H 2 SO 4 and particle production in a photolytic flow reactor: chemical modeling, cluster thermodynamics and contamination issues

Abstract. Size distributions of particles formed from sulfuric acid ( H 2 SO 4 ) and water vapor in a photolytic flow reactor (PhoFR) were measured with a nanoparticle mobility sizing system. Experiments with added ammonia and dimethylamine were also performed. H 2 SO 4(g) was synthesized from HONO, sulfur dioxide and water vapor, initiating OH oxidation by HONO photolysis. Experiments were performed at 296 K over a range of sulfuric acid production levels and for 16 % to 82 % relative humidity. Measured distributions generally had a large-particle mode that was roughly lognormal; mean diameters ranged from 3 to 12 nm and widths (ln σ ) were ∼0.3 . Particle formation conditions were stable over many months. Addition of single-digit pmol mol −1 mixing ratios of dimethylamine led to very large increases in particle number density. Particles produced with ammonia, even at 2000 pmol mol −1 , showed that NH 3 is a much less effective nucleator than dimethylamine. A two-dimensional simulation of particle formation in PhoFR is also presented that starts with gas-phase photolytic production of H 2 SO 4 , followed by kinetic formation of molecular clusters and their decomposition, which is determined by their thermodynamics. Comparisons with model predictions of the experimental result's dependency on HONO and water vapor concentrations yield phenomenological cluster thermodynamics and help delineate the effects of potential contaminants. The added-base simulations and experimental results provide support for previously published dimethylamine– H 2 SO 4 cluster thermodynamics and provide a phenomenological set of ammonia–sulfuric acid thermodynamics.