Role of OH-initiated oxidation of isoprene in aging of combustion soot.

We have investigated the contribution of OH-initiated oxidation of isoprene to the atmospheric aging of combustion soot. The experiments were conducted in a fluoropolymer chamber on size-classified soot aerosols in the presence of isoprene, photolytically generated OH, and nitrogen oxides. The evolution in the mixing state of soot was monitored from simultaneous measurements of the particle size and mass, which were used to calculate the particle effective density, dynamic shape factor, mass fractal dimension, and coating thickness. When soot particles age, the increase in mass is accompanied by a decrease in particle mobility diameter and an increase in effective density. Coating material not only fills in void spaces, but also causes partial restructuring of fractal soot aggregates. For thinly coated aggregates, the single scattering albedo increases weakly because of the decreased light absorption and practically unchanged scattering. Upon humidification, coated particles absorb water, leading to an additional compaction. Aging transforms initially hydrophobic soot particles into efficient cloud condensation nuclei at a rate that increases in the presence of nitrogen oxides. Our results suggest that ubiquitous biogenic isoprene plays an important role in aging of anthropogenic soot, shortening its atmospheric lifetime and considerably altering its impacts on air quality and climate.