Investigating atmospheric nitrate sources and formation pathways between heating and non-heating seasons in urban North China

In urban North China, nitrate (NO3-) is a primary contributor to haze formation. So far, the production processes and source apportionments of atmospheric NO3- during the heating season (i.e. the wintertime) have not yet been well understood. This study determined delta N-15-NO3-, delta O-18-NO3-, and ?O-17-NO 3- of aerosol samples to compare the potential sources and formation pathways of atmospheric NO3- during heating (November to March) and non-heating (April to May) seasons. Combining stable isotope composition with the MixSIAR model based on ?O-17-NO- (3) showed that NO3 + DMS/HC (dimethyl sulfate/hydrocarbon) pathway was the dominant process of atmospheric nitrate formation during the heating season (mean = 52.88 +/- 16.11%). During the non-heating season, the contributions of NO3 + DMS/HC (mean = 37.89 +/- 13.57%) and N2O5 + H2O (mean = 35.24 +/- 3.75%) pathways were comparable. We found that ?O-17-NO(3 )(-)was negatively correlated with wind speed and positively correlated with relative humidity during the heating season, possibly associated with the sources and production of atmospheric NO3-. In specific, in a dust storm event, the very low ?O-17-NO- (3) is likely associated with particles from land surface. Under the premise of considering 15N fractionation, the constraint-based on delta 15N-NO3- illustrated that coal combustion was the major source of NOx emission during the heating season, and the relative contribution of coal combustion decreased rapidly from the heating season (mean = 42.56 +/- 15.50%) to the non-heating season (mean = 21.86 +/- 4.91%). Conversely, the proportion of NOx emitted by soil microbes rose significantly from the heating (mean = 9.67 +/- 5.99%) to non-heating season (mean = 24.02 +/- 11.65%). This study revealed differences in the sources and formation processes of atmospheric NO-3 during the heating and non-heating seasons, which are of significance to atmospheric nitrogen oxide/nitrate pollution mitigation.

Automated summary

This study investigates the sources and formation pathways of atmospheric nitrate during the heating and non-heating seasons in urban North China. The results show that the dominant process of nitrate formation during the heating season is the NO3 + DMS/HC pathway, while during the non-heating season, both the NO3 + DMS/HC and N2O5 + H2O pathways contribute significantly. The study also reveals the correlation between ?O-17-NO(3 )(-) and wind speed, relative humidity, suggesting their association with nitrate sources and production during the heating season.