Water-Aerosol Interactions Downwind of Mexico City: Inferences about Mixing State, Droplet Growth Kinetics and Aging of Ambient Aerosol

We describe observations of size-resolved cloud droplet activation of partially aged Mexico City aerosol obtained during the MIRAGE campaign. These measurements provide unique insight into the integrated chemical properties and mixing state of the aerosol population, in addition to the water uptake kinetics of the particles as they grow into droplets, all of which are crucial constraints for understanding aerosol-cloud-climate interactions and chemical aging of polluted aerosol. A DMT Cloud Condensation Nucleus counter (CCNc) was operated in parallel to a particle counter with an upstream Differential Mobility Analyzer (DMA) to obtain the size-resolved activation fraction for a given water vapor supersaturation (SS). Activation spectra as a function of SS were obtained for the period of March 16-31, 2006. From the activation spectra, we determine the fraction of particles that act as CCN and the mean soluble mole fraction for these particles. We also estimate the variability in soluble mole fraction for these particles by analyzing the slope of the activation spectra, eliminating the effect of particle size variability inherent with the use of a DMA. We then perform "chemical closure" using the Aerosol Mass Spectrometer (AMS) dataset. The AMS data shows a large fraction of organic constituents, which may alter the cloud droplet growth kinetics. We monitor the droplet size distribution at the exit of the CCNc column, and, given the controlled conditions within the CCNc instrument, we can compare the observed droplet growth with the expected growth for classified, pure ammonium sulfate particles of the same critical supersaturation. A numerical model is used to parameterize the droplet growth kinetics in terms of a water vapor uptake coefficient.