Determination of volatile organic compounds in the atmosphere of Mexico City using optical systems and conventional methods

Since 1991, volatile organic compounds (VOC) have been determined in Mexico City Metropolitan Area (MCMA) using different analyzing methods. Samples collected in cartridges and canisters are analyzed by CC-MS and HPLC. Continuos monitoring by a long path differential absorption analyzer (DOAS) has also been performed. The Mexican Petroleum Institute (IMP) along with the USEPA and the collaboration of the Mexican environmental authorities have performed, following the EPA TO-14 method, four sampling campaigns of ambient air in canisters among 1992 and 1993. The campaigns were designed to be performed during March, month in which the worst ozone episodes have taken place. Additional sampling was performed in November in order to compare differences between seasons. In March 1993, samples in three different kinds of cartridges were used which selectively adsorb carbonyl compounds, light hydrocarbons (C-2-C-4) and heavier hydrocarbons (C-5-C-15). The three techniques employed, canisters, cartridges and the DOAS, reveal very high atmospheric concentrations of the volatile organic compounds, species identified as ozone precursors, these high concentration might appear as responsible for the ozone standards exceedances. The kinetics of the whole set of atmospheric reactions points then to nitrogen oxides as the limiting species since their relative smaller abundance. This fact engender a NOx limited atmosphere. The three techniques offer different advantages for the determination of species in the atmosphere, the canisters samples provides the total VOC concentration, the speciation of more than 200 compounds and the determination of CO and CH4. The sampling in cartridges could be used to detect occasional emissions. On the other hand the DOAS provides the means to follow the evolution of selected species during long periods and the influence of meteorological parameters as the wind speed and direction, on the concentration of each pollutant. The results presented here were obtained through collaboration of the Environmental Sciences group of the Mexican Petroleum Institute (GCA-IMP) with different people and groups.