Measurements of aromatic hydrocarbon ratios and NOx concentrations in the rural troposphere: Observation of air mass photochemical aging and NOx removal |
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| Affiliation: | 1. Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Environmental Research Laboratories, Boulder, CO 80303, U.S.A.;2. Department of Chemistry, University of Colorado, Boulder, CO 80309, U.S.A.;3. Cooperative Institute for Research in the Environmental Sciences, University of Colorado/NOAA, Boulder, CO 80309, U.S.A.;4. Currently a National Research Council Postdoctoral Associate, NOAA/ERL Aeronomy Laboratory, Boulder, CO 80303, U.S.A.;1. State Environmental Protection Key Laboratory of Urban Ambient Air Particulate Matter Pollution Prevention and Control & Tianjin Key Laboratory of Urban Transport Emission Research, College of Environmental Science and Engineering, Nankai University, Tianjin, 300350, China;2. CMA-NKU Cooperative Laboratory for Atmospheric Environment-Health Research, Tianjin, 300350, China;3. Department of Public Health Sciences, University of Rochester School of Medicine and Dentistry, Rochester, NY, 14642, USA;4. Institute for a Sustainable Environment, Clarkson University, Potsdam, NY, 13699, USA;1. State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing, 100012, China;2. Shandong Zibo Eco-Environmental Monitoring Center, Zibo, 255040, China;1. Department of Earth and Marine Sciences, Jeju National University, Jeju, 63243, Republic of Korea;2. Department of Environmental Engineering, Dong-Eui University, Busan, 47340, Republic of Korea;3. The Institute of Environmental Studies, Pusan National University, Busan, 46241, Republic of Korea;4. Department of Civil & Environmental Engineering, Hanyang University, Seoul, 04763, Republic of Korea;5. Environmental Health Center, University of Ulsan College of Medicine, Ulsan, 44033, Republic of Korea |
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| Abstract: | Measurements of the aromatic hydrocarbons (benzene, toluene, ethylbenzene and ortho(o)-xylene) at Niwot Ridge, Colorado have shown distinct correlations between the ratios of the concentrations of these compounds and the degree of direct urban influence. The major atmospheric removal mechanism of aromatic hydrocarbons is reaction with the hydroxyl radical, OH. This allows the decrease in the ratios of aromatic hydrocarbon concentrations to be related to the transport time and average OH number density within an air mass, if assumptions are made concerning background sources of aromatic hydrocarbons. Measured ratios of aromatic compounds at this site, along with ratios reported for several cities in the western United States, and estimates of transport times from these cities were used to calculate temporally and spatially averaged OH number densities. Hydroxyl radical number density estimates using toluene-, ethylbenzene-, and o-xylene-to-benzene ratios, were 1.2 ± 0.6 × 106, 1.0 ±0.8 × 106 and 0.48 ± 0.8 × 106 molecules cm−3, respectively. Considering the uncertainties in the assumptions used in the above estimates, we obtain a diurnal-average upper limit of 2.4 × 106 molecules cm−3. The correlations between measured ratios are found to yield slopes consistent with those predicted by experimental OH rate constants for benzene, toluene and ethylbenzene, and approximately a factor of two different in the case of benzene, toluene and o-xylene. The ratio of NOx: benzene was found to yield no correlation with toluene: benzene ratio for periods of westerly flow, but was well correlated with toluene: benzene ratio during periods of direct urban impact on the site (upslope easterly winds). The correlation of these ratios in urban plume air masses was consistent with NO2 + OH + M being the major daytime removal mechanism of NOx in the summertime. |
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