Isotopic constraints on heterogeneous sulfate production in Beijing haze

Abstract

Abstract. Discerning mechanisms of sulfate formation during fine-particle pollution (referred to as haze hereafter) in Beijing is important for understanding the rapid evolution of haze and for developing cost-effective air pollution mitigation strategies. Here we present observations of the oxygen-17 excess of PM 2.5 sulfate ( 17 O(SO 2− 4 )) collected in Beijing haze from October 2014 to January 2015 to constrain possible sulfate formation pathways. Throughout the sampling campaign, the 12-hourly averaged PM 2.5 concentrations ranged from 16 to 323 µg m −3 with a mean of (141 ± 88 (1σ )) µg m −3 , with SO Our estimate suggested that in-cloud reactions dominated sulfate production on polluted days (PDs, PM 2.5 ≥ 75 µg m −3 ) of Case II in October 2014 due to the relatively high cloud liquid water content, with a fractional contribution of up to 68 %. During PDs of Cases I and III-V, heterogeneous sulfate production (P het ) was estimated to contribute 41-54 % to total sulfate formation with a mean of (48 ± 5) %. For the specific mechanisms of heterogeneous oxidation of SO 2 , chemical reaction kinetics calculations suggested S(IV) (= SO 2 q H 2 O + HSO − 3 + SO 2− 3 ) oxidation by H 2 O 2 in aerosol water accounted for 5-13 % of P het . The relative importance of heterogeneous sulfate production by other mechanisms was constrained by our observed 17 O(SO 2− 4 ). Heterogeneous sulfate production via S(IV) oxidation by O 3 was estimated to contribute 21-22 % of P het on average. Heterogeneous sulfate production pathways that result in zero-17 O(SO 2− 4 ), such as S(IV) oxidation by NO 2 in aerosol water and/or by O 2 via a radical chain mechanism, contributed the remaining 66-73 % of P het . The assumption about the thermodynamic state of aerosols (stable or metastable) was found to significantly influence the calculated aerosol pH (7.6 ± 0.1 or 4.7 ± 1.1, respectively), and thus influence the relative importance of heterogeneous sulfate production via S(IV) oxidation by NO 2 and by O 2 . Our local atmospheric conditions-based calculations suggest sulfate formation via NO 2 oxidation can be the dominant pathway in aerosols at high-pH conditions calculated assuming stable state while S(IV) oxidation by O 2 can be the dominant pathway providing that highly acidic aerosols (pH ≤ 3) exist. Our local atmospheric-conditions-based calculations illustrate the utility of 17 O(SO 2− 4 ) for quantifying sulfate forPublished by Copernicus Publications on behalf of the European Geosciences Union. 5516 P. He et al.: Isotopic constraints on heterogeneous sulfate production in Beijing haze mation pathways, but this estimate may be further improved with future regional modeling work.

Role of Ammonia on the Feedback Between AWC and Inorganic Aerosol Formation During Heavy Pollution in the North China Plain

“…This is supported by the eNO z and eHNO 3 values, which increased from 0.07 and 0.03 at 1:00 LT on 3 December to 0.21 and 0.09 at LT 5:00 on 4 December (Figure d) during the nighttime, respectively. This is also consistent with that reported by He et al (), who used the oxygen isotope methods to find the importance of nocturnal chemistry in nitrate production in Beijing haze.…”

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

“…where NH + 4 , Na + , K + , Ca 2+ , Mg 2+ , SO 2− 4 , NO − 3 , and Cl − are the mass concentrations (µg m −3 ) of these ions in the atmosphere; [cations] and [anions] denote the sum of charge-equivalent total molar concentrations (µmol m −3 ) of cations and anions, respectively. Although they are straightforward to calculate, a few recent studies (Hennigan et al, 2015;Guo et al, 2016;Murphy et al, 2017) have demonstrated that the ion balance and equivalent ratio calculated from ambient particle measurements should not be used to predict the acidity of particles, especially under ammonia-rich conditions, for several reasons summarized as follows. (1) This would require all ions other than H + and OH − to be measured with both very high accuracy and precision, conditions unlikely to be achieved in practice.…”

“…In spite of its significance, ambient particle pH is still poorly constrained. The direct filter sampling approach is challenged by the nature of hydrogen ions in that its concentration in a solution does not scale in proportion to the level of dilution, and it is also subject to sampling errors (Hennigan et al, 2015). A few studies have determined the pH of laboratory-generated particles using colorimetry/spectrometry and Raman microspectroscopy (Li and Jang, 2012;Rindelaub et al, 2016;Craig et al, 2017), but such techniques have not been applied to ambient particles, partly due to their much more complex chemical and physical properties.…”

Observation of nitrate dominant PM<sub>2.5</sub> and particle pH elevation in urban Beijing during the winter of 2017

“…) and high RH while photochemistry is often less active (Wang et al, 2016b;Cheng et al, 2016b). Field observations, chamber experiments, source apportionments and simulation works all suggests that the joint effect of NO2, SO2, and NH3 is important in the sulfate formation processes in haze events (Cheng et al, 2016b;Wang et al, 2016b;He et al, 2018).Aqueous oxidation of SO2 by NO2 could be of major process of sulfate 55 formation in winter Beijing, as well as transition metal ions (TMI) catalyzed oxidation (Wang et al, 2016b;Cheng et al, 2016b). Besides, though the photochemistry were less active during winter haze periods, extra radical provided by HONO might enhance the atmospheric oxidation capacity and thus leads to extra SNA formation (Tan et al, 2018).…”