Abstract
Abstract. The emission of nitric oxide (NO) by soils (SNOx) is an important source of oxides of nitrogen (NOx=NO+NO2) in the troposphere, with estimates ranging from 4 to 21 Tg of nitrogen per year. Previous studies have examined the influence of SNOx on ozone (O3) chemistry. We employ the ECHAM5/MESSy atmospheric chemistry model (EMAC) to go further in the reaction chain and investigate the influence of SNOx on lower tropospheric NOx, O3, peroxyacetyl nitrate (PAN), nitric acid (HNO3), the hydroxyl radical (OH) and the lifetime of methane (τCH4). We show that SNOx is responsible for a significant contribution to the NOx mixing ratio in many regions, especially in the tropics. Furthermore, the concentration of OH is substantially increased due to SNOx, resulting in an enhanced oxidizing efficiency of the global troposphere, reflected in a ~10% decrease in τCH4 due to soil NO emissions. On the other hand, in some regions SNOx has a negative feedback on the lifetime of NOx through O3 and OH, which results in regional increases in the mixing ratio of NOx despite lower total emissions in a simulation without SNOx. In a sensitivity simulation in which we reduce the other surface NOx emissions by the same amount as SNOx, we find that they have a much weaker impact on OH and τCH4 and do not result in an increase in the NOx mixing ratio anywhere.
Highlights
Nitric oxide (NO) in the soil is produced by the microbial processes of nitrification and denitrification (Firestone and Davidson, 1989)
To investigate whether other surface NOx emissions result in similar effects, or if they differ due to differences in their distribution, we performed a third simulation (REDOTHER) in which we reduced the NOx emission from all other sources by the same amount as is emitted by the soils
The first point can be explained by the temperature dependence of SNOx and the second one by the greater landmasses in the Northern Hemisphere
Summary
Nitric oxide (NO) in the soil is produced by the microbial processes of nitrification and denitrification (Firestone and Davidson, 1989). Steinkamp et al.: Modelled NO soil emissions, related trace gases and oxidizing efficiency of methane (CH4), another greenhouse gas. Beyond these climate related issues, high NOx and O3 mixing ratios have a direct impact on human health and on the vegetation (Sitch et al, 2007). NOx is removed from the atmosphere by reaction with hydroxyl radicals (OH) or oxidation to dinitrogen pentaoxide (N2O5) and subsequent deposition as nitric acid (HNO3) It can react with organic tracers to form peroxyl nitrates, mainly peroxyacetyl nitrate (PAN), which, once it is lifted to higher altitudes, can be transported over large distances releasing NOx when it is transported back downward again. In the final section we present our conclusions and outlook
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