Analyzing the increasing importance of nitrate in Taiwan from long-term trend of measurements

Abstract

In recent years, many sample analyses have revealed that the proportion of nitrate (NO3−) in PM2.5 frequently exceeds that of other major PM2.5 species, such as SO42−, NH4+, and OC. This phenomenon has attracted considerable attention because it could change the direction of PM2.5 control policies. The present study analyzed the long-term trends of gaseous pollutants, PM2.5 and PM2.5 species. PM2.5 and precursor gases, such as SO2, NOX, and NMVOCs, showed obvious downtrends from 2005 to 2020, while O3 and NH3 remained roughly the mean level. In addition, the two stages (sampling period I: 2003–2009; sampling period II: 2015–2019) of PM2.5 composition analysis showed that the SO42−, OC, and EC concentrations obviously decreased annually while the NO3− concentrations did not. The proportion of NO3− in PM2.5 increased from 2.4% to 12.6% during sampling period I to 12.6%–23.9% during sampling period II when PM2.5 concentrations was higher than 35 μg m−3. NO3− and NH4+ were both highly correlated with PM2.5 in sampling period II, suggesting that NH4NO3 is the major chemical in PM2.5. Because most cities are under NH3-rich conditions, the control of NO3− will become the key to controlling PM2.5. According to the trends of O3, NO3−, and NH3, the amount of NH3, and the formation mechanism of NO3−, this study suggests that O3 can be regulated to control NO3− and thus control PM2.5. Methods of controlling O3 are beyond the scope of the current study but will be studied in the near future.