Chemical aging of single and multicomponent biomass burning aerosol surrogate particles by OH: implications for cloud condensation nucleus activity

Abstract

Abstract. Multiphase OH and O 3 oxidation reactions with atmospheric organic aerosol (OA) can influence particle physicochemical properties including composition, morphology, and lifetime. Chemical aging of initially insoluble or low-soluble single-component OA by OH and O 3 can increase their water solubility and hygroscopicity, making them more active as cloud condensation nuclei (CCN) and susceptible to wet deposition. However, an outstanding problem is whether the effects of chemical aging on their CCN activity are preserved when mixed with other organic or inorganic compounds exhibiting greater water solubility. In this work, the CCN activity of laboratory-generated biomass burning aerosol (BBA) surrogate particles exposed to OH and O 3 is evaluated by determining the hygroscopicity parameter, κ, as a function of particle type, mixing state, and OH and O 3 exposure applying a CCN counter (CCNc) coupled to an aerosol flow reactor (AFR). Levoglucosan (LEV), 4-methyl-5-nitrocatechol (MNC), and potassium sulfate (KS) serve as representative BBA compounds that exhibit different hygroscopicity, water solubility, chemical functionalities, and reactivity with OH radicals, and thus exemplify the complexity of mixed inorganic/organic aerosol in the atmosphere. The CCN activities of all of the particles were unaffected by O 3 exposure. Following exposure to OH, κ of MNC was enhanced by an order of magnitude, from 0.009 to ∼ 0.1, indicating that chemically aged MNC particles are better CCN and more prone to wet deposition than pure MNC particles. No significant enhancement in κ was observed for pure LEV particles following OH exposure. κ of the internally mixed particles was not affected by OH oxidation. Furthermore, the CCN activity of OH-exposed MNCcoated KS particles is similar to the OH unexposed atomized 1 : 1 by mass MNC : KS binary-component particles. Our results strongly suggest that when OA is dominated by watersoluble organic carbon (WSOC) or inorganic ions, chemical aging has no significant impact on OA hygroscopicity. The organic compounds exhibiting low solubility behave as if they are infinitely soluble when mixed with a sufficient number of water-soluble compounds. At and beyond this point, the particles' CCN activity is governed entirely by the watersoluble fraction and is not influenced by the oxidized organic fraction. Our results have important implications for heterogeneous oxidation and its impact on cloud formation given that atmospheric aerosol is a complex mixture of organic and inorganic compounds exhibiting a wide range of solubilities.

Cloud droplet activation through oxidation of organic aerosol influenced by temperature and particle phase state

“…The experimental setup and procedures follow our earlier study [ Slade et al ., ]. The setup consists of a horizontally‐mounted aerosol flow reactor (AFR) coupled to a cloud condensation nucleus counter.…”

“…Under these conditions, OH exposures, defined as [OH] × t, ranged from 0‐7 × 10 11 molecule cm ‐3 s, which corresponds to a maximum 12‐hr daytime OH exposure of ~8 d at a background [OH] ~ 2 × 10 6 molecule cm ‐3 . We note that under these levels of OH exposure, there were neither significant changes to the particle size distribution nor clear indication of particle volatilization and dependence on temperature, as determined by changes in the total particle volume concentration with OH exposure level [ Kessler et al ., ; Slade et al ., ]. In the supplementary material, Figure S4 shows the average particle size distributions for each exposure experiment, and Figure S5 shows the total volume concentration for each scan of the particle size distribution.…”

“…Modifications to the physical and chemical properties of OA particles through processes such as multiphase oxidation play a central role in their lifecycle and chemical aging [ Pöschl and Shiraiwa , ]. Especially important are the interactions between OA and reactive trace gases such as O 3 , OH, and NO 3 , which affect the volatility, oxidation state, size, and hygroscopicity or CCN and ice nuclei activity of aerosol particles [ Farmer et al ., ; Kroll et al ., ; Slade et al ., ; Wang and Knopf , ]. In general, chemical aging through OA oxidation enhances CCN activity due to the formation of more polar and hydrophilic functional groups in the particles, increasing their solubility in water and affecting the surface tension of cloud droplets [ Farmer et al ., ].…”

Condensed-phase biogenic–anthropogenic interactions with implications for cold cloud formation

“…However, the presence of small amounts of inorganic species increases particle hygroscopicity signicantly. [117][118][119][120] This may facilitate the formation of supercooled liquid droplets even at temperatures as low as 254 K (when neglecting other competing effects).…”

Influence of Relative Humidity on the Heterogeneous Oxidation of Secondary Organic Aerosol

“…Functionalization leads to formation of more oxygenated species in the particles Ruehl et al, 2013) while fragmentation generally leads to 20 production of more volatile species (Slade and Knopf, 2013). Heterogeneous oxidation can alter the optical properties (Cappa et al, 2011;Dennis-Smither et al, 2012), hygroscopicity (Petters et al, 2006;George et al, 2009;Cappa et al, 2011;Dennis-Smither et al, 2012;Harmon et al, 2013;Slade et al, 2015), density (George et al, 2007;Kroll et al, 2009;Nah et al, 2014), thermal properties (Emanuelsson et al, 2014;Hu et al, 2016) and the particle 25 size (George et al, 2009;Dennis-Smither et al, 2012;Donahue et al, 2012;Kroll et al, 2015) of OA .…”