Effects of Sources and Meteorology on Ambient Particulate Matter in Austin, Texas

We measured ambient submicron aerosol at an urban background site in Austin, Texas in June 2018 (summer) and October–November 2014 (fall). We report the NR-PM1 (non-refractory particulate matter with aerodynamic diameter under 1 μm) concentration, size distribution, and bulk composition measured with a quadrupole-aerosol chemical speciation monitor and a scanning electrical mobility spectrometer. Source apportionment of organic aerosol (OA) using positive matrix factorization (PMF) resolved two oxidized factors, local and regional-OOA, for the summer period and an additional factor, hydrocarbon-like OA (HOA), for the fall period. Differences in the diurnal variation of the local-OOA factor during the two periods and the absence of an HOA factor in the summer indicate that photochemical processing played an important role in governing summer OA composition and diurnal variation. Concentrations of particulate sulfate (SO4 2–) were higher in summer than fall, which may be due to emissions from cement kilns located upwind of the measurement location under the summer wind conditions. Size distribution data suggest frequent nucleation events in summer afternoons, possibly initiated by SO2 and high photochemical activity, and supported by low particle loading and, therefore, a low condensational sink. Overall, we demonstrate the importance of sources, meteorological conditions, and photochemistry in governing the composition and concentration of PM1 at this urban background site.