氯化钙种子气溶胶对甲苯二次有机气溶胶与氨形成含氮有机物的影响

苯系物光氧化反应形成的二次有机气溶胶(SOA)是大气细粒子的重要组成部分.SOA羧酸和二元醛组分能与氨反应形成有机酸铵和咪唑类含氮有机物,它们能够吸收205 nm和270 nm的紫外辐射,是棕色碳的主要组分.氯化钙等无机种子气溶胶具有较大的比表面积,可为气相羰基化合物和氨提供凝结与反应载体,从而影响含氮有机物的形成.基于此,本文利用烟雾腔研究氯化钙种子气溶胶存在时甲苯SOA与氨的反应,采用紫外-可见分光光度计测量产物溶液在205 nm和270 nm处的吸光度,并定性研究不同浓度、湿度和酸度的氯化钙种子气溶胶对含氮有机物形成的影响.结果表明:氯化钙种子气溶胶能够促进甲苯SOA含氮有机物的形成;含氮有机物的生成浓度随着氯化钙种子气溶胶浓度和pH值的增加而逐渐增大.但当氯化钙种子气溶胶为碱性时,OH~-会与凝结的有机酸发生酸碱中和反应并抑制二元醛化合物水合形成四醇产物,从而不利于含氮有机物的生成;水分子的增加占据了氯化钙种子气溶胶表面的吸附活性位点,氨被吸附和凝结的量减少,从而导致含氮有机物的生成浓度随着相对湿度的增大而降低.本研究可为人为源SOA棕色碳的形成机制和化学组成研究提供实验依据.

Effects of calcium chloride seed aerosol on the formation of nitrogen-containing organic compounds formed from toluene secondary organic aerosol and ammonia

Secondary organic aerosol (SOA) formed from the photooxidation of aromatic compounds is an important component of atmospheric fine particles. The carboxylic acids and dialdehydes of SOA can react with ammonia to generate organic acid ammonium and imidazole nitrogen-containing organic compounds, which can absorb ultraviolet radiation with wavelengths of 205 and 270 nm, are the main constituents of brown carbon. Calcium chloride and other inorganic seed aerosol particles have large specific surface area, which provides condensation and reaction center for gaseous carbonyl compounds and ammonia, thus affecting the formation of nitrogen-containing organic compounds. The reactions of toluene SOA with ammonia in the presence of calcium chloride seed aerosol were studied by smog chamber in this study. The absorbance of the reaction product solution at 205 and 270 nm was measured by UV-Vis spectrophotometer, and the effects of different concentrations, humidity and acidity of calcium chloride seed aerosol on the formation of nitrogen-containing organic compounds were investigated qualitatively. The experimental results shown that calcium chloride seed aerosol particles promote the formation of nitrogen-containing organic compounds formed from toluene SOA and ammonia. The concentration of nitrogen-containing organic products increased with the increasing of the concentration and pH of calcium chloride seed aerosol. However, when the calcium chloride seed aerosol is alkaline, OH^- would reaction with condensed organic acid and inhibits hydration of dialdehyde compound to form tetraol product, which is not conducive to the formation of nitrogen-containing organic products. The increase of water molecules occupied the adsorption active sites on the surface of the calcium chloride seed aerosol, and the amount of ammonia adsorbed and coagulated is reduced, which resulting in the concentration of nitrogen-containing organic products decreased with the increasing of relative humidity. These would provide experimental basis for the formation mechanism and chemical composition of brown carbon of anthropogenic SOA.