Abstract
Abstract. The heterogeneous oxidation of laboratory Secondary Organic Aerosol (SOA) particles by OH radicals was investigated. SOA particles, produced by reaction of α-pinene and O3, were exposed to OH radicals in a flow tube, and particle chemical composition, size, and hygroscopicity were measured to assess modifications due to oxidative aging. Aerosol Mass Spectrometer (AMS) mass spectra indicated that the degree of oxidation of 200 nm diameter SOA particles was significantly enhanced due to OH-initiated oxidation, as evidenced by the increase in the fraction of m/z 44 fragment of total organic mass concentration (F44). F44 values of the SOA particles, initially in the range F44=0.04–0.07, increased by up to ΔF44~0.01 under equivalent atmospheric aging timescales of 2 weeks, assuming a 24-h average OH concentration of 106 cm−3. Particle O/C ratios calculated from F44 values, initially in the range O/C=0.25–0.35, rose by a maximum of ΔO/C~0.04 units for 2 weeks of aging. Particle densities also increased with heterogeneous oxidation, consistent with the observed increase in the degree of oxidation. Minor reductions in particle size, with volume losses of up to 10%, were observed due to volatilization of oxidation products. The SOA particles activated more readily to form cloud droplets with an increase in the κ hygroscopicity parameter of up to a factor of two for the equivalent of 2 weeks of OH atmospheric exposure. These results indicate that OH heterogeneous oxidation of typical SOA needs to be considered as an atmospheric organic aerosol aging mechanism, most likely of higher relative importance away from VOC source regions, where other aging mechanisms are less dominant.
Highlights
Atmospheric aerosol particles play an important role in climate, visibility, atmospheric chemistry and human health
OH exposure led to an increase in m/z 18 (H2O+) and 44 (CO+2 ) mass fractions, which are typically associated with carboxylic acids, and a reduction in most other major peaks, such as masses that are typically associated with hydrocarbons (e.g. m/z 41, 55), indicating that Secondary organic aerosol (SOA) became more oxygenated with increasing OH exposure
A comparison of these aging studies along with our work on model Primary organic aerosol (POA) (George et al, 2007) leads to the generalized conclusion that POA-like particles undergo more dramatic chemical transformations than SOA on a relative scale due to OH oxidation. This is due to the fact that POA particles have low initial oxygen content (O/C
Summary
Atmospheric aerosol particles play an important role in climate, visibility, atmospheric chemistry and human health. A number of recent studies have investigated the heterogeneous oxidation of laboratory particles and films containing saturated organic matter by atmospheric gas-phase radicals, such as OH, NO3 and Cl (Bertram et al, 2001; Moise and Rudich, 2001; Eliason et al, 2004; Molina et al, 2004; Knopf et al, 2006; Lambe et al, 2007; George et al, 2007; Hearn et al, 2007; McNeill et al, 2008; Vlasenko et al, 2008; Gross and Bertram, 2009; Gross et al, 2009; Renbaum and Smith, 2009; Smith et al, 2009). The other reaction pathways lead to the addition of oxygenated functional groups to the organic compounds in the particle phase, thereby increasing the aerosol mass and degree of oxidation of the particle-phase organic material (Molina et al, 2004) We note that similar in nature to SOA oxidation experiments briefly presented in Jimenez et al (2009), our study examines the full effects of oxidative processing on the particle degree of oxidation, density, size, and hygroscopicity
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