Evidence for the formation of CCN by photochemical processes in Mexico City

Cloud condensation nuclei (CCN) concentrations in Mexico City have a diurnal cycle that is similar to those of condensation nuclei (CN) and PM2.5 but CCN, on average, lags the changes in CN and PM2.5 by almost an hour. The nature of these patterns is related to the onset of emissions from vehicular traffic in the morning followed by the photochemical production of secondary organics that condense on the primary particles. The rates at which particles grow and the detection thresholds of the instruments that measure them impose the apparent lag in CCN trends. A size-resolved aerosol model simulates the observed trend when instrument thresholds and boundary layer growth are taken into account. The measurements made in the year 2000 show no discernible decrease in maximum daily CCN concentrations when compared to similar measurements made in 1984, despite the efforts of local authorities to decrease pollution levels in Mexico City.