4.2.6.2 Atmospheric measurements and modelling of photochemistry

Figure 4.9: (left panel) Observed versus modelled (1) HO2 abundance (ppt), (2) OH abundance (ppt), and (3) HO2/OH ratio in the upper troposphere (8 to 12 km altitude) during SONEX. Observations are for cloud-free, daytime conditions. Model calculations are constrained with local observations of the photochemical background (H2O2, CH3OOH, NO, O3, H2O, CO, CH4, ethane, propane, acetone, temperature, pressure, aerosol surface area and actinic flux). The 1:1 line (solid) and instrumental accuracy range (dashed) are shown. Adapted from Brune et al. (1999). (right panel) Observed (4) HO2 abundance (ppt), (5) OH abundance (ppt), and (6) derived O3 production rate (ppb/day) as a function of the NOx (NO+NO2) abundance (ppt). Data taken from SONEX (8 to 12 km altitude, 40° to 60°N latitude) and adapted from Jaeglé et al. (1999). All values are 24-hour averages. The lines correspond to model-calculated values as a function of NOx using the median photochemical background during SONEX rather than the instantaneous values (points).

The 1997 SONEX aircraft campaign over the North Atlantic provided the first airborne measurements of HO_x abundances concurrent with the controlling chemical background: H₂O₂, CH₃OOH, CH₂O, O₃, NO_x, H₂O, acetone and hydrocarbons. These observations allowed a detailed evaluation of our understanding of HO_x chemistry and O₃ production in the upper troposphere. Figure 4.9 (panels 1-3) shows a comparison between SONEX measurements and model calculations (Jaeglé et al., 1999) for OH and HO₂ abundances and the ratio HO₂/OH. At each point the model used the local, simultaneously observed chemical abundances. The cycling between OH and HO₂ takes place on a time-scale of a few seconds and is mainly controlled by reaction of OH with CO producing HO₂, followed by reaction of HO₂ with NO producing OH. This cycle also leads to the production of ozone. As seen in Figure 4.9, the HO₂/OH ratio is reproduced by model calculations to within the combined uncertainties of observations (±20%) and those from propagation of rate coefficient errors in the model (±100%), implying that the photochemical processes driving the cycling between OH and HO₂ appear to be understood (Wennberg et al., 1998; Brune et al., 1999). The absolute abundances of OH and HO₂ are matched by model calculations to within 40% (the reported accuracy of the HO_x observations) and the median model-to-observed ratio for HO₂ is 1.12. The model captures 80% of the observed variance in HO_x, which is driven by the local variations in NO_x and the HO_x sources (Faloona et al., 2000, Jaeglé et al., 2000;). The predominant sources of HO_x during SONEX were reaction of O(¹D) with H₂O and photodissociation of acetone; the role of H₂O₂ and CH₃OOH as HO_x sources was small. This was not necessarily the case in some of the other airborne campaigns, where large differences between measured and modelled OH, up to a factor of 5, were observed in the upper troposphere. In these campaigns the larger measured OH concentrations were tentatively ascribed to enhanced levels of OH precursors, such as H₂O₂, CH₃OOH, or CH₂O, whose concentrations had not been measured.

Tropospheric O₃ production is tightly linked to the abundance of NO_x, and Figure 4.9 (panel 6) shows this production rate (calculated as the rate of the reaction of HO₂ with NO) for each set of observations as function of NO_x during the SONEX mission. Also shown in Figure 4.9 (panels 4-5) are the measured abundances of OH and HO₂ as a function of NO_x. The smooth curve on each panel 4-6 is a model simulation of the expected relationship if the chemical background except for NO_x remained unchanged at the observed median abundances. This curve shows the �expected� behaviour of tropospheric chemistry when only NO_x is increased: OH increases with NO_x abundances up to 300 ppt because HO₂ is shifted into OH; it decreases with increasing NO_x at higher NO_x abundances because the OH reaction with NO₂ forming HNO₃ becomes the dominant sink for HO_x radicals. Production of O₃ is expected to follow a similar pattern with rates suppressed at NO_x abundances greater than 300 ppt under these atmospheric conditions (e.g., Ehhalt, 1998). These SONEX observations indicate, however, that both OH abundance and O₃ production may continue to increase with NO_x concentrations up to 1,000 ppt because the high NO_x abundances were often associated with convection and lightning events and occurred simultaneously with high HO_x sources. By segregating observations according to HO_x source strengths, Jaeglé et al. (1999) identified the approach to NO_x-saturated conditions predicted by the chemical models when HO_x sources remain constant. A NO_x-saturated environment was clearly found for the POPCORN (Photo-Oxidant formation by Plant emitted Compounds and OH Radicals in north-eastern Germany) boundary layer measurements in Germany (Rohrer et al., 1998; Ehhalt and Rohrer, 2000). The impact of NO_x-saturated conditions on the production of O₃ is large in the boundary layer, where much of the NO_x is removed within a day, but may be less important in the upper troposphere, where the local lifetime of NO_x is several days and the elevated abundances of NO_x are likely to be transported and diluted to below saturation levels. This effective reduction of the NO_x-saturation effect due to 3-D atmospheric mixing is seen in the CTM modelling of aviation NO_x emissions where a linear increase in tropospheric O₃ is found, even with large NO_x emissions in the upper troposphere (Isaksen and Jackman, 1999).

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