A21G-01 INVITED
Radical production as a controlling factor for oxidant fields and secondary organic aerosol production in the polluted atmosphere
Recent field studies indicate that the gas-phase processing of primary VOCs, followed by condensation and reactive uptake of reaction products to aerosols is largely responsible for the high portion of organic aerosol mass in the Mexico City Metropolitan Area. The sources of the extremely fast and larger than expected formation of secondary organic aerosol are presently not clear, and could indicate missing SOA precursor VOCs or missing processes of SOA formation in current models. An additional uncertainty arises from our incapacity to adequately predict oxidant fields, in particular in the high NOx regime typical for morning hours, when observed levels of OH and HO2 radicals are inconsistent with our current understanding of HOx chemical cycles. Recent experimental and modeling evidence indicates that the processing of pollutants in the MCMA is NOx suppressed, and VOC limited, at different times of day. Chemical processing of primary VOCs in either of these chemical regimes is limited by the production of radicals. The oxidative capacity of the MCMA is being assessed in terms of the driving factors for OH, HO2 and RO2 radical production, as well as the recycling of radicals via the ROx cycle inside the city. Experimentally constrained model predictions indicate that the production of semivolatile VOC oxidation products due to uncertainties in our understanding of gas-phase radical processes adds significantly to the uncertainty in predicting SOA.
A21G-02
Correlations between Ozone and Secondary Organic Aerosol observed in Urban Locations
Observations in Mexico City during the MILAGRO 2006 campaign and in Houston during the TexAQS 2000 campaign have shown correlations between Ox (Ox = O3 + NO2) and the oxidized component of organic aerosol (OOA), which is interpreted as secondary organic aerosol (SOA). Such a correlation is expected given that ozone production and SOA formation are both fundamentally related to the oxidation of VOCs. We quantitatively investigate the observed correlation between OOA and Ox by comparison with the calculated ratio of the production rates of SOA and ozone. P(SOA)/P(Ox) is calculated using measurements of ambient VOCs, predicted concentrations of unmeasured compounds from vehicle exhaust, and known atmospheric oxidation mechanisms
A21G-03
Photochemical and meteorological conditions during the 2006 TexAQS II Radical and Aerosol Measurement Project (TRAMP)
The TexAQS II Radical and Aerosol Measurement Project (TRAMP) was an atmospheric chemistry field campaign from mid-August to early October 2006 with the primary objective to better understand processes important to the photochemical cycling of atmospheric radical and aerosol species in the Houston atmospheric environment. Photochemically important trace gas and aerosol species, as well as the relevant meteorological and solar conditions were measured on the roof of an 18-story building at the University of Houston. During the TRAMP campgain, multiple 1-hr and 8-hr ozone exceedences were observed. The basic photochemical conditions (CO, NO, NOx, O3, j-values, AOD) during the both clean and polluted days are compared with meteorological conditions (T, P, RH, clouds, wdir, ws) to identify the factors important to ozone events at this site. Chemical and meteorological conditions during the 2006 ozone season are compared to 2000 and 2005 when similar photochemical measurement campaigns were performed in Houston.
A21G-04
Fast airborne formaldehyde measurements during the Texas Air Quality Study (TexAQS) in 2006
Airborne, 1 second measurements of formaldehyde were performed using a tunable mid-IR difference frequency generation (DFG) laser spectrometer. The average detection limit (S/N=1) for all flights was 117-pptv (1 s) and 21- pptv (60 s), respectively. For most flights, the instrument was operated semi-autonomously with a duty cycle of 70% (60 s sample / 30 s background, including cell/inlet flushing) affording good spatial coverage and precision. The instrument worked with the quoted performance for over 94% of research flight time. Downwind plumes from power plants, the Houston shipping channel, and selected refineries have been intercepted and show a clear signature and correlation with ozone and NOy. We will present chemical processing signatures of formaldehyde and its critical role in governing the production of ozone in the greater Houston, TX area.
A21G-05
Impacts of primary formaldehyde on the photochemistry in Mexico City
Mexico City Metropolitan Area (MCMA) is characterized with unusually high emission rates of formaldehyde (HCHO), and the primary HCHO contributes significantly to the HCHO ambient level. As an important radical source, HCHO with a high concentration may have significant influence on the urban photochemistry. In this study, a 3-D chemical transport model (CAMx) is employed to examine the impacts of primary HCHO on the ambient HCHO concentration, the radical budget (radical initiation, propagation and termination as well as radical concentration), and ultimately on the ozone photochemical formation in the MCMA under different meteorological conditions. Simulated HCHO concentrations are compared with concurrent MCMA-2003 measurements, and the simulated partitioning between the primary and secondary sources is compared with results from a statistical analysis. The HONO contribution from the heterogeneous source is also considered.
A21G-06
In-situ Ground-Based and Airborne Formaldehyde Measurements in the Houston Area During TexAQS-II
Formaldehyde is considered to play a significant role in summertime photochemistry in the Houston area, in particular it is considered an important source for radicals. Secondary formation seems to be the most important fraction of ambient HCHO. Enhanced nighttime values may indicate primary sources. Potential sources may include mobile sources such as traffic exhaust, in particular not well maintained Diesel engines. Other possible sources may include point sources such as coffee roasting and flares from refineries. In this study we focused on the TexAQS-II continuous in-situ formaldehyde data set based on Hantzsch reaction which was obtained in the Ship Channel area (HRM3 and Lynchburg Ferry site) and at the Moody Tower for several weeks. We also include in-situ HCHO measurements obtained with the same technique aboard the Baylor aircraft during TexAQS-II flight missions. Formaldehyde data was compared to several trace gases that are supposed to be coemitted including CO (traffic), ethylene (flares), and SO2 (industry). In order to keep photochemical processes at a minimum special focus was on nighttime data. Case studies will be discussed where meteorological conditions including recirculation and boundary layer developments seem to play a major role in the redistribution of HCHO. Observations will be compared to CMAQ model studies.
A21G-07
Photochemical Modeling at Santiago, Chile (33.5° S, 70.6° W)
The greater metropolitan region of Santiago, Chile (6.5 million inhabitants) is located in a basin with complex topography that promotes pollutant trapping below the subsidence-based thermal inversion. The (diurnal) upwind land use consists of agriculture activities that contribute to the emissions of the city itself. Santiago is the 7th Latin American city in population, and 40% of the country's inhabitants live there. Steady economic growth in the last 20 years has resulted in a fast increment of car ownership, industrial activity, fuel consumption, etc. As a result of air quality regulations, ambient PM10 and PM2.5 concentrations have been reduced significantly between 1990 and 2000. However, ozone ambient concentrations do not show a downward trend, and the 98th percentile of the 8-h moving average consistently exceeds the 120 (μg/m3) standard. Also, the current annual ambient PM2.5 concentration is near 30 (μg/m3), twice the US standard. We have developed an emissions inventory for the greater metropolitan region of Santiago (base year 2005), including agriculture and biogenic emissions at the regional scale. We use the MM5 mesoscale modeling system coupled with the CAMx air quality model to: a) assess the quality of the emission inventory database, b) improve emission estimates by means of inverse modeling, c) model ozone formation and transport, with an emphasis on estimating ozone sensitivities with respect to different geographical regions and emission sources. We do this analysis for two multi-day episodes in spring and summer seasons. Results of constraining CO, VOC, NOx and primary PM emissions with ambient monitoring data using a Kalman filter approach will be shown, along with the results for the ozone sensitivity estimates.
A21G-08
Is There a Megacity Effect on SOA Production?
Mexico City has a more concentrated set of emission sources than found in most, if not all, areas of the U.S., resulting in very high concentrations of gas phase pollutants. It is of interest to determine whether secondary organic aerosol (SOA) per unit precursor depends on absolute concentration. If it did, then the growth of megacities would lead to a different set of impacts compared to the situation where the same number of people and the same amount of industrial activity remain spread out over several smaller population centers. An expectation that SOA production might be greater in regions with high emissions can be justified on the basis of the absorptive/partitioning model, in which aerosol yields increase when there is a large amount of organics in the aerosol phase into which low volatility VOCs can partition. SOA production in Mexico City urban air masses is determined from the change in OA/CO as a function of photochemical age. Dilution is accounted for by normalizing results to CO, an urban tracer that is assumed to be emitted in proportion to SOA precursors. SOA production, so determined during the 2006 Milagro campaign, is compared with that determined in the eastern U.S. by ourselves and others during the 2002 and 2004 NEAQS campaigns. To provide a context for this comparison, we contrast chemical conditions in Mexico City with the NEAQS domain. Subject to several caveats, the Mexico City value for SOA per unit CO is similar to that found in the eastern U.S.