On the presence of high nitrite (NO₂⁻) in coarse particles at Mt. Qomolangma

Atmospheric reactive nitrogen cycling, with nitrous acid (HONO) and particulate nitrite (NO2-) as important intermediates, is crucial for maintaining the atmospheric oxidation capacity of background atmosphere on the Tibetan Plateau. During an 11 d field campaign at the Base Camp of Mt. Qomolangma in spring of 2022, we observed significant enrichments of NO2- in total suspended particulate (TSP) with a mean concentration of 375±386 ng m−3, while NO2- was absent in fine particles (PM2.5). The comparison revealed that NO2- predominately exists in coarse particles. Local surface soil at the sampling site also exhibited high levels of NO2-, with δ15N value similar to NO2- in TSP. This isotopic similarity suggests that wind-blown soil is probably the primary source of NO2- in TSP, accounting for the background levels. While concentration changes of water-soluble inorganic ions in TSP and PM2.5 in response to shifts in air mass back-trajectories imply that atmospheric pollutants transported from South Asia may further elevate the NO2-, the specific mechanisms of long-range transport resulting in NO2- accumulation in TSP rather than PM2.5 remain unknown and need to be investigated. The elevated levels of TSP NO2- may readily participate in atmospheric reactive nitrogen cycling through gas-particle partitioning or photolysis, leading to the production of HONO, OH and NO and thereby influencing oxidation chemistry. Further efforts on the sources and atmospheric chemistry of particulate nitrite are warranted, particularly in the pristine Tibetan Plateau, where even small inputs of NOx or HONO can disproportionately affect oxidant budgets and reactive nitrogen cycling.