Evidence for large average concentrations of the nitrate radical (NO3) in Western Europe from the HANSA hydrocarbon database
TNO Bouw en Ondergrond
The nitrate radical (NO3) was first measured in the atmosphere in the 1970s and suggestions were made that it could play a major role in oxidising many unsaturated hydrocarbons, such as those emitted from the biosphere. Analysis of the hydrocarbon mix over the North Atlantic Ocean suggested subsequently that the influence of NO3 radical chemistry at night was even more extensive, being on a par with hydroxyl radical chemistry at some times of the year. The paper presents a detailed analysis of an extensive database of many nonmethane hydrocarbons collected at various sites around the North Sea in the mid 1990s during the HANSA project. By comparing the relative rates of oxidation of iso and normal pentane with that of toluene and benzene it clearly shows that the efficiency of NO3 radical chemistry and hydroxyl radical chemistry over northwest Europe are similar in springtime and predicts an average nighttime NO3 concentration of the order of 350 pptv, assuming an annual average OH concentration of 0.6×106 cm-3. This value is very dependant on the average emission ratios of the different hydrocarbons and values between 200 and 600 pptv are possible. It is much larger than direct measurements made in Europe at the surface, but is of the same magnitude as concentrations measured recently from aircraft in the boundary layer over the northeast USA, and previously in vertical profiles by remote sounding over Europe. A simple analytical expression can be derived to calculate the NO3 concentration at night with the only variables being ozone and the loss rate of N2O5, either to the ground or to aerosol surfaces. The concentrations of NO3 calculated in this manner are similar to those derived from the analysis of the HANSA hydrocarbon database for typical conditions expected over Europe, but they are very dependant on the efficiency of the aerosol sink for N2O5. It is shown that NO3 oxidation of many unsaturated hydrocarbons can indeed be more efficient than OH oxidation, especially at times of the year outside the summer season. Direct evidence for hydrocarbon oxidation by NO3 radicals is shown by a series of peroxy radical measurements where the nighttime concentrations can be significantly higher than daytime concentrations in polluted air on occasion. Also the winter/summer (W/S) ratios of many unsaturated hydrocarbons are much lower than those expected from their removal purely by hydroxyl radical chemistry. The consequences of these findings are profound especially as satellite measurements of NO2, a major precursor to NO3, suggest that these high average concentrations of several hundred pptv could be widespread over most of the continents. This needs to be confirmed by direct in-situ measurement of nitrate radicals but it suggests a much larger role for NO3 chemistry in the oxidation capacity of the atmosphere than realised hitherto. © 2006 Elsevier Ltd. All rights reserved.
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Atmospheric Environment, 41 (16), 3465-3478