Transport pathways of peroxyacetyl nitrate in the upper troposphere and lower stratosphere from different monsoon systems during the summer monsoon season
Atmospheric Chemistry and Physics Discussions
The Asian summer monsoon involves complex transport patterns with large scale redistribution of trace gases in the upper troposphere and lower stratosphere (UTLS). We employ the global chemistry-climate model ECHAM5-HAMMOZ in order to evaluate the transport pathways and the contributions of nitrogen oxide species PAN, NO<sub><i>x</i></sub>, and HNO<sub>3</sub> from various monsoon regions, to the UTLS over Southern Asia and vice versa. Simulated long term seasonal mean mixing ratios are
... ratios are compared with trace gas retrievals from the Michelson Interferometer for Passive Atmospheric Sounding aboard ENVISAT(MIPAS-E) and aircraft campaigns during the monsoon season (June–September) in order to evaluate the model's ability to reproduce these transport patterns. <br><br> The model simulations show that there are three regions which contribute substantial pollution to the South Asian UTLS: the Asian summer monsoon (ASM), the North American Monsoon (NAM) and the West African monsoon (WAM). However, penetration due to ASM convection reaches deeper into the UTLS as compared to NAM and WAM outflow. The circulation in all three monsoon regions distributes PAN into the tropical latitude belt in the upper troposphere. Remote transport also occurs in the extratropical upper troposphere where westerly winds drive North American and European pollutants eastward where they can become part of the ASM convection and be lifted into the lower stratosphere. In the lower stratosphere the injected pollutants are transported westward by easterly winds. <br><br> The intense convective activity in the monsoon regions is associated with lightning and thereby the formation of additional NO<sub><i>x</i></sub>. This also affects the distribution of PAN in the UTLS. According to sensitivity simulations with and without lightning, increase in concentrations of PAN (~ 40%), HNO<sub>3</sub> (75%), NO<sub><i>x</i></sub> (70%) and ozone (30%) over the regions of convective transport, especially over equatorial Africa and America and comparatively less over the ASM. This indicates that PAN in the UTLS over the ASM region is primarily of anthropogenic origin.