Polymer Formation on Atmospheric Nanoparticles and Its Effect on Aerosol Hygroscopicity

Although the dominant source of global aerosol production is new particle formation, the growth mechanisms for the freshly nucleated nanoparticles remain largely uncertain. In the present study, we observed large size growth, dependent on relative humidity and particle size, when 4-20 nm sulfuric acid nanoparticles are exposed to epoxide vapors. After mass spectrometric chemical analysis of the size-enlarged particles, high molecular weight organosulfate and polymer products are revealed. Based on the large formation yield of epoxides from photochemical oxidation of biogenic volatile organic compounds (VOCs), they could play an important role in the growth of freshly nucleated nanoparticles. To investigate the effect of polymers on aerosol hygroscopicity, we used surrogates to mimic atmospherically relevant polymers, including glyoxal trimer dihydrate, methylglyoxal trimer dihydrate, sucrose, and organic-sulfuric acid mixtures. Low κ values were observed for polymers than monomers. Our results indicate that polymer formation decrease hygroscopicity and CCN activity of particles and provide guidance in analyzing ambient aerosols.