A41F-01 INVITED
Modeling Particulate Evolution from Urban to Synoptic Scales Downwind of Mexico City
High concentrations of pollutants, such as ozone, ozone precursors, and particulates, have been observed in Mexico City for many years. Field campaign measurements and modeling studies over the past twenty years have examined the local scale processes responsible for the evolution of these pollutants. However, the effect of these pollutants on air quality and climate downwind of Mexico City was not known. To address this issue, extensive meteorological, chemical, and particulate measurements were made during March 2006 over central Mexico and the Gulf of Mexico as part of the Megacity Initiative Local and Global Regional Observations (MILAGRO). MILAGRO was comprised of several complementary field campaigns including MCMA (supported by Mexican agencies, NSF, and DOE), MAX-Mex (supported by DOE), MIRAGE-Mex (supported by NSF), and INTEX-B (supported by NASA) that were conducted at different spatial scales ranging from local-scale measurements over the Mexico City basin to synoptic-scale measurements several hundred kilometers downwind. The impact, or "footprint", of Mexico City emissions on synoptic-scale trace gases and particulate distributions may be somewhat different since gas-aerosol partitioning processes and aerosol-radiation effects couple trace gas and particulate evolution. The "footprint" also varies from day to day as a result of the evolving meteorological conditions in the region. The downwind impact of Mexico City pollutants is investigated by combining MILAGRO measurements with a fully-coupled meteorology-chemistry-particulate model (WRF-chem) to examine the how particulate mass, composition, and size distribution evolves over several days downwind of Mexico City. Understanding the regional- to synoptic-scale impacts requires understanding local-scale processes that affect non-linear chemistry in the vicinity of urban primary emissions; therefore, nested domains are employed that consist of an outer grid encompassing all of Mexico with a grid spacing of 12 km and an inner grid that encompasses central Mexico with a grid spacing of 3 km. The performance of the model is evaluated with measurements obtained from surface, aircraft, and satellite instrument platforms. The contribution of anthropogenic emissions from Mexico City in relation to other sources, including anthropogenic emissions outside of Mexico City, as well as biogenic, biomass burning, and dust sources is examined. Other modeling studies that are currently being conducted to examine the impact of Mexico City on air quality and climate will also be described.
A41F-02
Observations of volatile organic compounds downwind of Mexico City during MIRAGE- MEX
Non-methane hydrocarbons (NMHCs) and oxygenated volatile organic compounds (OVOCs) were measured aboard the NCAR C-130 during MIRAGE-MEX. Airborne NMHCs result from primary emissions whereas airborne OVOCs result from both primary emissions and from the oxidation of NMHC precursors. The photochemical age of air downwind of Mexico City is estimated using an ensemble of VOC measurements and this is compared with back-trajectory estimates. The OVOCs are explored in terms of observed mixing ratios and expected mixing ratios calculated from a box model and from MOZART, a 3-D global chemical transport model. Correlations between various OVOC species, CO, formaldehyde, and NMHCs are examined. Relationships between PAN and acetaldehyde and PPN and propionaldehyde are explored and implications are noted for active photochemistry downwind of Mexico City.
A41F-03
The impact of biomass burning on air quality in and down wind of Mexico City
Observations of highly elevated concentrations of cyanides in the atmosphere above Mexico City during March 2006, demonstrate that biomass burning significantly impacts the regional air quality. In particular, such fires significantly enhance the amount of benzene, carbon monoxide, and organic aerosol in the outflow from the basin. The biomass burning emissions coupled with the anthropogenic NOx , fuel ozone production in the outflow. Significant improvements in visibility may be realized with burning management in and around the basin.
A41F-04
Airborne Formaldehyde Measurements Onboard the NCAR C-130 Aircraft During the 2006 MIRAGE Campaign Using a Difference Frequency Spectrometer
The 2006 MIRAGE-Mex (Megacity Impacts on Regional and Global Environments – Mexico City case study) campaign was designed to improve our understanding of chemical transformations involving photo-oxidation intermediates produced in aging plumes downwind of major tropical megacities such as Mexico City. Formaldehyde (CH2O) is one such key intermediate involved in a number of important atmospheric processes. Despite extensive air quality measurements of this and other gases in Mexico City, there is very little information about CH2O distributions and formation downwind in the surrounding areas. This talk will present CH2O mixing ratios, acquired by a newly developed airborne infrared absorption spectrometer operated onboard NCAR's C- 130 aircraft during the MIRAGE-Mex campaign. This spectrometer employed an infrared difference frequency generation laser source. Airborne CH2O distributions and correlations with other reactive intermediates over a wide geographic range over Mexico will be presented. Comparisons and contrasts with results from other major metropolitan areas will also be discussed.
A41F-05
Factors Influencing the Large-Scale Distribution of Hg(0) in the Mexico City Area and Over the North Pacific
Gas-phase elemental mercury (Hg°) was measured aboard the NASA DC-8 aircraft during the NASA Intercontinental Chemical Transport Experiment Phase B (INTEX-B) campaign in spring 2006. Flights were conducted around Mexico City and on two subsequent deployments over the North Pacific based out of Honolulu, Hawaii and Anchorage, Alaska. Data was obtained during March-May 2006 and covered 0.15-12 km altitude. The vertical distribution in all three study regions showed that Hg° exhibited a relatively constant mixing ratio centered around 100 ppqv. Highly concentrated pollution plumes emanating from Mexico City revealed that mixing ratios of Hg° as large as 500 ppqv were correlated with combustion tracers such as CO, but not SO2 which is presumably released from coal burning, refineries, and volcanoes. Our analysis of Mexico City plumes indicated that widespread multi-source urban/industrial emissions may have a more important influence on Hg° than specific point sources. Over the Pacific, correlations with CO, CO2, CH4, and C2Cl4 were diffuse overall, but recognizable on flights out of Anchorage and Honolulu. In distinct plumes originating from the Asian continent the Hg° - CO relationship was better defined and yielded an average value of ~0.0050 ng Hg° m-3/ppbv, in good agreement with previous findings. A prominent feature of the INTEX-B dataset was frequent total depletion of Hg° in the upper troposphere when stratospherically influenced air was encountered. When O3 mixing ratios exceeded 300 ppbv, Hg° was rarely detected. It appears that in the tropopause and stratosphere Hg° is oxidized on the order of days followed by heterogeneous transformation to particulate mercury. Our data confirm efficient chemical cycling of Hg° in the tropopause region and show that it is strongly anti-correlated with O3. These processes constitute an effective chemical sink for Hg° at the top of the troposphere, which may recycle mercury back to the gas phase as stratospheric aerosols are eventually dispersed into the troposphere. Ozone data obtained with the differential absorption lidar (DIAL) showed that the stratospheric impact on the tropospheric column was a common and pervasive feature on all flights out of Honolulu and Anchorage. Mixing of stratospheric air, containing little or no Hg°, with tropospheric air should effectively reduce the mixing ratio of Hg° in the free troposphere. We propose that this is likely a major factor driving large-scale seasonality in Hg° mixing ratios, especially at mid-latitudes, and an important process that should be incorporated into global chemical transport models.
A41F-06
Assessing the regional impact of Mexico City on air quality
This research analyzes the effect of Mexico City emissions on air quality, photochemistry, and ozone production efficiencies. Model sensitivity studies suggest that Mexico City emissions can contribute to 20-30 percent of NO2 two degrees North of the city, but at higher altitudes this can contribute to 4-8 percent of NO2 into the Gulf of Mexico. This research provides evidence of long range transport of Mexico City emissions off the coast of Texas using a combination of aircraft observations and model products for a March 19, 2006 C-130 flight. Observations were interpolated by kriging, showing significant enhancements of O3 and NOy collocated with enhancements of MTBE, a tracer for mobile source emissions. Sensitivity studies and source tag tracer model products suggest large contribution of Mexico City emissions to those samples. The simulated effect of MC aerosol emissions on photochemistry showed a regional decrease of 40 percent in J[NO2 to NO+O.], and ozone production by 5-10 percent. Interpolation of model error of NO2 photolysis rates show these estimates may be conservative. This paper will explore the interactions between aerosol loadings in relationship to ozone production efficiency using C-130, DC-8, and G-1 data.
A41F-07
Reconstruction of Trajectories, Mixing, and Dispersion of a Mexico City Pollution Outflow Event Using In-Situ Observations From Free-Floating Altitude-Controlled Balloons
The phenomenal growth of megacities, particularly in the developing world, has fueled interest in their effects on climate and air quality on the local, regional, and global scales. During the MILAGRO 2006 campaign, aircraft, satellites, and ground stations were coordinated to make the most intensive measurements to date of the transport and transformation of emissions from a tropical megacity. Likely the most certain case of long-range transport observed during the campaign occurred on March 18-19 when the DOE G1 and NCAR C-130 aircraft made coordinated observations within the Mexico City Metropolitan Area and the C-130 then intercepted the remnants of this urban air 24 hours later and 800 kilometers downwind near the U.S. boarder. Confidence in this event was significantly increased by two free-floating altitude-controlled balloons that remained embedded in the airmass while making repeated profile measurements of winds, thermal structure, and humidity during the transport process. This time series of quasi-Lagrangian soundings is probably the most comprehensive set of in-situ meteorological observations made in a long-range transport event. The profile data from the balloons is used to reconstruct trajectories and estimate mixing and dispersion throughout an advecting slab of the atmosphere. When combined with aircraft, satellite, and surface measurements, the balloon data provide a unique view of an advecting megacity plume that can be used to constrain both meteorological and photochemical models.