Ozone production during the field campaign RISFEX 2003 in the sea of Japan: analysis of sensitivity and behaviour based on an improved indicator

Abstract

The ratio Φ=<i>k</i>_HC+OH[HC]/<i>k</i>_{NO_x+OH}[NO_x] is used as an indicator for the sensitivity of ozone production (<i>P</i>(O₃)) to HC and NO_x in the field campaign RISFEX 2003 (RIShiri Fall EXperiment 2003) at Rishiri Island (45.07° N, 141.12° E, and 35 m a.s.l.) in the sea of Japan during September 2003. Four different sensitivity regimes are obtained based on the indicator. The sensitivity is found to show a distinctive pattern in each regime. In Regime I (Φ<1), <i>P</i>(O₃) almost linearly increases with increasing HC and almost linearly decreases with increasing NO_x. In Regime II (1<&Phi;<9 &plusmn; 5), there is a less-than-linear increase in <i>P</i>(O₃) with HC and a less-than-linear decrease with NO_x. <i>P</i>(O₃) less-than-linearly increases with both HC and NO_x in Regime III (9 &plusmn; 5<&Phi;<45 &plusmn; 7), and near linearly increases with NO_x and is nearly constant with increasing HC in Regime IV (&Phi;>45 &plusmn; 7). During the campaign, 91 percent of <i>P</i>(O₃) data appear in Regime III and IV, indicating that NO_x is a limiting factor of ozone production. Hence, it may be an efficient strategy to control NO_x emission for ozone abatement at the site. <br><br> Comparisons between the observed <i>P</i>(O₃) and the ones modelled have represented general agreement. However, the model tends to underestimate <i>P</i>(O₃) in Regime II, implying that an important source of peroxy radicals is possibly missed. In Regime IV, the modelled <i>P</i>(O₃) is systematically larger than the measured one under a low <i>j</i>(O¹D) condition, which may be caused by the over-estimated yields of peroxy radicals from the reactions of monoterpenes with ozone. A budget analysis indicates that sensitivity of <i>P</i>(O₃) is declining with HC and enhancing with NO when the condition shifts from Regime II to Regime IV, which is also observed through the analysis of <i>P</i>(O₃) sensitivity using &Phi;. Sensitivity studies for <i>P</i>(O₃) are conducted to determine the effect of NO_x and monoterpenes on ozone production and the conclusions are very consistent with those derived from the indicator. This study demonstrates that the ratio &Phi; could be a useful index to ascertain the sensitivity of <i>P</i>(O₃) to HC and NO_x in the clean marine boundary layer.