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.

<p><strong>Abstract.</strong> Size distributions of particles formed from sulfuric acid (H<sub>2</sub>SO<sub>4</sub>) and water vapor in a Photolytic Flow Reactor (PhoFR) were measured with a nano-particle mobility sizing system. Experiments with added ammonia and dimethylamine were also performed. H<sub>2</sub>SO<sub>4(g)</sub> was synthesized from HONO, sulfur dioxide, and water vapor, initiating OH oxidation by HONO photolysis. For standard reactant flows and conditions, 296 K, 52 % relative humidity, and a ~ 40 s residence time, the calculated concentration of H<sub>2</sub>SO<sub>4</sub> peaked at 1.2 × 10<sup>10</sup> cm<sup>−3</sup>, measured particle mean diameter was ~ 6 nm and total number density was ~ 10<sup>4</sup> cm<sup>−3</sup>. Measured distributions were influenced by molecular clusters at small sizes, less than or equal to 2 nm diameter, but were generally dominated by large particles that are roughly log-normal with mean diameters ranging up to 12 nm and a relatively constant lnσ of ~ 0.3. Particle number density and their mean size depended on relative humidity, HONO concentration, illumination, and SO<sub>2</sub> level. Particle formation conditions were stable over many months. Addition of single-digit pmol/mol mixing ratios of dimethylamine led to very large increases in number density. Ammonia at levels up to 2000 pmol/mol showed that NH<sub>3</sub> is less effective than dimethylamine at producing particles. A two-dimensional simulation of PhoFR reveals that H<sub>2</sub>SO<sub>4</sub> scales with HONO and its level builds along the length of the flow reactor. Experimentally, particle growth scaled with HONO, in accord with model-predicted H<sub>2</sub>SO<sub>4</sub> levels. Additional comparison between experiment and model indicates that reaction of HO<sub>2</sub> with SO<sub>2</sub> could be a significant source of H<sub>2</sub>SO<sub>4</sub> in this experiment. The effects of potential contaminants on particle formation rates near room temperature are addressed and provide context in comparisons with previous experiments. The added-base experimental results provide support for previously published dimethylamine-H<sub>2</sub>SO<sub>4</sub> cluster thermodynamics but do not support previously published ammonia-sulfuric acid thermodynamics.</p>

We have identified a "leading edge" of the size distribution which is "leading" in terms of particle growth. This "leading edge" terminology refers to the large-particles at the leading edge of the distribution and here we clarify this further. This is beyond what one would naturally think of as the leading edge particularly as we fit lognormals to this data which requires to some extent data on both sides of a peak. So in terms of the fit, data should extend from the "leading edge" at the far right all the way down in size to the inflection point between the large particle peak and the minimum towards smaller sizes. Note that N p is for "large particles" that are defined differently: all those with D p >= 2.4 nm. It is important to maintain the distinction of which criterion was used to derive any particular parameter (e.g. D le , not D p). In practice, the integration of the lognormal fit and N p do not differ appreciably. Also some distributions do not exhibit an inflection point and were not able to be fit automatically. In these cases, lognormals were fitted by eye, using an average width from contemporary distributions. See section S2.x for examples.