A poorly quantified, but still potentially important source of NO, is the oxidation of atmospheric ammonia (NH3). A major uncertainty regarding this source is the lack of information on the tropospheric distribution of NH3. NH3 is initially oxidized by OH to form NH2. NH2 may go on to form NO, through reaction with 03, however, it may also react with NO, to form N2 or N20. Based on differences in rate coefficients, NH3 oxidation should provide a net source of NO, when ambient NO, is less than 200 to 500 ppt, a condition that is prevalent in the remote troposphere. Recently, boundary layer NH3 mixing ratios in the 50 to 900 ppt range (median 250pptv) have been found over large stretches of the South Pacific and the Southern Ocean (J. Bradshaw, unpublished data). A reasonable estimate for this source is about 0.6TgN/yr, assuming a background tropospheric NH3 value of 150pptv and a 4-month lifetime for oxidation via OH.
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