The time evolution of aerosol composition over the Mexico City plateau

[摘要] The time evolution of aerosol concentration and chemical composition in amegacity urban plume was determined based on 8 flights of the DOE G-1aircraft in and downwind of Mexico City during the March 2006 MILAGRO fieldcampaign. A series of selection criteria are imposed to eliminate datapoints with non-urban emission influences. Biomass burning has urban andnon-urban sources that are distinguished on the basis of CH₃CN and CO.In order to account for dilution in the urban plume, aerosol concentrationsare normalized to CO which is taken as an inert tracer of urban emission,proportional to the emissions of aerosol precursors. Time evolution isdetermined with respect to photochemical age defined as −Log₁₀(NO_x/NO_y). The geographic distribution of photochemical age and COis examined, confirming the picture that Mexico City is a source region andthat pollutants become more dilute and aged as they are advected towards T1and T2, surface sites that are located at the fringe of the City and 35 kmto the NE, respectively. Organic aerosol (OA) per ppm CO is found toincrease 7 fold over the range of photochemical ages studied, correspondingto a change in NO_x/NO_y from nearly 100% to 10%. In the oldersamples the nitrate/CO ratio has leveled off suggesting that evaporation andformation of aerosol nitrate are in balance. In contrast, OA/CO increaseswith age in older samples, indicating that OA is still being formed. Theamount of carbon equivalent to the deduced change in OA/CO with age is 56 ppbCper ppm CO. At an aerosol yield of 5% and 8% for low and highyield aromatic compounds, it is estimated from surface hydrocarbonobservations that only ~9% of the OA formation can be accountedfor. A comparison of OA/CO in Mexico City and the eastern U.S. gives noevidence that aerosol yields are higher in a more polluted environment.

[学科分类] 大气科学