Evolution in physiochemical and cloud condensation nuclei activation properties of crop residue burning particles during photochemical aging.

As a main form of biomass burning in agricultural countries, crop residue burning is a significant source of atmospheric fine particles. In this study, the aging of particles emitted from the burning of four major crop residues in China was investigated in a smog chamber. The particle size distribution, chemical composition and cloud condensation nuclei (CCN) activity were simultaneously measured. The properties of crop residue burning particles varied substantially among different fuel types. During aging, the particle size and mass concentration increased substantially, suggesting condensational growth by formation of secondary aerosols. The particle composition was dominated by organics. Aging resulted in considerable enhancement of organics and inorganics, with enhancement ratios of 1.24-1.44 and 1.33-1.76 respectively, as well as a continuous increase in the oxidation level of organics. Elevated CCN activity was observed during aging, with the hygroscopicity parameter κ varying from 0.16 to 0.34 for fresh particles and 0.19 to 0.40 for aged particles. Based on the volume mixing rule, the hygroscopicity parameter of organic components (κ) was derived. κ exhibited an increasing tendency with aging, which was generally consistent with the tendency of the O:C ratio, indicating that the oxidation level was related to the hygroscopicity and CCN activity of organic aerosols from crop residue burning. Our results indicated that photochemical aging could significantly impact the CCN activation of crop burning aerosols, not only by the production of secondary aerosols, but also by enhancing the hygroscopicity of organic components, thereby contributing to the aerosol indirect climate forcing.