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The Role of Isoprene in Aging of Atmospheric Soot Aerosols

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Tuesday, 8 January 2013
The Role of Isoprene in Aging of Atmospheric Soot Aerosols
Alexei Khalizov, Texas A&M University, College Station, TX; and Y. Lin, C. Qiu, S. Guo, and R. Zhang

The emissions of isoprene from terrestrial and marine biogenic sources can impact the composition and properties of atmospheric aerosols. We have investigated the contribution of isoprene to atmospheric aging of combustion soot. Atmospheric soot is often found to be internally mixed with other aerosol constituents, yet the processes and chemical species responsible for the soot aging are not well understood. Aging experiments were conducted in a fluoropolymer chamber on size-classified soot aerosol in the presence of isoprene and photolytically generated hydroxyl radical (OH). The evolution in the aging state of soot was monitored from measurements of the particle size, mass, hygroscopicity, cloud condensation nuclei (CCN) activity, and optical properties. Upon photooxidation of isoprene in the chamber the soot particle mass was found to increase promptly due to condensation of semi-volatile and low-volatility reaction products on the particle surface. The increase in the particle mass was accompanied by a decrease in the particle mobility diameter and an increase in the effective density, indicating that the condensed material partially restructured the fractal soot aggregates, making them more compact. Upon humidification, the coating material absorbed water, increasing in volume and causing an additional restructuring of the soot aggregates. Furthermore, the presence of a water-soluble coating has transformed initially hydrophobic soot into efficient CCN at atmospherically relevant water supersaturations. However, the absorption and scattering of light by the aged soot remained practically unchanged because of the relatively low coating mass. The rate of aging and corresponding changes in the properties of soot were significantly enhanced in the presence of nitrogen oxides (NOx = NO + NO2), a common combustion co-pollutant of soot. On the basis of our experimental results we conclude that isoprene may play a significant role in the atmospheric aging of combustion soot aerosols, altering their atmospheric lifetime and impacts on air quality and cloud formation.