Effect of heterogeneous aqueous reactions on the secondary formation of inorganic aerosols during haze events

Abstract

h i g h l i g h t sHeterogeneous aqueous reactions during haze events was investigated. The conversion of gas-phase of S and N to particle-phase was analyzed. Relationships were given between conversion ratio of S, N with RH and O 3 . Evolution of aerosol composition and particle size were analyzed. a b s t r a c tThe effect of heterogeneous aqueous reactions on the secondary formation of inorganic aerosols during haze events was investigated by analysis of comprehensive measurements of aerosol composition and concentrations [e.g., particular matters (PM 2.5 ), nitrate (NO 3 ), sulfate (SO 4 ), ammonium (NH 4 )], gas-phase precursors [e.g., nitrogen oxides (NOx), sulfur dioxide (SO 2 ), and ozone (O 3 )], and relevant meteorological parameters [e.g., visibility and relative humidity (RH)]. The measurements were conducted in Beijing, China from Sep. 07, 2012 to Jan. 16, 2013. The results show that the conversion ratios of N from NOx to nitrate (N ratio ) and S from SO 2 to sulfate (S ratio ) both significantly increased in haze events, suggesting enhanced conversions from NOx and SO 2 to their corresponding particle phases in the late haze period. Further analysis shows that N ratio and S ratio increased with increasing RH, with N ratio and S ratio being only 0.04 and 0.03, respectively, when RH < 40%, and increasing up to 0.16 and 0.12 when RH reached 60e80%, respectively. The enhanced conversion ratios of N and S in the late haze period is likely due to heterogeneous aqueous reactions, because solar radiation and thus the photochemical capacity are reduced by the increases in aerosols and RH. This point was further affirmed by the relationships of N ratio and S ratio to O 3 : the conversion ratios increase with decreasing O 3 concentration when O 3 concentration is lower than <15 ppb but increased with increasing O 3 when O 3 concentration is higher than 15 ppb. The results suggest that heterogeneous aqueous reactions likely changed aerosols and their precursors during the haze events: in the beginning of haze events, the precursor gases accumulated quickly due to high emission and low reaction rate; the occurrence of heterogeneous aqueous reactions in the late haze period, together with the accumulated high concentrations of precursor gases such as SO 2 and NOx, accelerated the formation of secondary inorganic aerosols, and led to rapid increase of the PM 2.5 concentration.

Vertically-resolved Characteristics of Air Pollution during Two Severe Winter Haze Episodes in Urban Beijing, China

“…One explanation is the enhanced formation of 25 secondary aerosol species during severe haze episodes that were associated with the oxidation of precursors (e.g., SO 2 and NO 2 ). This is particularly important for SO 2 that can be rapidly oxidized to form sulfate via aqueous-phase and/or fog processing (Sun et al, 2013c;Quan et al, 2015), while the role of NO 2 oxidation is generally small due to its much slower aqueous-phase processing rates (Seinfeld and Pandis, 2006), which is also supported by the shorter lifetime of SO 2 compared with that of NO 2 in the atmosphere. In addition, the scavenging rates of gaseous species and particles can be substantially 30 different during the high RH event (Wayne et al, 1991;Dan et al, 1999;Dan et al, 2003;Mcmurry et al, 2004).…”

Persistent sulfate formation from London Fog to Chinese haze

“…Hence, the acidity, hygroscopicity, and RH represent the key factors for sulfate formation on fine PM, explaining the differences in the various ambient measurements (14,(20)(21)(22)(23)(24)(25). For example, the acidity effect on fine aerosols is effectively overcome by NH 3 neutralization in Xi'an and Beijing (Fig.…”

“…A model simulation of dissolution of NO 2 in cloud droplets under NO x -rich environments has shown enhanced regional wintertime sulfate by up to 20%, resulting in better agreement between simulations and observations (13). Also, atmospheric measurements have revealed high sulfate production during severe haze events in China (21)(22)(23)(24)(25), which cannot be explained by current atmospheric models and suggests missing sulfur oxidation mechanisms (14). Typically, high sulfate levels during haze events in China occurred with concurrently elevated RH, NO x , and NH 3 (24,25), implicating an aqueous sulfur oxidation pathway.…”

“…Also, atmospheric measurements have revealed high sulfate production during severe haze events in China (21)(22)(23)(24)(25), which cannot be explained by current atmospheric models and suggests missing sulfur oxidation mechanisms (14). Typically, high sulfate levels during haze events in China occurred with concurrently elevated RH, NO x , and NH 3 (24,25), implicating an aqueous sulfur oxidation pathway. However, elucidation of the sulfur oxidation mechanisms from available atmospheric measurements remains challenging, particularly under polluted conditions because of multiple highly coemitted primary gaseous pollutants (1,21).…”

Intermittent turbulence contributes to vertical diffusion of PM&lt;sub&gt;2.5&lt;/sub&gt; in the North China Plain

“…3b and 3c for Case-2 verify this conclusion as well. Besides, an ambience with high relative humidity (RH) is favorable for the increase of PM2.5 concentration in the ABL through secondary formation by heterogeneous reactions (Quan et al, 2015;Wang et al, 2012;Faust et al, 2017) and hygroscopic growth (Engelhart et al, 2011;Petters and Kreidenweis, 2008). For Case-1, the RH during the CS keeps high with a mean value of 53% but sharply 25 falls into a very low level once entering the TS.…”