Fine particle pH during severe haze episodes in northern China

Abstract

Aerosol acidity plays an important role in atmospheric chemistry. China emits large amounts of SO2, NOx, and NH3 into the atmosphere, but aerosol acidity is poorly characterized. In this study, simultaneous 1 h measurements of particulate and gaseous compositions along with the ISORROPIA‐II thermodynamic equilibrium model were used to study aerosol acidity during severe haze episodes in northern China. The summed concentration of sulfate, nitrate, and ammonium was 135 ± 51 μg/m3 with a maximum of 250 μg/m3, and the gas‐phase NH3 mixing ratio was 22 ± 9 ppb. Fine particles were moderately acidic, with a pH range of 3.0–4.9 and an average of 4.2, which was higher than those in the United States and Europe. Excess NH3 and high aerosol water content are responsible for the relatively lower aerosol acidity. These results suggest that the new pathways for sulfate production in China proposed by recent studies should be revisited.

Fine particle pH for Beijing winter haze as inferred from different thermodynamic equilibrium models

“…The studies using the standard ISORROPIA model with the stable state assumption have predicted unrealistic pH values of around 7 and should be reevaluated He et al, 2017). Previous ISORROPIA calculations using the metastable state assumption 15 obtained average pH values from 4.1 to 5.4 (He et al, 2017;Liu et al, 2017a;Shi et al, 2017;Cheng et al, 2016;Guo et al, 2017b;Tan et al, 2018), agreeing reasonably with our results (an average pH value of 4.6). It is noted that concentrations of Ca 2+ and Mg 2+ were not measured in this study and a sensitivity test suggests that including these crustal cations in thermodynamic calculations would increase predicted pH values by about 0.1 unit (Fig.…”

“…This suggests that ISORROPIA may overestimate pH if E-AIM is taken as a benchmark. Liu et al (2017a) also found that the pH values from ISORROPIA were higher (0.3 unit on average) than those from E-AIM (version II) under winter haze conditions. One assumption in ISORROPIA is that γ H + equals unity (Sect.…”

“…4d). Comparable NH3 levels have been measured at multiple stations over the NCP during the winter months Liu et al, 2017a;Zhao et al, 2016), indicating that ammonia-rich conditions are very common. The NHx sources include agriculture, fossil fuel use, and green space, and their contributions vary in different environments Sun et al, 2017;Teng et al, 2017;Pan et al, 2016).…”

“…Recent studies (Guo et al, 2017b;Liu et al, 2017a) have emphasized the important role of ammonia in determining winter haze particle pH. As shown in Fig.…”

“…CC BY 4.0 License. deviation) are high whereas HNO3 (0.08 ± 0.04 ppb) and HCl (0.25 ± 0.07 ppb) are observed to be very low, consistent with Liu et al (2017a). The inputs for the forward mode calculations involve the measured total (gas plus aerosol) concentrations of NH3, H2SO4, HCl, HNO3, Na + , and K + .…”