I am currently working on satellite-based NOx emission inversions for the East Asia region. I noticed that when I increase NOx emissions in East Asia, the concentration of nitrate doesn’t significantly rise but rather decreases.
Although I roughly understand that the formation mechanism of nitrate and its relationship with nitrogen oxides is not absolutely positively correlated. I still don’t fully comprehend the underlying chemical mechanism. Why does the concentration of nitrate decrease when nitrogen oxides increase? I wonder if there are any relevant references to consult or if this is caused by certain physical or chemical mechanisms.
The important daytime production pathway for HNO3 is the NO2 + OH reaction and nighttime HNO3 production is the heterogeneous hydrolysis of N2O5. Some VOC reactions with NO3 radical can produce nighttime HNO3. However, they are generally small but can be important in specific areas. My understanding is that East Asia is dominated by high NOx and low VOC; thus, the impact of VOC reactions on HNO3 in East Asia is likely small. ISORROPIA calculates partitioning of total nitrate (HNO3 + aerosol nitrate) into HNO3 and aerosol nitrate. First, you can look into total nitrate and determine how it changes with increased NOx. It is possible that total nitrate increases with increased NOx but aerosol nitrate may not increase due to partitioning by ISORROPIA. Second, East Asia is a high NOx area. When you increase NOx, it likely reduces daytime O3 through titration which may lead to lower OH. Lower OH leads to lower daytime HNO3 and aerosol nitrate production. Lower O3 may led to lower NO3 radical at night which ultimately leads to lower N2O5 and lower HNO3 and aerosol nitrate production. Thus, you can check how O3, OH and N2O5 changes with increased NOx. If they are lower with increased NOx, then HNO3 and subsequently aerosol nitrate will be lower.
Hi gsarwar,
Thank you very much for providing such a detailed and comprehensive response.
I now have a general understanding of the overall chemical reaction mechanisms and the issues involved.
As you mentioned, the ozone levels in East Asia are indeed showing a decreasing trend. It’s possible that a unilateral increase in nitrogen oxides may lead to a reduction in nitrates.
I will conduct further in-depth evaluations in the future.
Thank you again!