Variability in ultraviolet total optical depth during the Southern California Ozone Study (SCOS97) |
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| Institution: | 1. Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA;2. Civil and Environmental Engineering Department, University of California, Berkeley, CA 94720, USA;3. Natural Resource Ecology Laboratory, Colorado State University, CO, USA;1. Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, and School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China;2. Department of Chemistry, South China University of Agriculture, Guangzhou 510642, China;1. State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;2. Centre for Clean Environment and Energy, Griffith Scholl of Environment, Griffith University, Queensland 4222, Australia;3. School of Life Sciences, The Chinese University of Hong Kong, Shatin, NT, Hong Kong SAR, China;4. University of Chinese Academy of Sciences, Beijing 100049, China;1. Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua, Zhejiang, 321004, PR China;2. Hefei National Laboratory for Physical Sciences at the Microscale, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui, 230026, PR China;1. State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun, Jilin 130022, People''s Republic of China;2. University of Chinese Academy of Sciences, Beijing 100049, People''s Republic of China;3. College of Chemical Engineering and Material, Quanzhou Normal University, Quanzhou 362000, Fujian, People''s Republic of China;4. Guangdong Institute of Analysis, Guangdong Provincial Key Laboratory of Emergency Test for Dangerous Chemicals, Guangzhou 510070, People''s Republic of China;5. State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, University of Science and Technology of China, Changchun 130022, People''s Republic of China;1. School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, PR China;2. School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China;3. School of Environmental and Safety Engineering, Changzhou University, Changzhou 213164, PR China |
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| Abstract: | Formation of photochemical air pollution is governed in part by the solar ultraviolet actinic radiation flux, but wavelength-resolved measurements of UV radiation in polluted urban atmospheres are rarely available. As part of the 1997 Southern California Ozone Study, cosine weighted solar irradiance was measured continuously at seven UV wavelengths (300, 306, 312, 318, 326, 333 and 368 nm) at two sites during the period 1 July to 1 November 1997. The first site was at Riverside (260 m a.s.l.) in the Los Angeles metropolitan area, which frequently experiences severe air pollution episodes. The second site was at Mt Wilson (1725 m a.s.l.), approximately 70 km northwest of Riverside, and located above much of the urban haze layer. Measurements of direct (i.e., total minus diffuse) solar irradiance were used to compute total atmospheric optical depths. At 300 nm, optical depths (mean±1 S.D.) measured over the entire study period were 4.3±0.3 at Riverside and 3.7±0.2 at Mt Wilson. Optical depth decreased with increasing wavelength, falling at 368 nm to values of 0.8±0.2 at Riverside and 0.5±0.1 at Mt Wilson. At all wavelengths, both the mean and the relative standard deviation of optical depths were larger at Riverside than at Mt Wilson. At 300 nm, the difference between the smallest and largest observed optical depths corresponds to over a factor 2 increase in the direct beam irradiance for overhead sun, and over a factor 7 increase for a solar zenith angle of 60°. Principal component analysis was used to reveal underlying factors contributing to variability in optical depths. PCA showed that a single factor (component) was responsible for the major part of the variability. At Riverside, the first component was responsible for 97% of the variability and the second component for 2%. At Mt Wilson, 89% of the variability could be attributed to the first component and 10% to the second. Dependence of the component contributions on wavelength allowed identification of probable physical causes: the first component is linked to light scattering and absorption by atmospheric aerosols, and the second component is linked to light absorption by ozone. These factors are expected to contribute to temporal and spatial variability in solar actinic flux and photodissociation rates of species including ozone, nitrogen dioxide, and formaldehyde. |
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