Vione Davide, Maurino Valter, Minero Claudio, Pelizzetti Ezio, Harrison Mark A J, Olariu Romeo-Iulian, Arsene Cecilia
Dipartimento di Chimica Analitica, Università di Torino, Via P. Giuria 5, 10125 Torino, Italy.
Chem Soc Rev. 2006 May;35(5):441-53. doi: 10.1039/b510796m. Epub 2006 Feb 16.
This paper is a tutorial review in the field of atmospheric chemistry. It describes some recent developments in tropospheric photochemistry in the aqueous phase and on particulate matter. The main focus is regarding the transformation processes that photochemical reactions induce on organic compounds. The relevant reactions can take place both on the surface of dispersed particles and within liquid droplets (e.g. cloud, fog, mist, dew). Direct and sensitised photolysis and the photogeneration of radical species are the main processes involved. Direct photolysis can be very important in the transformation of particle-adsorbed compounds. The significance of direct photolysis depends on the substrate under consideration and on the colour of the particle: dark carbonaceous material shields light, therefore protecting the adsorbed molecules from photodegradation, while a much lower protection is afforded for the light-shaded mineral fraction of particulate. Particulate matter is also rich in photosensitisers (e.g. quinones and aromatic carbonyls), partially derived from PAH photodegradation. These compounds can induce degradation of other molecules upon radiation absorption. Interestingly, substrates such as methoxyphenols, major constituents of wood-smoke aerosol, can also enhance the degradation of some sensitisers. Photosensitised processes in the tropospheric aqueous phase have been much less studied: it will be interesting to assess the photochemical properties of Humic-Like Substances (HULIS) that are major components of liquid droplets. The main photochemical sources of reactive radical species in aqueous solution and on particulate matter are hydrogen peroxide, nitrate, nitrite, and Fe(iii) compounds and oxides. The photogeneration of hydroxyl radicals can be important in polluted areas, while their transfer from the gas phase and dark generation are usually prevailing on an average continental scale. The reactions involving hydroxyl radicals can induce very fast transformation of compounds reacting with (*)OH at a diffusion-controlled rate (10(10) M(-1) s(-1)), with time scales of an hour or less. The hydroxyl-induced reactivity in solution can be faster than in the gas phase, influencing the degradation kinetics of water-soluble compounds. Moreover, photochemical processes in fog and cloudwater can be important sources of secondary pollutants such as nitro-, nitroso-, and chloro-derivatives.
本文是大气化学领域的一篇教程综述。它描述了对流层水相和颗粒物中光化学的一些最新进展。主要关注点是光化学反应在有机化合物上引发的转化过程。相关反应可发生在分散颗粒表面和液滴（如云、雾、霭、露）内部。直接光解和敏化光解以及自由基物种的光生是主要涉及的过程。直接光解在颗粒吸附化合物的转化中可能非常重要。直接光解的重要性取决于所考虑的底物以及颗粒的颜色：深色碳质材料会遮挡光线，从而保护吸附的分子免受光降解，而对于颗粒中浅色矿物部分的保护则要低得多。颗粒物中还富含光敏剂（如醌类和芳香羰基化合物），部分源自多环芳烃的光降解。这些化合物在吸收辐射后可诱导其他分子的降解。有趣的是，诸如甲氧基酚类（木烟气溶胶的主要成分）等底物也可增强某些敏化剂的降解。对流层水相中敏化过程的研究要少得多：评估作为液滴主要成分的类腐殖质（HULIS）的光化学性质将很有意思。水溶液和颗粒物中活性自由基物种的主要光化学来源是过氧化氢、硝酸盐、亚硝酸盐以及铁（III）化合物和氧化物。羟基自由基的光生在污染区域可能很重要，而它们从气相的转移和暗生成在平均大陆尺度上通常占主导地位。涉及羟基自由基的反应可导致与（*）OH以扩散控制速率（10（10）M（-1）s（-1））反应的化合物在一小时或更短的时间尺度内发生非常快速的转化。溶液中羟基诱导的反应性可能比气相中更快，影响水溶性化合物的降解动力学。此外，雾和云水的光化学过程可能是硝基、亚硝基和氯衍生物等二次污染物的重要来源。
