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

Abstract

Abstract. Size distributions of particles formed from sulfuric acid (H2SO4) 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. H2SO4(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 (lnσ) 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 NH3 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 H2SO4, 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–H2SO4 cluster thermodynamics and provide a phenomenological set of ammonia–sulfuric acid thermodynamics.

Measurement Report: Sulfuric Acid Nucleation and Experimental Conditions in a Photolytic Flow Reactor

“…Yet these nucleation rates have not been corroborated in independent laboratory investigations. In fact there is disagreement with other recent work, albeit based on extrapolated data and/or theoretical treatments (see Zollner et al 2012;Hanson et al 2019;Kürten et al 2016) . For the binary system, Ehrhart et al [2016] showed that the CLOUD data at low temperatures agrees well with theoretical 40 nucleation rates from the SAWNUC thermodynamics [Lovejoy et al 2004].…”

“…Certainly contamination is a concern for experimental work in the binary system [e.g. Zollner et al 2012;Hanson et al 2019]. Here, we report more measurements in the putative binary system to gain further information on the effects of potential contaminants in our Photolytic Flow Reactor (PhoFR) [Hanson et al 2019].…”

“…Yet over the decades there is a wide divergence in nucleation rates measured in laboratory experiments, particularly in the 30 binary system -see figure 6 in Zollner et al [2012] and figure 9 in Hanson et al [2019]. While many of these discrepancies may be attributable to contamination or other experimental conditions, this leaves nucleation rates for atmospheric conditions somewhat uncertain.…”

“…We present measurements of NO and NO x , growth measurements, and measurements from a second particle counter that improve knowledge of the conditions in PhoFR. For the ternary system, Kürten [2019] and Hanson et al [2019] used cluster models to derive thermodynamic information from the CLOUD and the Photolytic Flow Reactor (PhoFR) results, respectively. Both concluded that the thermodynamics derived by Hanson et al [2017] for H 2 SO 4 -NH 3 clusters from the bulk flow reactor study of Glasoe et al [2015] are 50 inaccurate; Hanson et al [2019] speculated this was due to a cross-contamination by an amine in the ammonia delivery lines.…”

referee comment on Hanson et al. (2020)

“…(1) Section 2: Although the chemistry model is described in the earlier publication by Hanson et al (2019) it would be good to add a paragraph, which summarizes the chemistry treated by the model.…”