Size-resolved hygroscopic behavior of atmospheric aerosols during heavy aerosol pollution episodes in Beijing in December 2016

et al. 2020

Atmos. Chem. Phys.

Abstract. The Chinese government has exerted strict emission controls to mitigate air pollution since 2013, which has resulted in significant decreases in the concentrations of air pollutants such as SO2. Strict pollution control actions also reduced the average PM2.5 concentration to the low level of 39.7 µg m−3 in urban Beijing during the winter of 2017. To investigate the impact of such changes on the physiochemical properties of atmospheric aerosols in China, we conducted a comprehensive observation focusing on PM2.5 in Beijing during the winter of 2017. Compared with the historical record (2014–2017), SO2 decreased to the low level of 3.2 ppbv in the winter of 2017, but the NO2 level was still high (21.4 ppbv in the winter of 2017). Accordingly, the contribution of nitrate (23.0 µg m−3) to PM2.5 far exceeded that of sulfate (13.1 µg m−3) during the pollution episodes, resulting in a significant increase in the nitrate-to-sulfate molar ratio. The thermodynamic model (ISORROPIA II) calculation results showed that during the PM2.5 pollution episodes particle pH increased from 4.4 (moderate acidic) to 5.4 (more neutralized) when the molar ratio of nitrate to sulfate increased from 1 to 5, indicating that aerosols were more neutralized as the nitrate content elevated. Controlled variable tests showed that the pH elevation should be attributed to nitrate fraction increase other than crustal ion and ammonia concentration increases. Based on the results of sensitivity tests, future prediction for the particle acidity change was discussed. We found that nitrate-rich particles in Beijing at low and moderate humid conditions (RH: 20 %–50 %) can absorb twice the amount of water that sulfate-rich particles can, and the nitrate and ammonia with higher levels have synergetic effects, rapidly elevating particle pH to merely neutral (above 5.6). As moderate haze events might occur more frequently under abundant ammonia and nitrate-dominated PM2.5 conditions, the major chemical processes during haze events and the control target should be re-evaluated to obtain the most effective control strategy.

Section: Aerosol Ph's Response To the Elevation Of Nitrate Fraction Imentioning

confidence: 96%

Section: Comparison Of Major Inorganic Compositions During the Early mentioning

confidence: 99%

Nitrate-dominated PM<sub>2.5</sub> and elevation of particle pH observed in urban Beijing during the winter of 2017

Xie

et al. 2020

Atmos. Chem. Phys.

“…LH‐ and MH‐mode particles thus represent different aerosol sources and chemical processes. Implied aerosol chemical information from the GF‐PDF/ κ ‐PDF can be used to analyze particle formation processes for particles of different sizes, which is necessary to study the haze formation mechanism (X. Wang et al, ; Xie et al, ).…”

Section: Introductionmentioning

confidence: 99%

Distinct Ultrafine‐ and Accumulation‐Mode Particle Properties in Clean and Polluted Urban Environments

Geophysical Research Letters

Zhang

et al. 2019

In this study, we report the phenomenon of fast changes in aerosol hygroscopicity between clean and polluted periods observed frequently in urban Beijing during winter using a hygroscopicity tandem mobility analyzer. The cause of this phenomenon and the formation process of particles in different modes are discussed. During clean periods, ultrafine‐mode particles (i.e., nucleation and Aitken modes) stem mainly from nucleation events with subsequent growth. During heavily polluted periods, ultrafine‐mode particles originate chiefly from primary emissions. Larger‐mode particles like accumulation‐mode particles are mainly from primary emissions during clean periods and aqueous reactions during polluted periods. This finding based on hygroscopicity tandem mobility analyzer measurements can make up the deficiency of mass‐dependent instruments in analyzing sources and chemical processes of ultrafine‐mode particles.

“…X. Wang et al (2018) measured aerosol hygroscopic growth (30 %-90 % RH) of ambient aerosols on the campus of the Chinese Academy of Meteorological Sciences located between the second and third ring roads in western Beijing. Measurements were conducted on a building roof (∼ 53 m above ground level) in December 2016, and the distance between the site and a major road with heavy traffic was < 200 m. Aerosol hygroscopic growth displayed unimodal distributions when RH did not exceed 60 %, while bimodal distributions were usually observed at 70 % and 80 % RH; in addition, aerosol hygroscopic growth occasionally exhibited trimodal distribution at 85 % and 90 % RH (X. .…”

Section: Cams Sitementioning

confidence: 99%

“…Number fractions of hydrophobic particles exceeded 50 % at 50 and 100 nm, while hydrophilic particles frequently became dominant in terms of number concentrations at 150 and 200 nm (X. Wang et al, 2018). In addition, hygroscopicity decreased at 50 nm but increased at 200 nm during heavily polluted periods (X.…”

Section: Cams Sitementioning

confidence: 99%

Tropospheric aerosol hygroscopicity in China

Peng

et al. 2020

Atmos. Chem. Phys.

Abstract. Hygroscopicity largely determines phase state, chemical reactivity, optical properties, and cloud nucleation activities of aerosol particles, thus significantly affecting their impacts on visibility, atmospheric chemistry, and climate. In the last 20 years, a large number of field studies have investigated the hygroscopicity of tropospheric aerosols in China under subsaturated and supersaturated conditions. Aerosol hygroscopicity measurements in China are reviewed in this paper: (1) a comprehensive summary and critical discussion of aerosol hygroscopicity measurements in China are provided; (2) available measurement data are compiled and presented under a consistent framework to enhance their accessibility and usability; and (3) current knowledge gaps are identified, and an outlook which could serve as guidelines for planning future research is also proposed.