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.
Highlights
Particle formation in the atmosphere has long been studied (McMurry et al, 2005; Kulmala et al, 2004) to ascertain potential impacts on health (Nel, 2005) and on climate processes (IPCC, 2013)
Previous work on particle nucleation in the binary system (Kirkby et al, 2011; Ball et al, 1999; Zollner et al, 2012; Ehrhart et al, 2016; Yu et al, 2017) has concluded that binary nucleation can be significant at low temperatures, such as at high latitudes and in the upper troposphere
We studied the effects of adding ammonia or dimethylamine; both are known to greatly enhance particle production rates (Almeida et al, 2013; Glasoe et al, 2015; Yu et al, 2012; Ortega et al, 2012; Nadykto and Yu, 2011)
Summary
Particle formation in the atmosphere has long been studied (McMurry et al, 2005; Kulmala et al, 2004) to ascertain potential impacts on health (Nel, 2005) and on climate processes (IPCC, 2013). Nanoparticles (characterized as < 10 nm in diameter) can have special health effects, as their small size allows for efficient transport into lung tissue (Kreyling et al, 2006) They influence climate by growing to sizes large enough to affect radiative forcing and the properties of clouds. Despite numerous and wideranging studies devoted to understanding new particle formation, mechanisms and nucleation rates applicable to many regions of the atmosphere remain uncertain. Sulfuric-acid-driven nucleation is a prime source of nanoparticles in the atmosphere (Kuang et al, 2012; Sipilä et al, 2010); it is the starting point for many laboratory studies. Good knowledge of the formation and stability of binary nanoparticles is needed to understand their subsequent growth via other compounds
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