Chemical evolution of secondary organic aerosol from OH-initiated heterogeneous oxidation

The heterogeneous oxidation of laboratory Secondary Organic Aerosol (SOA) particles by OH radicals was investigated. SOA particles, produced by reaction of alpha-pinene and O-3, 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 Delta F44 similar to 0.01 under equivalent atmospheric aging timescales of 2 weeks, assuming a 24-h average OH concentration of 10(6) 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 Delta O/C similar to 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 kappa 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.