High summer background O3 levels in the desert of northwest China.

Generally speaking, the precursors of ozone (O3), nitrogen oxides and volatile organic compounds are very low in desert areas due to the lack of anthropogenic emissions and natural emissions, and thus O3 concentrations are relatively low. However, high summer background concentrations of about 100 µg/m3 or 60 ppb were found in the Alxa Desert in the highland of northwest China based on continuous summer observations from 2019 to 2021, which was higher than the most of natural background areas or clean areas in world for summer O3 background concentrations. The high O3 background concentrations were related to surface features and altitude. Heavy-intensity anthropogenic activity areas in desert areas can cause increased O3 concentrations or pollution, but also generated O3 depleting substances such as nitrous oxide, which eventually reduced the regional O3 baseline values. Nitrogen dioxide (NO2) also had a dual effect on O3 generation, showing promotion at low concentrations and inhibition at high concentrations. In addition, sand-dust weather reduced O3 clearly, but O3 eventually stabilized around the background concentration values and did not vary with sand-dust particulate matter.