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Photochemical production and evolution of selected C2-C5 alkyl nitrates in tropospheric air influenced by Asian outflow

  • Author(s): Simpson, IJ
  • Blake, NJ
  • Blake, DR
  • Atlas, E
  • Flocke, F
  • Crawford, JH
  • Fuelberg, HE
  • Kiley, CM
  • Meinardi, S
  • Rowland, FS
  • et al.

The photochemical production and evolution of six C -C alkyl nitrates (ethyl-, 1-propyl-, 2-propyl-, 2-butyl-, 2-pentyl-, and 3-pentyl nitrate) was investigated using selected data from 5500 whole air samples collected downwind of Asia during the airborne Transport and Chemical Evolution over the Pacific (TRACE-P) field campaign (February-April 2001). Air mass age was important for selecting appropriate field data to compare with laboratory predictions of C alkyl nitrate production rates. In young, highly polluted air masses, the ratio between the production rates of 3-pentyl nitrate and 2-pentyl nitrate from n-pentane was 0.60-0.65. These measured ratios show excellent agreement with results from a field study in Germany (0.63 ± 0.06), and they agree better with predicted ratios from older laboratory kinetic studies (0.63-0.66) than with newer laboratory results (0.73 ± 0.08). TRACE-P samples that did not show influence from marine alkyl nitrate sources were used to investigate photochemical alkyl nitrate evolution. Relative to 2-butyl nitrate/n-butane, the measured ratios of ethyl nitrate/ethane and 2-propyl nitrate/propane showed notable deviations from modeled values based on laboratory kinetic data, suggesting additional Asian sources of their alkyl peroxy radical precursors. By contrast, the measured ratios of 1-propyl-, 2-pentyl-, and 3-pentyl nitrate to their respective parent hydrocarbons were fairly close to modeled values. The 1-propyl nitrate findings contrast with field studies in North America, and suggest that air downwind of Asia was not significantly impacted by additional 1-propyl nitrate precursors. The sensitivity of modeled photochemical processing times to hydroxyl radical concentration, altitude, city ventilation times, and dilution is discussed. 2 5 5

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