ATMOCHEM - Interaction of trace gases with mineral dust: Impact on atmospheric photochemical cycles and dust properties
Zusammenfassung der Projektergebnisse
Our box model studies show, that the uptake of gaseous O3 and NO2 on mineral dust does not have a significant impact on the background ozone concentration. Even if an unresisted uptake of the gas molecules is assumed, the atmospheric ozone concentration is not visibly changing. Therefore, an improved value for the uptake coefficient of NO2 , increased by a factor of 16 and found by the experimental coworkers of our project, shows no effect on the ozone concentrations. The impact of HNO3 uptake depends on the degree of particle concentration on one hand and on the existing NOx -regime on the other hand. The uptake of HNO3 leads to a decrease of O3 in the presence of a low NOx-regime and to an increase of ozone in the high NOx-regime by about 15% on simulation day 3. This effect is amplified by N2O5 , as this trace gas is partly converting to HNO3 in the presence of H2O. The unresisted uptake of NO3 causes a clear reduction of O3 up to 60% on day 3 in the case of high particle number densities. Aldehyde and cresol are mainly involved in the corresponding reaction cycle, but due to the non-linearity of the ozone chemistry, the combination of all reactions treating NO3 is necessary to explain the simulated ozone reduction. For NO3 , a variation of the uptake coefficient changes the effect on the ozone chemistry. Thus, the update of the NO3 coefficient value by laboratory studies in the framework of this project is an important improvement and leads to more reliable results of our box model simulations. Recent studies of further authors emphasized, that heterogeneous uptake of O3 and NO2 has no remarkable influence on the background ozone. Our studies confirm this context. Besides, recent publications assume a significant influence of HNO3 and also of N2O5 on ozone, resulting in a reduction of O3 by 2% - 4%. Our box model simulations show, that the impact of both species is important, but depends on the NOx-regime. Regarding NO3 , the influence of its uptake was assessed as not really important so far. Using improved uptake coefficients and taking into account a complex hydrocarbon chemistry in the box model, we find an essential impact of the heterogeneous NO3-uptake which may partly explain the observed atmospheric ozone reduction during mineral dust events. One has to keep in mind, that a box model does only treat chemical processes and no atmospheric dynamics. The latter attenuate the effect of chemistry. The correlations of O3 and NOy illustrate the implication of the degree of particle pollution for the relation between the atmospheric NOy reservoir and the ozone chemistry. In the case of moderate particle number densities (scenario 1), the borderline between the low and high NOx regime is identical for all considered species. In the case of high particle number densities (scenario 2), species with a high ozone destruction potential (O3 with γinit and NO3 ) indicate a borderline shifted to lower NOy concentrations. Besides, the impact of uptake of each trace gas does not change linearly with varying NO emission rates (i.e. at different values of f ). Scenario 1 shows larger discrepancies only for f=1. Scenario 2 implies substantial larger variations for most of the presented N O emission rates with a maximum of discrepancies for f=0.9, which corresponds to N O-emissions of 1.17 · 10^−2 ppb/min.
