Two years of online measurement of fine particulate nitrate in the western Yangtze River Delta: influences of thermodynamics and N<sub>2</sub>O<sub>5</sub> hydrolysis

Abstract

Abstract. Particulate nitrate contributes a large fraction of secondary aerosols. Despite understanding of its important role in regional air quality and global climate, long-term continuous measurements are rather limited in China. In this study, we conducted online measurement of PM2.5 (particulate matter with diameters less than 2.5 µm) nitrate for 2 years from March 2014 to February 2016 using the Monitor for AeRosols and Gases in ambient Air (MARGA) in the western Yangtze River Delta (YRD), eastern China, and investigate the main factors that influenced its temporal variations and formation pathways. Compared to other sites in China, an overall high concentration of particulate nitrate was observed, with a mean value of 15.8 µg m−3 (0.5 to 92.6 µg m−3). Nitrate on average accounted for 32 % of the total mass of water-soluble ions and the proportion increased with PM loading, indicating that nitrate is a major driver of haze pollution episodes in this region. Sufficient ammonia drove most nitrate into the particle phase in the form of ammonium nitrate. A typical seasonal cycle of nitrate was observed, with the concentrations in winter on average 2 times higher than those in summer mainly due to different meteorological conditions. In summer, the diurnal variation of particulate nitrate was determined by thermodynamic equilibrium, resulting in a much lower concentration during daytime despite a considerable photochemical production. Air masses from the polluted YRD and biomass burning region contributed to the high nitrate concentration during summer. In winter, particulate nitrate did not reveal an evident diurnal variation. Regional transport from northern China played an important role in enhancing nitrate concentration. A total of 18 nitrate episodes were selected to understand the processes that drive the formation of high concentration of nitrate. Rapid nitrate formation was observed during the pre-episode (the day before nitrate episode day) nights, and dominated the increase of total water-soluble ions. Calculated nitrate from N2O5 hydrolysis was highly correlated to and accounted for 80 % of the observed nitrate, suggesting that N2O5 hydrolysis was a major contributor to the nitrate episodes. Our results suggested that rapid formation of nitrate could be a main cause for extreme aerosol pollution events in the YRD during winter, and illustrated the urgent need to control NOx emission.

Contributions of different sources to nitrous acid (HONO) at the SORPES station in eastern China: results from one-year continuous observation

“…Overall, the average POH(HONO) during 8:00-16:00 LT is 0.90 ± 0.27 ppb/h, more than twice the value of POH(O3), the mean value of which is 0.41 ± 0.25 ppb/h. The impressive role of HONO in the atmospheric oxidizing capacity should benefit photochemical ozone production (Ding et al, 2013;Xu et al, 2017;Xu et al, 2018), new particle formation and secondary aerosol formation (Xie et al, 2015;Sun et al, 2018) in Nanjing, the western YRD region.…”

Significant reduction of PM<sub>2.5</sub> in eastern China due to regional-scale emission control: evidence from SORPES in 2011–2018

“…Fine particulate matter, with an aerodynamic diameter smaller than 2.5 µm (PM 2.5 ), which impacts human health and visibility negatively (Cao et al, 2012;Zhang et al, 2017;Malm et al, 2004), has been considered to be one of the main air pollutants in China (He et al, 2001;Yao et al, 2002;Sun et al, 2006;Pathak et al, 2009;Zhang et al, 2015;. To tackle this great challenge, China has been implementing the "Action Plan for Air Pollution Prevention and Control" (i.e., the so-called national "Ten Measures of air") in several developed critical regions since 2013 (Sheehan et al, 2014;Wang et al, 2017;Liu et al, 2018Liu et al, , 2019Zheng et al, 2018).…”

Significant reduction of PM<sub>2.5</sub> in eastern China due to regional-scale emission control: Evidences from the SORPES station, 2011–2018

“…Some recent studies reported the change of PM2.5 and its chemical components in some YRD cities in a short period, e.g. 2-3 years (Wang H. et al, 2016;Sun et al, 2018), however, so far there is a lack of long-term observational study with 25 comprehensive measurements covering the entire period of national "Ten measures" period, i.e. 2013-2017, in this region.…”

“…sulfate (SO4 2-), nitrate (NO3 -), ammonium (NH4 + ), potassium(K + ), 15 and calcium (Ca 2-), etc.) has been measured since 2013 (Xie et al, 2015;Sun et al, 2018;Wang et al, 2018;Shen et al, 2018). Details of the data, including instrumentation, measurement period, and data coverage, used in this study are given in Table S1 in the Supporting Information (SI).…”

“…PM2.5 mass concentration 20 is measured by the online analyzer based on the light scattering and beta ray absorption method (Thermo Fisher Scientific, 5030SHARP, USA). The water-soluble inorganic ions, including SO4 2-, NO3 -, NH4 + , and K + etc., are detected by the instrument for Measuring AeRosols and GAses (MARGA, Metrohm, Switzerland) (Xie et al, 2015;Sun et al, 2018). BC was measured using a 7-wavelenth Aethalometer (AE-31, Magee Scientific) and the data at a wavelength of 25 880 nm was used in this study (Shen et al, 2018;Virkkula et al, 2015).…”