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

Abstract

In urban North China, nitrate () is a primary contributor to haze formation. So far, the production processes and source apportionments of atmospheric during the heating season (i.e. the wintertime) have not yet been well understood. This study determined δ15N–, δ18O–, and Δ17O– of aerosol samples to compare the potential sources and formation pathways of atmospheric during heating (November to March) and non-heating (April to May) seasons. Combining stable isotope composition with the MixSIAR model based on Δ17O– 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 Δ17O– 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 . In specific, in a dust storm event, the very low Δ17O– is likely associated with particles from land surface. Under the premise of considering 15N fractionation, the constraint-based on δ15N– illustrated that coal combustion was the major source of NO x 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 during the heating and non-heating seasons, which are of significance to atmospheric nitrogen oxide/nitrate pollution mitigation.