Urban flux and concentration measurements of volatile organic compounds and carbon dioxide in Mexico City

Measurements of ambient concentrations and fluxes of volatile organic compounds (VOCs) and CO2 in the atmosphere of Mexico City are reported here. These measurements were part of the MCMA-2002 and 2003 field campaigns. In both campaigns, a wide array of VOC measurements were conducted using a variety of methods with different spatial and temporal scales at locations in the urban core, in a heavily industrial area and at boundary sites. The VOC data were analyzed to document their distribution, diurnal pattern, origin and reactivity in Mexico City. In the 2003 field campaign, an eddy covariance flux system was deployed at the CENICA site to perform direct measurements of emissions of CO2 and selected VOCs (olefins, acetone, methanol, toluene and C2-benzenes) from sources in an urban neighborhood. This work demonstrates the use of micrometeorological techniques coupled with fast-response sensors to measure fluxes of trace gases from urban landscapes, where the spatial variability of emission sources, surface cover and roughness is very complex. The capability to evaluate emission inventories using these techniques as described in this work is a valuable and new tool for improving air quality management.; Overall, it was found that the VOC burden is dominated by alkanes (60%), followed by aromatics (15%) and olefins (5%). However, in terms of ozone production olefins are the most important hydrocarbons in Mexico City. Ambient concentrations and fluxes of VOCs and CO2 exhibited clear diurnal patterns with distinct correlation with vehicular traffic. The flux measurements showed that the urban landscape is nearly always a net source of VOCs and CO 2. The exception was acetone, which showed negative fluxes before sunrise. Fluxes of olefins, acetone, toluene and C2-benzene were compared to the emissions reported in the local emissions inventory. For the grid where the CENICA site was located, the emissions inventory generally agreed with the measured fluxes. Furthermore, a comparison of the ambient VOC concentrations in terms of lumped modeling VOC classes to the emissions inventory suggests that some, but not all classes are underestimated in the inventory by factors of 3, as suggested in previous studies.