43A-09
First Experimental Campaign For Meteorological And Ozone Vertical Soundings And Surface Pollutants Measurements In The Metropolitan Area Of São Paulo, Fall-2006.
Frequently the air quality standards are exceeded in the Metropolitan Area of São Paulo, MASP, mainly due to the emitted gases of the vehicles, reason why great emphasis has been given to the control of the vehicle emissions. Regarding ozone, the dominant situation leads to the need of the control of the volatile organic compounds and oxides of nitrogen, which are the ozone precursors through photochemical processes. Besides the ozone, these processes also generate a range of aggressive substances, generally denominated photochemical oxidizers. Additionally they generate a considerable amount of secondary organic aerosols that, due to their small size, have significant health importance. A pioneering campaign was carried out in the Metropolitan Area of São Paulo (MASP), Brazil. Hourly measurements of the gases NOx, NO, NO2 (on the surface) and O3 (on the surface and in altitude) were carried out from May 12th to May 29th, 2006. This campaign was organized by the group LAPAT together with Instituto Nacional de Pesquisas Espaciais and São Paulo Environmental Protection Agency. Meteorological radiosondes coupled with ozonesoundes were released. The sounds were launched on May 15th (14h50min and 21h18min), May 16th (02h30min, 16h30min and 18h30min), May 17th (00h50min, 10h30min and 14h50min) and on May 18th (06:00 and 11:00), totaling 10 releases. They measured the following variables in the vertical structure of the atmosphere: pressure (mb), relative humidity (%), temperature (°C), altitude (m), currents and ozone (nb). Comparing the measurements of ozone carried out on the surface with those of the first point in altitude that the radiosondes measured, it was observed that there is a good correlation among these values of concentration of ozone (r2 = 0,746). The May 15th and May 16th measurements presented an increase in the concentration of the ozone during dawn, the same doesn't happen on the following days. Since the ozone is not formed during the night, this increase is probably originated from the transport of this pollutant from other levels. This study has great importance for it will be used as input in the validation of air quality models and to describe the evolution and formation of secondary pollutants.
A43A-10
Levels, Composition and Sources of PM in the Mexico City Metropolitan Area During the MILAGRO Campaign
Particle air pollution in urban agglomerations comes mostly from anthropogenic sources, mainly traffic, industrial processes, energy production, domestic and residential emissions, construction, but also a minor contribution from natural sources may be expected (bioaerosols, soil dust, marine aerosol). Once emitted into the atmosphere, this complex mixture of pollutants may be transformed as a function of the ambient conditions and the interaction among the different PM components, and also between PM components and gaseous pollutants. This system is especially complex in mega-cities due to the large emission volumes of PM components and gaseous precursors, the high variability and broad distribution of emission sources, and the possible long range transport of the polluted air masses. Speciation studies help to identify major sources of PM components with the end objective of applying plans and programs for PM pollution abatement. In this framework, concentration levels and compositions of particulate matter (PM2.5, PM10 and TSP) have been measured simultaneously at two sites in the Mexico City Metropolitan Area (T0 and CENICA) and at one site 50 km away from Mexico City (T1) during the MILAGRO campaign (1st to 31st March 2006). Spatial and time (day and night) variations have been analysed. Coarse fraction levels were higher at T1 than at CENICA and T0, contrary to what was expected. This was due to the important soil re-suspension at T1, contributing significantly to the crustal load. Moreover, crustal levels were higher during daytime than during nights at all sites, while some secondary compounds (sulphate and ammonium) presented an opposite trend. Regarding trace elements, levels of Pb, Zn and Cd were higher at T0 than at CENICA and T1, probably due to traffic contribution. Arsenic levels did not show a clear pattern, being alternatively higher at CENICA and T0. Two intense episodes of Hg particulate have been recorded, more noticeable at T1 than at the urban sites. V and Ni showed the same evolution at all sites and fractions, being alternatively higher at the three sites. In order to identify the sources of the studied pollutants, a statistical analysis has been carried out. Crustal, regional and industrial sources were identified at the three sites. Moreover, traffic and fuel combustion sources were found at the urban sites. Finally, a metallurgy source was detected at T1 and CENICA. Nevertheless these results must be considered as indicative of the possible sources but not completely definitive due to the relative low number of samples.
A43A-11
PBL Aerosols SE of Mexico City in the dry Season: Biomass Burning and Windblown Dust and its Impact on Photolysis Frequencies
During the dry season in March 2006 airborne investigations on aerosol distributions, ultraviolet actinic radiation and ozone profiles were performed southeast of Mexico City using an ultralight aircraft as a mobile platform. The area investigated covered the rural area southeast of Mexico City, the Chalco Valley, Huexca and Atlixco south of the volcano Popocatepetl, east of Paso de Cortés to the airport of Puebla and the pass between Puebla and Mexico City north of the volcano Ixtachiuatl. The Chalco valley is the main venting valley of the Mexico City basin to the south. Intense biomass burning was observed on both slopes of the volcanoes leading to strong pyrocumulus cloud production in the northern part of the national reserve and above the motorway Puebla-Mexico. Fine particle (> 10 nm) numbers reached up to 80000/cm3 close to the burning plumes with significant reduction to ~ 30-40000/cm3 in the Chalco valley where coarse particles (> 300 nm) dominated the total mass. Dust devils transporting coarse soil particles up to elevations of more than 4000 m a.s.l. were frequently observed. Particles and air masses of pollution sources in the area can be characterized by aerosol size distributions and/or spectral absorption from multi-wavelength aethalometer measurements as well as from ozone mixing ratios and meteorological data measured onboard. The aerosol impact on photolysis rates and air chemistry is derived from vertical profiles of actinic radiation in the JO1D and JNO2 spectral regimes at 300 nm and 380 nm, respectively. Profiles were flown on both sides of the volcano ridge, south of Popocatepetl and above Tenango del Aire where aircraft measurements were supported by ceilometer aerosol vertical profiles.
A43A-12
PM Levels, Composition and Evolution in a Highly Industrialised Area. Objectives of Improvement
Evolution of levels and speciation of PM10 in the ceramic producing area of Castello (East Spain) was studied from April 2002 until December 2005. To this end, daily PM10 sampling was carried out at three urban sites and one suburban site of the area and chemical analyses were made in about 35 % of the samples. Average PM10 levels varied between 27-36 µg/m3 for the study period. The major constituent was mineral matter, exceeding by 5-12 µg/m3 the usual ranges of annual mineral loads in PM10 at similar Spanish urban or regional background sites with no industrial influence. Based on this comparison and on the efficiency of emission abatement techniques, a reduction target of 3-5 µgPM10/m3 of the annual mean seems to be achievable at the urban sites. Moreover, levels of Li, Sc, Co, Zn, As, Se, Rb, Zr, Cd, Cs, Ce, Tl and Pb were higher than the usual range of concentration in urban areas of Spain. Of these elements, Zr, Zn, Pb and As may be considered as tracers of the ceramic emissions from the study area. Their levels showed a simultaneous decrease with the progressive implementation of emission abatement techniques in frit (glaze component for the manufacture of glazed tiles) fusion kilns of the area. Given the high proportion of facilities with implemented abatement techniques at the end of the study period, the reduction margin for these elements is very low.
A43A-13
Trends In The Elemental Composition Of PM2.5 In Santiago, Chile From 1998 To 2006
Santiago, Chile is one of the most polluted cities in South America. As a response, over the past 15 years, numerous pollution reduction programs have been implemented by the environmental authority, Comision Nacional del Medio Ambiente (CONAMA). This paper assesses the effectiveness of these interventions by examining the trends of PM2.5 and its elemental composition. Daily fine particle filter samples were collected in Santiago, Chile at a downtown location from April 1998 through March 2003. Additionally, meteorological variables were measured continuously. Annual average concentrations of PM2.5 decreased only marginally, from 41.8 μg/m3 for the 1998/1999 period to 35.4 μg/m3 for the 2002/2003 period. PM2.5 concentrations exceeded the annual U.S. EPA Standard of 15 μg/m3. Additionally, about 20% of the days exceeded the old standard of 65 μg/m3, while about half of the days exceeded the new standard (effective in 2006) of 35 μg/m3. Mean PM2.5 levels during the cold season (April through September) were three times as high as those observed in the warm season (October through March). Particulate mass and elemental concentration trends were investigated using regression models, controlling for year, month, weekday, wind speed, temperature and relative humidity. Significant decreases were observed for Pb, Br, and S concentrations, while minor but still significant decreases were observed for Ni, Al, Si, Ca and Fe. The larger decreases were associated with specific remediation policies implemented to remove lead from gasoline, the reduction of sulfur levels in diesel fuel, and the introduction of natural gas. These results suggest that the pollution reduction programs, specially the ones related to transport, have been effective in reducing various important components of PM. However, particle mass and other associated element levels remain high and it is thus imperative to continue the efforts to improve air quality, particularly, focusing on industrial sources. New results including data collected until August 2006 will also be presented.
A43A-14
Analyses of Mexico City Surfaces Properties Using MODIS and Sun-photometer Data During MCMA-2006/MILAGRO Campaign
Monitoring the air quality in megacities around the world and understanding the impact of the emitted pollutants on the local and global climate is a challenge for the scientific community. The air quality monitoring system in megacities has been based almost exclusively on ground-based station networks. Satellites can be used as a complementary tool to the ground-based stations by providing in a systematic way aerosol property with a higher spatial resolution than the continuous ground-based stations. With the growing concern over aerosol particle pollution in megacities, interest in the higher resolution ô data from satellite retrievals is increasing. However, to achieve a higher spatial resolution from the MODIS instrument, it is essential to have more accurate information on the surface reflectance and aerosol optical properties. The heterogeneity of the surface cover in an urban environment only increases the uncertainties in the estimation of the surface reflectance and therefore aerosol optical depth. In this work we perform an analysis of the surface reflectance specifically for the Mexico City urban area. We also present the improvement that the new estimation can provide for the ô retrievals over the region. We performed this analyses based on an unprecedented measurement of ô from a network of sun photometers deployed in Mexico City during the MILAGRO Campaign experiment in 2006. The Milagro (Megacity Initiative: Local and Global Research Observations) campaign of air pollutant measurements was carried out during the month of March 2006 in Mexico City. It has four main components, this work is part of the MCMA-2006 (Mexico City Metropolitan Area - 2006) led by the Molina Center on Energy and the Environment.
A43A-15
Monitoring ultrafine particulate material in Santiago de Chile
A monitoring campaign was performed in Santiago de Chile during a winter month of 2003 and 2006 (July) using several instruments to measure particulate material. For the first time, the size distribution of ultrafine particles was measured in Santiago, and an estimation of its sources was done by analyzing its temporal variation. The study was performed in three sites; one of them is located in the eastern part of Santiago, a sector with low particle concentration and about 100 m from a busy street. The other site is located in the western part, which is the sector that has the highest concentration of fine and coarse particle matter during winter, also located far from a street. The third site is located less than 5 m from the busiest street in Santiago. In all stations traffic is the dominating source for fine and ultrafine particles, however the size distribution in site near the street is peaked towards lower sizes. The size distribution measurements presented here indicate that aerosol dynamics play a more important role for the Santiago case as compared to cleaner cities in Europe. The size distribution in the sites far from streets during most of the day is peaked around 70 nm, indicating that there is a dominance of the soot mode over the nuclei mode. Changes in the particle size during different hours of the day reflect both variations in meteorological mixing conditions as well as effects of aerosol dynamic processes such as coagulation, condensation and dry deposition. A relative increase in the number of large particles (d > 70 nm), as compared to the number of smaller particles (d < 70 nm) is an indication of pollution transport with aged particles from other parts of the city.
A43A-16
Seasonal Variation of Polyciclic Aromatic Hydrocarbons in the Atmosphere of Mexico City
With the aim to determine concentrations levels and seasonal variation of polyaromatic hydrocarbons (PAH) in the northern of Mexico City, three sampling campaigns were performed during cold dry season, rain season and hot dry season in 2005. Particle-bound polycyclic aromatic hydrocarbons were collected on quartz fiber filters and gas-phase PAH was collected into polyurethane foam (PUF) with XAD-4 resin. Both types of samples were extracted with a dichloromethane and acetonitrile (2:1). Quantification of 17 PAH was carried out using gas chromatography .mass spectrometry (GC-MS). Recovery factors were determined by spiking filters with standard (NIST SRM 1647c). Average PM10 concentrations during the three seasons were 62.3, 68.9 and 131 μ gm- 3 for hot dry season, rainy season and hot dry season respectively. In the hot dry season the total of quantified PAH in particles was in average 1.99±0.82 ngm-3 and the dominant PAH compounds were Pyr, BghiP and Cry; the rainy season presented a total of 2.8± 1.2 ngm-3 of quantified PAH in particles and the pore abundant PAH were BaA, BbF, and BghiP; the cold dry season presented the highest concentrations with an average of quantified PAH in particles of 15.5± 4.9 ng-3 and BbF, BaA, and BghiP were the most abundant compounds. The most abundant PAH in the gaseous phase in the three seasons were naphtalene, pyrene and acenaphtylene with average concentrations of 132, 2.7 and 2.1 ngm-3 in the hot dry season, 18.1, 0.47 and 1.8 ngm-3 in the rainy season and 58, 1.71 and 2.37 ngm-3 in the cold dry season. Principal component analyses and cluster analyses were applied to relate the PAH concentrations with their sources.
A43A-17
Comparison of Aerosol Optical Properties and High Spatial Resolution Retrievals with MODIS in Sao Paulo, Mexico City and Beijing Megacities
The degradation of air quality is a consequence of the fast-growing rates in the urban areas and has harmful effects on human health. It is known that the aerosol particles are important contributors to the atmospheric energy budget, cloud properties and atmospheric chemistry. There is still a large uncertainty in estimating the global aerosol total (direct and indirect) effect on the climate, and the uncertainty is even larger in understanding the urban pollution effect on local or global climate. The MODIS sensor algorithm of aerosol optical properties over continent brought a new light to the monitor of aerosols from urban areas. The current operational product with 10 km x 10km is focused on the climatic effects of the global aerosol distribution. The analyses of aerosol distribution over an urban area require a higher spatial resolution. This work explores the use of MODIS in the retrieval of AOD over urban areas with higher spatial resolution. Aerosol Optical Depth (AOD) was retrieved with 1.5x1.5 km resolution over the megacities Sao Paulo, Mexico City and Beijing in a long time series (2002-2005). The products were compared with the CIMEL from the AERONET Global Aerosol Robotic Network. Advantages and limitations have been analyzed in the methodology. This study analyze the aerosol optical properties such as single scattering albedo, asymmetry parameter and size distribution and compare between the different sites of measurement. The MODIS AOD products with finer spatial resolution generated in a systematic way is a powerful tool to complement the ground-based monitoring stations measurements.
A43A-18
Physical Properties and Chemical Composition of Aerosols sampled in T1 site during MILAGRO Campaign
Results from pollutant measurements and meteorological variables corresponding to the month of March of 2006 during the MILAGRO campaign at site T1 are presented (Tecamac, State of Mexico). Three 8-stage cascade impactors (MOUDI) were employed to obtain aerosol samples of different sizes. For organic species analysis, samples were collected with a PM2.5 High Volume sampler. Mass and chemical composition (inorganic and organic species) were obtained with the use of analytical techniques. Particle morphology analysis was done with a TEM-EDAX System. Physical properties of aerosols were measured with a PSAP, a nephelometer and a CPC. According with area meteorology, days with Mexico City urban influence on T1 (March 9-12) and without influence (March 14 and 15) were analyzed. The particle average concentration during the whole campaign was 20,000 particles/cm3. For the days with and without urban influence the average concentrations were 17,500 and 8,000 particles/cm3 respectively. From the MOUDI data the highest particle concentration through the campaign was during the morning in the mode d50=0.32 μm. On the other hand, the cumulative highest concentration of all the stages was observed for March 19 followed by March 9. Scattering and absorption coefficients average obtained on T1 were 5.1x10-5 m-1 and 2.54x10-5 m-1 respectively and single scattering albedo was 0.676. These values show T1 as a polluted atmosphere, just as happens with megacities. Morphology of particles captured in a MOUDI impactor was studied. Particles between d50=0.18 μm and d50=1.8 μm sampled in T1 associated with urban influence (March 9) tended to show less irregular shapes through different periods of that day. These findings suggest the presence of large numbers of secondary aerosols and aged agglomerated particles. Particles ranging from d50=0.18 μm to d50=1.8 μm sampled in T1 and associated mainly with surrounding areas influence, e.g. Tizayuca Industrial Park (March 15) showed variation in morphology with size. More irregular particles were found in the smallest size, and less irregular particles were present in the largest size. The highest concentrations of ions present in all 8 stages of the MOUDI were chlorine, sodium, calcium, and magnesium. For stages less than 1 μm, the dominant ions were sulfate, ammonium, and potassium. The nitrate ion showed an irregular behavior through the campaign. For the days with influence (March 10 to 12), the nitrate ion presented its higher concentration. During episodes without influence (March 14 and 15), potassium, chlorine, and ammonium ions showed high concentration. On average, polycyclic aromatic hydrocarbons present in PM2.5 (from acenaphthylene to benzo[ghi]perylene) had concentrations between 0.2 to 0.5 ng/m3. In particular, naphthalene had a concentration of 2.5 ng/m3. Our results show that Tecamac population is certainly exposed to high levels of pollution from Mexico megacity.
A43A-19
Aerosol optical depth, backscattering profiles and column NO2 and SO2 measurements during March 2006 in Tenango del Aire, Mexico.
The Tenango del Aire site (19.16°N, 98.86°W, 2380 masl) is located some 40 km SE from Mexico City in the Chalco Valley, where some pollution otflow has been predicted by air quality models. Direct and diffuse ground-based irradiance measurements have been made at this site during the MILAGRO field campaign by a UV-MultiFilter Rotating ShadowBand Radiometer (UV-MFRSR) at seven channels (300-, 305-, 311-, 317-, 325-, 332- and 368-nm with 2-nm nominal bandpass) from which several parameters could be calculated, including the aerosol optical depths (AOD), total ozone column (TOC), asymmetry factor (g) and aerosol single scattering albedo (SSA) via an optimal estimation algorithm. NO2 and SO2 column concentrations were measured with the zenith-sky DOAS (differential absorption spectrometer) technique. LIDAR (light detection and ranging) remote sensing of aerosols was done with a Vaisala Ceilometer Model LD40 from which mixing layer heights were estimated. The results of these measurements and a preliminary analysis of the possible sources of the aerosols are presented.
A43A-20
Measurements of Total Gaseous and Particulate Mercury in Mexico City During the MCMA- 2006/MILAGRO Campaign
Mercury, a highly toxic chemical with a complex biogeochemical cycle, is widely distributed and can be found in gaseous and aqueous phases as well as particulate matter. According to the preliminary atmospheric emissions inventory of Hg in Mexico (1999 base), around 31 tons/year are emitted of which approximately 67 % are from gold/Hg mining and refining, followed by 16 % from chlor-alkali plants. TGM levels in two pristine environments in Mexico have been reported to be less than 2.5 ng.m-3, while for urban sites in Mexico City a wider range has been found between 1.5 to 108 ng.m-3. Continuous mercury measurements were carried out within the Mexico City Metropolitan Area (MCMA) during the MCMA-2006/MILAGRO Campaign. Total gaseous mercury (TGM) was determined at two sites, and mercury in the particulate phase was determined at five sites. TGM measurements were done in two supersites: the Northern (T0) and Northeastern (T1) parts of the Metropolitan area, using continuous Ultra-Trace Mercury Vapour Analyzers (Tekran Model 2537A) from March 1 until March 31 with a sampling resolution of 2 hours and 5 minutes, respectively. During two days, both analyzers were run with a 5 minute time resolution at T0 for inter-comparison showing a good correlation (R2 = 0.88). A comparison with previous TGM measurements in Mexico City is also presented. Particles were collected using Andersen and Wedding Hi-Vol samplers at T0, T1, at CENICA (in Southeastern Mexico City), at UNAM (in Southern Mexico), and at the Biznaga Ranch (T2) supersite, a semi-rural area located at the north east of T1. Particles chemical speciation included: Hg and other elements by ICP-MS; ions by ion chromatography; and total, elemental and organic carbon using thermal optical reflectance. In this work, only Hg analyses are reported, elemental composition is discussed elsewhere. Some samples were also analyzed for morphology and elemental analysis using electron microscopy with EDS. Except for the UNAM and T2 sites, Hg in particles correlated with industrial elements possibly associated with the metal smelting industry, such as Pb, Zn, Cu, Cd, and As. Two episodes of possible regional transport of particles between T0, CENICA and T1, on the 3rd and 17th of March, were identified. Lagrangian, stochastic, back-trajectories were simulated using FLEXPART together with mesoscale meteorological simulations from MM5 for T0 and T1 sites. Based on these, an analysis of TGM, Hg in particles and criteria pollutants is presented to identify potential sources of mercury emissions.
A43A-21
A Comprehensive study of Metal Rich Aerosol in an Industrial Region of Mexico City
Single particle measurements in Northern Mexico City have indicated the presence of fine particles that are internal mixtures of Pb, Zn, Cl, P and S. This unique set of markers is consistent with source profiles provided by studies of municipal waste incinerators (MWIs). Previous studies in Mexico City have indicated the presence of Zn and Cl particles that exhibited similar time series to the particles observed in this work. The significance of the current results is that they are derived from single aerosol particles, proving that many of these components are internally mixed and likely from the same source. A comparative analysis was performed on Mexico City aerosols using Aerosol Time-of-Flight Mass Spectrometry (ATOFMS), Computer Controlled Scanning Electron Microscopy/Energy Dispersive X-Ray analysis (CCSEM/EDX), Scanning Transmission X-Ray Microscopy/Near Edge X-ray Absorption Fine Structure Spectroscopy (STXM/NEXAFS) and Proton Induced X-Ray Emission (PIXE). Using a rule based search method, single particle characteristics of ATOFMS and CCSEM/EDX data sets are found to be in complementary agreement. PIXE data indicate that Zn, Pb, Na and Cl are strongly correlated over entire period of study and that there are higher concentrations of Zn compared to all other detected metals. Morphology measurements indicate these metal-rich aerosols are a mixture of spherical and non-spherical particles. STXM/NEXAFS measurements showed that the needle-like particles appear to be composed of Zinc Nitrate and Zinc Oxide. This is consistent with the observations by CCSEM/EDX and single particle mass spectrometry. This talk will focus on the chemistry and temporal variability of these particles. Given that similar events have been regularly observed in previous field studies, this source of particles is likely important in this growing megacity.
A43A-22
Sensitivity of the forecast skill to the combination of physical parameterizations in the WRF/Chem model: A study in the Metropolitan Region of São Paulo (MRSP)
The Planetary Boundary Layer (PBL) is the region of the atmosphere that suffers the direct influence of surface processes and the evolution of their characteristics during the day is of great importance for the pollutants dispersion. The aim of the present work is to analyze the most efficient combination of PBL, cumulus convection and cloud microphysics parameterizations for the forecast of the vertical profile of wind speed over Metropolitan Region of São Paulo (MRSP) that presents serious problems of atmospheric pollution. The model used was the WRF/Chem that was integrated for 48 h forecasts during one week of observational experiment that take place in the MRSP during October-November of 2006. The model domain has 72 x 48 grid points, with 18 km of resolution, centered in the MRSP. Considering a mixed-physics ensemble approach the forecasts used a combination of the parameterizations: (a) PBL the schemes of Mellor-Yamada-Janjic (MYJ) and Yonsei University Scheme (YSU); (b) cumulus convections schemes of Grell-Devenyi ensemble (GDE) and Betts-Miller-Janjic (BMJ); (c) cloud microphysics schemes of Purdue Lin (MPL) and NCEP 5-class (MPN). The combinations tested were the following: MYJ-BMJ-MPL, MYJ-BMJ-MPN, MYJ-GDE-MPL, MYJ-GDE-MPN, YSU-BMJ-MPL, YSU-BMJ-MPN, YSU-GDE-MPL, YSU-GDE-MPN, i.e., a set of 8 previsions for day. The model initial and boundary conditions was obtained of the AVN-NCEP model. Besides this data set, the MRSP observed soundings were used to verify the WRF results. The statistical analysis considered the correlation coefficient, root mean square error, mean error between forecasts and observed wind profiles. The results showed that the most suitable combination is the YSU-GDE-MPL. This can be associated to the GDE cumulus convection scheme, which takes into consideration the entrainment process in the clouds, and also the MPL scheme that considers a larger number of classes of water phase, including the ice and mixed phases. For PBL the YSU presents the better approaches to represent the wind speed, where the atmospheric gradients are stronger and the atmosphere is less mixed.
A43A-23
Application of Receptor Models for the Identification of Metal Sources in Tampico, Mexico.
A factor analysis model based in a multivariate statistical analysis was used as a complement of correlation coefficients to determine the number and the type of metal sources associated to the receptor monitoring site in the urban zone of Tampico, Tamaulipas, Mexico. To obtain the possible metal sources, 11 variables including metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb, Ti, V and Zn) and particulate matter (PM10) data were used. Results of correlation coefficients showed strong relation between metals as V-Ni and Cu-Zn with 0.89 and 0.85, respectively. In Addition, Cd-Cr and Fe-Ti showed good correlation coefficients of 0.73 and 0.74, respectively. Pb and Zn showed correlation among several metals as Cu, Ni, Fe and Mn. These correlations were verified with the help of factor analysis. 4 factors (possible sources) explaining 85% of the total variance were found. Cu, Mn and Zn were principally found in the first factor, whereas Cd and Cr were mainly found in the second source. V-Ni and Fe-Ti were related to the next sources. Thus, the metals associated could be related to industrial activities and dust suspension near the sampling zone. Meteorological parameters as wind direction and wind velocity were used in addition to correlation coefficients and factor analysis to study the possible sources of these metals.