Spectroscopic study of organic coatings on fine particles, exposed to ozone and simulated sunlight

摘要

This work intends to quantify the variation in optical properties of aerosol by in-situ spectroscopic monitoring the ozonolysis of a mixture of typical biomass burning compounds. The reaction occurs on silica and glass particles in the presence of simulated sunlight. Fused silica particles (Aerosil) were coated with a thin film of a 1:1 mixure of 4-phenoxyphenol with 4-carboxyphenone as a photosensitizer. UV—VIS spectra of dichloromethane extracts from the particles recorded before and after treatment, show development of a new band after prolonged ozone and light exposure. Changes in optical properties are reported, and variations of spectroscopic features are discussed. We show that the ozone-induced heterogeneous photochemical reaction does produce species absorbing light in the solar spectral range. Further, we demonstrate that the heterogeneous photosensitized reactions at 200 ppb ozone (strongly ozone polluted regions) for a time period of 7 h aging process, can increase light absorption of atmospheric aerosols in the tropospheric actinic window (>290 nm) by 0.4 absorption units ng-C~(-1) O_3 ppm~(-1) in the region 290-358 nm and by 1.0 absorption units ng-C~(-1) O_3 ppm~(-1) in the region 360—448 nm. Chemical changes of such surface films were identified by diffuse reflectance infrared Fourier transform spectroscopy of coated glass spheres, and we suggest formation of humic-like substances comparable to those reported in continental aerosol.