Optical Properties of Polymers Relevant to Secondary Organic Aerosols

Aerosols play a critical role in climate directly by scattering and absorbing solar radiation, and indirectly by altering cloud formation; both representing the largest uncertainties in climate predictions. Secondary organic aerosols (SOA), which are produced by chemical reactions and gas-to-particle conversion of volatile organic compounds in the troposphere, represent an important fraction of the total aerosol burden. Some SOA particles are light-absorbing species, known as a Brown Carbon (BrC). The overall contribution of SOA to BrC and the related climate forcing are poorly understood and not currently included in global climate models. This is due in part to the chemical complexity of SOA, and the lack of understanding of SOA formation, transformation, and optical properties. Recent studies indicate the importance of aqueous chemistry in the light-absorbing and high molecular weight oligomeric species, which increase the SOA mass production, and alter the direct and indirect effect of aerosols. In the present study, the multi-phase aerosol chemistry was simulated with bulk phase reactions between small α-dicarbonyl compounds with amines, producing light-absorbing species. Spectroscopic techniques were used to quantify the optical properties and to characterize the product distribution of model SOA formed by these aqueous-phase reactions. Differences between primary, secondary and tertiary amines with glyoxal and methylglyoxal were observed in terms of SOA browning efficiency. Also, an enhancement of the absorption properties was observed at shorter wavelengths. Atmospheric implications of our present work for understanding the formation of light-absorbing SOA will be presented.