A42A-01
Comparison of Airborne Sunphotometer and Satellite Sensor Retrievals of Aerosol Optical Depth during MILAGRO/INTEX-B
The 14-channel Ames Airborne Tracking Sunphotometer (AATS-14) was operated on a Jetstream 31 (J31) aircraft based in Veracruz, Mexico in March 2006 during MILAGRO/INTEX-B (Megacity Initiative-Local And Global Research Observations Phase B of the Intercontinental Chemical Transport Experiment). AATS measured aerosol optical depth (AOD) at 13 wavelengths (354-2139 nm) and water vapor column content in 13 flights that sampled clean and polluted airmasses over the Gulf of Mexico and Mexico City. J31 flights were coordinated with overflights of several satellites, including Aqua, Aura, Terra, and Parasol. In this paper we will focus on comparing AATS retrievals of AOD with corresponding AOD values retrieved from spatially and temporally coincident or near- coincident measurements acquired by the satellite sensors MODIS (Aqua and Terra), MISR (Terra), and OMI (Aura). Our preliminary analyses of 37 coincident observations by AATS and MODIS-Terra and 18 coincident observations between AATS and MODIS-Aqua indicate that for MODIS-Terra, 98 percent of near-IR AOD retrievals fall within the estimated uncertainty range ± 0.03 ± 0.05 AOD, while 100 percent of MODIS-Aqua near-IR AOD retrievals fall within this uncertainty range. Collocated measurements by AATS, MODIS-Terra and MISR within three MISR retrieval grid cells over the Gulf of Mexico on March 10 have been analyzed. Mid-visible (at 446 and 558 nm) AOD retrievals produced by the MISR standard operational algorithm Version 19 compare well both with AATS and with MODIS-Terra observations in two of the three MISR grid cells, while the MISR AOD retrievals in the third cell exceed those obtained from AATS or MODIS. Aerosol data products produced by OMI include AOD, single scattering albedo (SSA), and aerosol indices. These OMI aerosol products are derived using wavelengths and algorithms that differ significantly from those of MODIS and MISR. We have identified four Aura overpasses during MILAGRO/INTEX-B for which OMI AOD spectra have been retrieved using the UV and the multiwavelength retrieval algorithms and AATS AOD spectra have been calculated at coincident or near-coincident times and locations. Three of these (March 3, 10 and 17) were over the Gulf of Mexico, and one (March 19) was over Mexico City. In general, OMI AOD retrievals exceed corresponding AATS AOD values.
A42A-02
Impacts of Megacities on Regional Air Quality from MOPITT Observations and MOZART Model Results
The emissions from large cities, such as Mexico City, Los Angeles and Tokyo, as well as densely populated regions in India, China, etc., can clearly be seen in the CO retrievals from the Measurements of Pollution in the Troposphere (MOPITT) instrument on the Terra satellite and will be illustrated in this presentation. To assist in the flight planning and analysis of the MILAGRO field campaigns in Mexico during March 2006, MOPITT CO retrievals were assimilated in the global chemical transport model MOZART, using fire emissions based on satellite observations. To understand the impacts of Mexico City and other megacities on regional air quality, additional simulations of MOZART have been performed. The CO emissions from different types of sources (biomass burning, industry, etc.) are "tagged" in the model to show their relative contribution to the regional atmospheric composition. In addition, NO emissions from a single megacity or region are tagged to identify the contribution of ozone from a given source. The contribution from Mexico City pollution to the regional and global atmosphere will be compared to other megacities.
A42A-03
Megacity Radiative Forcing: A Mexico City Case Study
We assess the radiative forcing of the largest megacity in North America, Mexico City. While particular aspects of
the regional environmental impacts of cities on their surroundings have been thoroughly investigated, e.g., air
quality and acid rain, relatively little effort has been focused on the net radiative impact of a megacity on global
climate. The range of radiative impacts from a megacity covers many spatial and temporal scales from short-term
regional-scale effects due to aerosols and relatively short-lived gases (ozone) to long-term global-scale impacts
due to longer-lived trace gases (e.g., carbon dioxide, methane). In this study we combine chemistry-transport
model simulations from the Model for Ozone And Related Chemical Tracers (MOZART-2) with in situ and satellite
observations from the Aerosol Robotic Network (AERONET) and the Moderate Resolution Imaging
Spectroradiometer (MODIS) to calculate the global radiative forcing of megacity emissions. We also explore the
radiative impact of various emission control strategies that focus on improving regional air quality. Our results
suggest that the warming by greenhouse gases like carbon dioxide and ozone can be moderated or exacerbated
by aerosols depending on their optical properties. As the size and number of megacities increase and clean air
regulations are implemented, metrics such as the net radiative forcing may become increasingly important in
comparing the impact of urban centers and assessing the trade-offs between improving local air quality and
minimizing global radiative impacts.
http:aerosols.lanl.gov
A42A-04
Regional and Global Megacity Impacts: A comparison of boundary layer and free troposphere airmasses over Mexico, the Gulf of Mexico, and the Eastern North Pacific
During March 2006 both the NASA/UND DC-8 and NSF/NCAR C-130 aircraft were used to characterize natural and anthropogenic aerosols over Central Mexico and the Gulf as part of MILAGRO. In late April and early May the same aircraft were used to characterize airmasses over the remote Eastern North Pacific (20-60N, 120-180W) as part of INTEX. The upper troposphere (6-12 km) over Mexico and the Gulf was found to be relatively pristine compared to mid- latitudes reflecting the well aged nature of the airmasses; a combination of long-range transport over the sub- tropical Pacific, and dilution due to inter-hemispheric exchange. The continental boundary layer over Mexico was heavily influenced by anthropogenic pollution from Mexico City. Measurements of scattering and absorption Angstrom exponent as well as the increase in light scattering as a function of relative humidity, f(RH) show that aerosols in the boundary layer are complex mixtures of both fresh and aged pollution, biomass burning and mineral dust. Few episodes of deep convection were observed during MILAGRO. However, anthropogenically influenced boundary layer and lower free troposphere air was transported from Mexico City toward the Gulf of Mexico forming a 1-2 km deep transitional layer above the marine boundary layer. The marine boundary layer over the Gulf is heavily influenced by both anthropogenic and biomass burning emissions from the United States and Mexico. The upper troposphere over the North Pacific was found to be influenced by pollution and dust transported from Asia. Measurements in the Pacific MBL ranged from relatively pristine to polluted due to the entrainment of Asian pollution and dust from the lower troposphere. Vertical profiles of gas and aerosol phase tracers are compared with an emphasis on the in-situ measurements' ability to discriminate between airmasess of different origin. We examine the aerosol size distribution, chemistry and optical properties with an emphasis on the Angstrom exponent for light scattering and absorption as well as f(RH) as a function of relative humidity. The events are put into a broader context through the use of models and satellite imagery.
A42A-05
Evaluation of a Three-Dimensional Chemical Transport Model (PMCAMx) in the Mexico City Metropolitan Area
Atmospheric aerosols have adverse effects on human health, contribute to the visibility reduction and influence the energy balance of the planet. A three-dimensional chemical transport model (PMCAMx) (Gaydos et al., 2007) is used to simulate the particular matter (PM) mass composition distribution in the Mexico City Metropolitan Area (MCMA). PMCAMx uses the framework of CAMx (ENVIRON, 2002) modelling the processes of horizontal and vertical advection, horizontal and vertical dispersion, wet and dry deposition, and gas-phase chemistry. In addition to the above, PMCAMx includes three detailed aerosol modules: inorganic aerosol growth (Gaydos et al., 2003; Koo et al., 2003a), aqueous-phase chemistry (Fahey and Pandis, 2001), and secondary organic aerosol formation and growth (Koo et al., 2004). The aerosol thermodynamic model ISORROPIA has been improved as it now simulates explicitly the chemistry of Ca, Mg, and K salts and is linked to PMCAMx. The hybrid approach (Koo et al., 2003b) for modelling aerosol dynamics is applied in order to accurately simulate the inorganic components in coarse mode. This approach assumes that the smallest particles are in equilibrium while the condensation/evaporation equation is solved for the larger ones. The new CMU organic aerosol model, which is based on the splitting of the organic aerosol volatility range in discrete bins, is also used. The model predictions are evaluated against the PM and vapour concentration measurements from the MCMA-2003 Campaign (Molina et al., 2007). References Gaydos, T., Pinder, R., Koo, B., Fahey, Κ., Yarwood, G., and Pandis, S. N., (2007). Development and application of a three-dimensional Chemical Transport Model, PMCAMx. Atmospheric Environment, in press. ENVIRON (2002). User's guide to the comprehensive air quality model with extensions (CAMx). Version 3.10. Report prepared by ENVIRON International corporation, Novato, CA Gaydos, T., Koo, B., and Pandis, S. N., (2003). Development and application of an efficient moving sectional approach for the solution of the atmospheric aerosol condensation/evaporation equations. Atmospheric Environment, 37, 3303-3316. Fahey, K. and Pandis, S. N., (2001). Optimizing model performance: variable size resolution in cloud chemistry modelling. Atmospheric Environment 35, 4471-4478. Koo, B., Pandis S. N., and Ansari, A. (2003a). Integrated approaches to modelling the organic and inorganic atmospheric aerosol components. Atmospheric Environment, 37, 4757-4768. Koo, B., Gaydos, T.M., Pandis, S.N., (2003b). Evaluation of the equilibrium, hybrid, and dynamic aerosol modeling approaches. Aerosol Science and Technology 37, 53-64 Molina, L.T., Kolb, C.E., de Foy, B., Lamb, B., Brune, W., Molina, M.J., (2007). Air Quality in North Americas Most Populous City Overview of MCMA-2003 Campaign. Atmos. Chem. Phys. Discuss. 7.
A42A-06
Model Sensitivity Analysis of Ozone to Mobile Emissions in the MCMA
Studies of the impacts of air pollution in urban areas with chemical transport models require detailed information on the properties of emitted gases and particles. Moreover, the evaluation of the accuracy of predictions from complex models relies on the quantification of the uncertainties in the main model parameters, and emissions in particular. During the 2002/2003 MCMA and the MCMA-2006/MILAGRO field campaigns in the Mexico City Metropolitan Area (see http:www.mce2.org), the Aerodyne Research Inc. (ARI) Mobile Laboratory characterized on-road vehicle fleet emission indices in fleet-average mode for various vehicle classes using fast-response instrumentation. The measurements obtained by ARI and other groups have been used to validate the emissions inventory in Mexico City and to quantify the uncertainties of emissions as inputs to photochemical models. In this study we perform a sensitivity analysis of the ozone levels predicted from the chemical transport model CAMx to mobile sources by perturbing the base-case scenario of NOX and VOC emissions. Using the Decoupled Direct Method (DDM) in CAMx, we evaluate the model sensitivity coefficients of ozone to its precursors from mobile sources as well as for other emission sources. The sensitivity coefficients obtained with the DDM method are evaluated against the Brute Force Method (BFM). The predicted ozone changes from the sensitivity analysis are compared with observed historical trends of ozone levels resulting from perturbations on the mobile emission sources.
A42A-07
A Multiscale Four-Dimensional Data Assimilation System Applied in the Mexico City Valley
Several modeling studies have shown that four-dimension data assimilation (FDDA) has the ability to improve the simulations of wind, temperature, moisture and mixed layer depth. These works concluded that modeling with FDDA can produce spatially consistent solutions without degrading important dynamical processes. Additionally, it is widely recognized that MM5 with FDDA can be used to develop realistic three-dimensional fields that are completely suited as inputs to air quality or diagnostic meteorological models. In this work, MM5 and FDDA were used to model the weather conditions in the Mexico City Metropolitan Area (MCMA). The surface information was obtained from "Red Automática de Monitoreo Atmosférico (RAMA)" data bases. Sounding data were obtained from "Servicio Meteorológico Nacional". Four simulation domains were used with spatial resolutions of 27, 9, 3, and 1 Km2 respectively. In this work, 15 surface weather stations and 30 sounding points were employed. With this technique, weather predictions have correlation levels higher than 80%. Additionally, these predictions were used as input of the photochemical model MCCM and an important influence on pollutants concentration and dispersion predictions was observed.
A42A-08
Sodar and Lidar Observations and Modelling of the Pollutants Dynamics in a Strongly Industrialized Coastal Area during a Sea-Breeze Event
The understanding of the atmospheric dynamics under sea breeze is crucial for predicting pollution transport and dispersion in coastal areas. We present here a study of the boundary-layer dynamics and the redistribution of the industrial SO2 emissions in the Dunkerque region (51 N, 2.20 E), in the north of France, at the eastern limit of the English Channel. The French Flanders coastal area is strongly industrialized with many refineries and metallurgic factories. This region is a flat area with small hills (up to 200 meters high) located 30 km away from the coast where sea breezes develop frequently all along the year. Thus, the pollutants transport and pollution episodes are mainly governed by the sea-breeze phenomenon under sunny days. By using optical and acoustic remote-sensing instruments (lidar and sodar), the vertical structure of the atmospheric boundary layer has been observed during a whole sea-breeze event in September 2003. The structure and dynamics of the atmosphere and the pollutants transport within has been simulated with a 3D non hydrostatic model Meso-NHC, developed by the C.R.N.M. and Laboratoire d Aerologie, and was compared with the remote-sensing observations. We present the results of our numerical simulations as well as the data of the remote sensing instruments and stations of the air quality network. The thermal internal boundary layer, the gravity current, the atmospheric boundary layer, the nocturnal boundary layer and the residual layer, were observed by the remote sensing instruments and computed by the model. High values of SO2 concentration were observed at the sea-breeze front passage, and the pollutants emitted were uplifted by updrafts in the front. The acceleration of the sea-breeze flow seems to be responsible of the vertical redistribution of the pollutants emitted inside the sea-breeze system by means of vertical ascents in the sea-breeze front. High values of SO2 concentrations computed in the model were accumulating at the vicinity of the sea-breeze gravity current, whose height was determined by sodar, and at the altitude of which the lidar detected a layer of high reflectivity. The polluted air mass, above the sea and the gravity current and which was containing the pollutants emitted from the industrial area, seemed to turned lately in a stable multilayered structure. Such dynamical conditions are keen to enhance the concentration of secondary pollutants during sunny days, as suggested by the vertical profiles of ozone concentration measured by the lidar.