Influence of modelled soil biogenic NO emissions on related trace gases and the atmospheric oxidizing efficiency

Abstract

The emission of nitric oxide (NO) by soils (SNOx) is an important source of oxides of nitrogen (NO_x=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 NO_x, O₃, peroxyacetyl nitrate (PAN), nitric acid (HNO₃), the hydroxyl radical (OH) and the lifetime of methane (τ_CH₄). We show that SNOx is responsible for a significant contribution to the NO_x 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 τ_CH₄ due to soil NO emissions. On the other hand, in some regions SNOx has a negative feedback on the lifetime of NO_x through O₃ and OH, which results in regional increases in the mixing ratio of NO_x despite lower total emissions in a simulation without SNOx. In a sensitivity simulation in which we reduce the other surface NO_x emissions by the same amount as SNOx, we find that they have a much weaker impact on OH and τ_CH₄ and do not result in an increase in the NO_x mixing ratio anywhere.