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

The photochemical production and evolution of six C 2 –C 5 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 5 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.