Chemical evolution of volatile organic compounds in the outflow of the Mexico City Metropolitan area

摘要

The volatile organic compound (VOC) distribution in the Mexico CityMetropolitan Area (MCMA) and its evolution as it is uplifted and transportedout of the MCMA basin was studied during the 2006 MILAGRO/MIRAGE-Mex fieldcampaign. The results show that in the morning hours in the city center, theVOC distribution is dominated by non-methane hydrocarbons (NMHCs) but with asubstantial contribution from oxygenated volatile organic compounds (OVOCs),predominantly from primary emissions. Alkanes account for a large part ofthe NMHC distribution in terms of mixing ratios. In terms of reactivity,NMHCs also dominate overall, especially in the morning hours. However, inthe afternoon, as the boundary layer lifts and air is mixed and aged withinthe basin, the distribution changes as secondary products are formed. TheWRF-Chem (Weather Research and Forecasting with Chemistry) model and MOZART(Model for Ozone and Related chemical Tracers) were able to approximate theobserved MCMA daytime patterns and absolute values of the VOC OH reactivity.The MOZART model is also in agreement with observations showing that NMHCsdominate the reactivity distribution except in the afternoon hours. TheWRF-Chem and MOZART models showed higher reactivity than the experimentaldata during the nighttime cycle, perhaps indicating problems with themodeled nighttime boundary layer height.A northeast transport event was studied in which air originating in the MCMAwas intercepted aloft with the Department of Energy (DOE) G1 on 18 March anddownwind with the National Center for Atmospheric Research (NCAR) C130 oneday later on 19 March. A number of identical species measured aboard eachaircraft gave insight into the chemical evolution of the plume as it agedand was transported as far as 1000 km downwind; ozone was shown to bephotochemically produced in the plume. The WRF-Chem and MOZART models wereused to examine the spatial extent and temporal evolution of the plume andto help interpret the observed OH reactivity. The model results generallyshowed good agreement with experimental results for the total VOC OHreactivity downwind and gave insight into the distributions of VOC chemicalclasses. A box model with detailed gas phase chemistry (NCAR MasterMechanism), initialized with concentrations observed at one of the groundsites in the MCMA, was used to examine the expected evolution of specificVOCs over a 1–2 day period. The models clearly supported the experimentalevidence for NMHC oxidation leading to the formation of OVOCs downwind,which then become the primary fuel for ozone production far away from theMCMA.