Vertical profiles and receptor modeling of volatile organic compounds over Southeastern Mexico City

Tethered balloon flights were carried out over Southeastern Mexico City in April 2003, providing vertical profiles of ozone (0–1000m above ground) and volatile organic compounds (VOCs, 0–200m above ground). The objective of this study was to provide insight into the processes that drive ozone formation in Mexico City. In this paper, we discuss the atmospheric chemistry, emission sources and transport processes that influence the observed profiles. Characteristic ozone profiles were measured in the morning with the highest concentrations recorded in the residual layer. Maximum ozone concentrations, as high as 170ppb, were measured at ground level in the afternoon. For the shape of VOC vertical profiles, no unique patterns were found. High concentrations were measured for propane, butane and toluene. Diurnal variations in VOC concentrations imply a strong effect of vertical dilution with an increasing mixing layer. Two different receptor models were used to relate observed VOC concentrations to emission sources. Results from a chemical mass balance (CMB) model suggest that the main contributors to total VOCs and the related ozone formation potential are gasoline vehicle exhaust and liquefied petroleum gas (LPG). A linear unmixing analysis reproduced satisfactorily the measured source profiles of the main sources. The relative source activities calculated by the CMB and the linear unmixing models coincide well for the LPG contribution; however, the vehicle exhaust contribution is underestimated and solvent emissions are overestimated by linear unmixing relative to the CMB model.