Atmospheric heterogeneous reactions on soot: A review

Soot particles, composed of elemental carbon and organic compounds, have attracted widespread attention in recent years due to their significant impacts on climate, the environment and human health. Soot has been found to be chemically and physically active in atmospheric aging processes, which leads to alterations in its composition, morphology, hygroscopicity and optical properties and thus changes its environmental and health effects. The heterogeneous reactions on soot also have a significant impact on the transformation of gaseous pollutants into secondary aerosols. Therefore, the interactions between soot and atmospheric substances have been widely investigated to better understand the environmental behaviors of soot. In this review, we systematically summarize the progress and developments in the heterogeneous chemistry on soot over the past 30 years. Atmospheric trace constituents such as NO2, O3, SO2, N2O5, HNO3, H2SO4, OH radical, HO2 radical, peroxyacetyl nitrate etc., are presented in detail from the aspect of their heterogeneous reactions on soot. The possible mechanisms and the effects of environmental conditions on these heterogeneous reactions are also addressed. Further, the impacts of the heterogeneous reactions of soot on the atmospheric environment are discussed, and some aspects of soot-related research which require further investigation are proposed as well.

HETEROGENEOUS CHEMISTRY OF CARBON AEROSOLS

Atmospheric carbon particles originate from natural sources and from human activity. The processes that lead to their formation are varied and include fossil fuel combustion, biomass burning, and mechanical stress and wear of carbonaceous materials. In this review, we examine recent work on the structure and composition of carbon aerosol particles, and we describe how they react with the atmospherically abundant gases ozone, oxygen, sulfur dioxide, nitric acid, and nitrogen oxides. The study of carbon particles in the laboratory has shown that chemical reactivity depends strongly on the type of carbon used and on experimental conditions such as temperature and humidity. The variability in the results demonstrates the difficulty in extrapolating laboratory results to atmospheric conditions and in explaining the role of carbon particles in processes such as global warming and environmental chemical cycling.

Efficient Low-Temperature Oxidation of Carbon-Cluster Anions by SO2

Carbon-cluster anions, CN-, are very reactive toward SO2 (sticking probability of 0.012 +/- 0.005 for C27- at 25 degrees C), in contrast to their inertness toward other common atmospheric gases and pollutants. In flow reactor experiments at ambient temperature and near atmospheric pressure, primary adsorption of SO2 by the carbon cluster anions, N = 4-60, yields CNSO2- or CN-1S-. The inferred elimination of neutral CO2 is also detected as meta-stable decay in collision-induced dissociation. At higher temperatures, the reaction of SO2 with nascent carbon clusters yields CN-1SO- as well as undetected CO. The size-dependent initial reactivity reflects the previously established structural transitions (i.e., from chain to cyclic to cage structures). Such carbon clusters are formed in sooting flames and may act as nuclei for the formation of primary soot particles and serve as models for the local structural features of active soot particle sites for black-carbon soot. The facile generation of reactive carbon-sulfide and -sulfinate units may therefore have implications for understanding the health and environmental effects attributed to the coincidence of soot and SO2.