Measurements of VOCs in Mexico City (1992–2001) and evaluation of VOCs and CO in the emissions inventory

Measurements of ambient volatile organic compounds (VOCs) are reported from several field campaigns of simultaneous measurements in the Mexico City Metropolitan Area (MCMA). Conducted between 1992 and 2001, these measurements represent the longest and most complete record of VOC measurements available for Mexico City. Ambient air VOC samples were collected at surface air quality monitoring sites in electro-polished stainless-steel canisters, and analyzed by Gas Chromatography with Flame Ionization Detection for the C2 to C12 hydrocarbons and MTBE. This paper presents the total VOCs in morning measurements (06:00–09:00 AM) and its trend through time. Average concentrations of total VOCs in each campaign were between 3130 and 6711 ppbC at Xalostoc (northeast of the city center), 2994 and 4935 ppbC at La Merced (center), and 1136 and 2815 ppbC at Pedregal (southwest). A linear regression of the total VOCs reveals a slight decreasing trend from 1992 to 2001 which is statistically significant only at Xalostoc; the decreasing trends at the two other sites are not statistically significant and therefore inconclusive. This apparent stabilization and possible decrease in ambient concentrations, despite the growth in the vehicular fleet and other activities during this period, suggests that VOC emission control measures have been effective at reducing emissions. Co-located measurements of CO and NOx from the Metropolitan Atmospheric Monitoring Automatic Network are used to calculate ratios of total VOCs/NOx and CO/NOx, in order to evaluate the official 1998 emissions inventory for the MCMA. The ambient ratios of VOCs/NOx and CO/NOx are found to be factors of 2–3 times higher than the corresponding inventory ratios. This discrepancy suggests that VOC and CO emissions may be significantly underestimated.