Photolytic Sulfuric Acid Particle Formation: Dependence on Precursor Species

Nucleation of particles from sulfuric acid and water was studied using a cylindrical Photolytic Flow Reactor (PhoFR). Sulfuric acid was formed from the photo-oxidation of sulfur dioxide, via HONO photolysis near 360 nm. Particles were detected with a nano-Mobility Particle Sizer coupled to an Ultrafine Condensation Particle Counter. The particle number density Np and the peak of the size distribution Dp depended on [HONO], light level, water content, and also upon sulfur dioxide concentration. Much of the variations of Np and Dp are in line with well-understood photolytic pathways and chemical kinetics, however variations with sulfur dioxide point to some complications. The formation rates of particles will be compared to previous laboratory results. In addition, there were very large effects on formation rates when small amounts of alkylamines were added; factors of hundreds to thousands. The presence of sub-pmol/mol mixing ratios of alkylamines can significantly affect nucleation rates measured in PhoFR. The effects of these types of impurities need to be evaluated in any experimental nucleation setup. Information on the extent of particle growth and independently determined cluster thermodynamics lend veracity to the results and to their extrapolation to atmospheric conditions.