Mutual promotion effect between aerosol particle liquid water and nitrate formation lead to severe nitrate-dominated particulate matter pollution and low visibility

Abstract

<p><strong>Abstract.</strong> As has been the case in North America and Western Europe, the SO<sub>2</sub> emissions substantially reduced in North China Plain (NCP) in recent years. A dichotomy of reductions in SO<sub>2</sub> and NO<sub>x</sub> concentrations result in the frequent occurrences of nitrate (pNO<sub>3</sub><sup>−</sup>)-dominated particulate matter pollution over NCP. In this study, we observed a polluted episode with the nitrate mass fraction in non-refractory PM<sub>1</sub> (NR-PM<sub>1</sub>) up to 44 % during wintertime in Beijing. Based on this typical pNO<sub>3</sub><sup>−</sup>-dominated haze event, the linkage between aerosol water uptake and pNO<sub>3</sub><sup>−</sup> formation, further impacting on visibility degradation, have been investigated based on field observations and theoretical calculations. During haze development, as ambient relative humidity (RH) increased from ~ 10 % up to 70 %, the aerosol particle liquid water increased from ~ 1 μg/m<sup>3</sup> at the beginning to ~ 75 μg/m<sup>3</sup> at the fully-developed haze period. Without considering the water uptake, the particle surface area and the volume concentrations increased by a factor of 4.1 and 4.8, respectively, during the development of haze event. Taking water uptake into account, the wet particle surface area and volume concentrations enhanced by a factor of 4.7 and 5.8, respectively. As a consequence, the hygroscopic growth of particles facilitated the condensational loss of dinitrogen pentoxide (N<sub>2</sub>O<sub>5</sub>) and nitric acid (HNO<sub>3</sub>) to particles contributing pNO<sub>3</sub><sup>−</sup>. From the beginning to the fully-developed haze, the condensational loss of N<sub>2</sub>O<sub>5</sub> increased by a factor of 20 when only considering aerosol surface area and volume of dry particles, while increasing by a factor of 25 considering extra surface area and volume due to water uptake. Similarly, the condensational loss of HNO<sub>3</sub> increased by a factor of 2.7~2.9 and 3.1~3.5 for dry and wet aerosol surface area and volume from the beginning to the fully-developed haze period. Above results demonstrated that the pNO<sub>3</sub><sup>−</sup> formation is further enhanced by aerosol water uptake with elevated ambient RH during haze development, in turn, facilitating the aerosol taking up water due to the hygroscopicity of nitrate salt. Such mutual promotion effect between aerosol particle liquid water and nitrate formation can rapidly degrade air quality and halve visibility within one day. Reduction of nitrogen-containing gaseous precursors, e.g., by control of traffic emissions, is essential in mitigating severe haze events in NCP.</p>

Tropospheric aerosol hygroscopicity measurements in China

“…Diurnal variation of aerosol hygroscopicity was also explored (Achtert et al, 2009): hygroscopicity was found to be higher in the daytime than the nighttime for the Aitken mode, whereas no significant difference in hygroscopicity was observed between daytime and nighttime for the accumulation mode. growth (at 90% RH) was measured at this site in January-March (Wang et al, 2020b). The average κ were determined to be 0.162-0.208 for 50-300 nm particles (Wang et al, 2020b), and mass fractions of nitrate, which contributed significantly to aerosol hygroscopic growth, reached 44% during polluted episodes.…”

“…growth (at 90% RH) was measured at this site in January-March (Wang et al, 2020b). The average κ were determined to be 0.162-0.208 for 50-300 nm particles (Wang et al, 2020b), and mass fractions of nitrate, which contributed significantly to aerosol hygroscopic growth, reached 44% during polluted episodes.…”

“…Aerosol hygroscopic growth were measured at two rural sites in Beijing, including Yufa (Achtert et al, 2009) and Huairou (Wang et al, 2020b). The Yufa site (39.51ºN, 116.31ºE) is ~1.2 km away from a high-traffic expressway and ~50 km south to urban Beijing, and can be considered as a representative rural and regional background site.…”

Stable15N isotopes in fine and coarse urban particulate matter

“…According to the German Environment Agency (2019), sulfur dioxide emissions in Germany decreased by 94.3% from 1990 to 2017, whereas nitrogen oxide emissions decreased by 59% and ammonia emissions only decreased by 10.9%. Thus, nitrate has become increasingly important in the fine aerosol range (Wang et al 2019).…”

Amplification of black carbon light absorption induced by atmospheric aging: temporal variation at seasonal and diel scales in urban Guangzhou

“…Hourly backward trajectories for the past 72 h were calculated using NOAA's HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory, version 4) model (Draxier and Hess, 1998) for both dry and wet seasons. Backward trajectory cluster analysis was conducted using Me-teoInfo (Wang, 2014(Wang, , 2019. Fire count data from the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi NPP weather satellite (Csiszar et al, 2014) were downloaded from the NASA FIRMS website (https://firms.…”