An investigation of aerosol physical properties in Houston, Texas

Abstract

From June through October 2001, three Tandem Differential Mobility Analyzer (TDMA) systems were operated around Houston, Texas, to obtain a large, high-quality dataset in order to explore characteristics of aerosol size distributions and hygroscopicity in an urban environment. Mean aerosol size distributions from each of the monitoring sites portray the differences observed among a site upwind of the major direct aerosol sources, a site near many major aerosol sources, and a site downwind much of Houston. Diurnal cycles were observed with afternoon peaks in aerosol number concentrations along with additional peaks corresponding to morning and afternoon automobile traffic. Two case studies of abnormally high number concentration events are described, one in which a change in the local weather pattern contributed to the increase in concentrations observed and one in which it appears that high amounts of condensable gas phase species led to intense particle nucleation and subsequent growth. The hygroscopicity data show a clear size-dependence, as particles with D[] < 100 nm are mostly non-hygroscopic while those with D[] > 100 nm exhibit bimodal growth patterns. Furthermore, as size increases, the more-hygroscopic mode becomes more dominant than the less-hygroscopic mode.