In situ measurements of sulfur dioxide (SO2), carbon monoxide (CO) and sulfate mass provide evidence that aerosol composition in Mexico City is affected by emissions from the neighboring volcano, Popocatepetl. The data collected over a two-week period in November 1997 suggest that there are two distinct pathways by which SO2 is incorporated into particulates. Periods of high humidity, fog, and rain are accompanied by elevated sulfate mixing ratios, attributed to aqueous chemistry. Similarly, elevated sulfate concentrations in low humidity periods appear to be a result of adsorption onto existing particles. These two mechanisms are important for understanding the processes associated with particle formation in this highly polluted urban area. Under the influence of volcanic emissions, SO2 concentration can reach more than four times the monthly mean daily average (20ppbv) and particulate sulfate can be a factor of 2 larger. ¿ 1999 American Geophysical Union