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Abstract. Nucleation rates involving sulfuric acid and watermeasured in a photolytic flow reactor have decreased considerably over atime period of several years. Results show that the system – flow reactor,gas supplies and lines, flow meters, valves, H2SO4 photo-oxidantsources – has reached a baseline stability that yields nucleationinformation such as cluster free energies. The baseline nucleation rate ispunctuated by temporary bursts that in many instances are linked to cylinderchanges, delineating this source of potential contaminants. Diagnostics wereperformed to better understand the system, including growth studies to assessH2SO4 levels, chemiluminescent NO and NOx detection toassess the HONO source, and deployment of a second particle detector toassess the nanoparticle detection system. The growth of seed particles showstrends consistent with the sizes of nucleated particles and provides ananchor for calculated H2SO4 concentrations. The chemiluminescentdetector revealed that small amounts of NO are present in the HONO source,∼ 10 % of HONO. The second condensation-type particlecounter indicates that the nanoparticle mobility sizing system has a bias atlow sulfuric acid levels. The measured and modeled nucleation ratesrepresent upper limits to nucleation in the binary homogeneous system,H2SO4-H2O, as contaminants might act to enhance nucleationrates and ion-mediated nucleation may contribute. Nonetheless, theexperimental nucleation rates, which have decreased by an order of magnitudeor larger since our first publication, extrapolate to some of the lowest ratesreported in experiments with photolytic H2SO4. Results fromexperiments with varying water content and with ammonia addition are alsopresented and have also decreased by an order of magnitude from our previouswork; revised energetics of clusters in this three-component system arederived which differ from our previous energetics mainly in the five-acid andlarger clusters. 

Abstract. Size distributions of particles formed from sulfuric acid(H2SO4) and water vapor in a photolytic flow reactor (PhoFR) weremeasured with a nanoparticle mobility sizing system. Experiments with addedammonia and dimethylamine were also performed. H2SO4(g) wassynthesized from HONO, sulfur dioxide and water vapor, initiating OHoxidation by HONO photolysis. Experiments were performed at 296 K over arange of sulfuric acid production levels and for 16 % to 82 % relativehumidity. Measured distributions generally had a large-particle mode thatwas roughly lognormal; mean diameters ranged from 3 to 12 nm and widths(lnσ) were ∼0.3. Particle formation conditions werestable over many months. Addition of single-digit pmol mol−1 mixing ratios ofdimethylamine led to very large increases in particle number density.Particles produced with ammonia, even at 2000 pmol mol−1, showed that NH3is a much less effective nucleator than dimethylamine. A two-dimensionalsimulation of particle formation in PhoFR is also presented that starts withgas-phase photolytic production of H2SO4, followed by kineticformation of molecular clusters and their decomposition, which is determined by theirthermodynamics. Comparisons with model predictions of the experimentalresult's dependency on HONO and water vapor concentrations yieldphenomenological cluster thermodynamics and help delineate the effects ofpotential contaminants. The added-base simulations and experimental resultsprovide support for previously published dimethylamine–H2SO4cluster thermodynamics and provide a phenomenological set ofammonia–sulfuric acid thermodynamics.