Air humidity affects secondary aerosol formation in different pathways

Highlights

• Impact of RH and water vapor content on nitrate and sulfate formation was investigated.

• Air humidity affects nitrate formation by both gaseous oxidation and aqueous reaction.

• Lower temperature and higher RH more facilitate SO₄²⁻ formation.

Abstract

Haze pollution characteristics and PM_2.5 chemical composition were distinctive in different air humidity-dependent haze episodes in winter of North China Plain (NCP). The impact of air humidity on particulate chemical composition was investigated based on the in situ observation in winter of 2017–2018 in Tianjin. Relative humidity (RH) and absolute humidity affect the secondary aerosol generation in different ways. Particularly, nitrate changes more obviously with absolute humidity, while sulfate changes more obviously with RH. In the daytime, at certain conditions, high water vapor content, O₃ concentration and stronger solar radiation may promote the gas-phase oxidation of NO_x by the addition of OH formed though O₃ photolysis, especially during the transition periods between winter and autumn or spring. Whereas in the nighttime, temperature drop generated the high RH, which was favorable for the gas-particle portioning of HNO₃ and the occurrence of the N₂O₅ heterogeneous hydrolysis reaction. At lower temperature and higher RH (T < 0 °C, RH > 80%) condition, SO₄²⁻ mass fraction was relatively higher. Lower temperature can result in more SO₂ dissolved in equilibrium and the relatively higher initial aerosol pH, which both generate faster aqueous oxidation rate. Given the currently low SO₂ concentration in the regional scale, the meteorological condition in which the occurrence of sulfate formation through aqueous reaction may be more stringent.

Introduction

During the 2013–2018, the annual PM_2.5 mass concentration in Jing-Jin-Ji region has dropped to 60 μg m⁻³ (2018) from 106 μg m⁻³ (2013) (China Ambient Air Quality Bulletin). Particulate air pollution has been significantly mitigated. However, frequent haze episodes still occurred, especially during winter. Chemically, in Jing-Jin-Ji region, the inorganic ions typically account for 40%–50% of the total PM_2.5 (Zou et al., 2018; Huang et al., 2017; Gao et al., 2018), which are major hygroscopic species in particles, high loading of inorganic ions and high RH can generate strengthen of light extinction (Ding et al., 2019; Zhao et al., 2019).

Measurements have shown that secondary aerosol formation during the haze episodes contributes to the high loading of fine particles (Tie et al., 2017; Huang et al., 2014; Quan et al., 2015; Wen et al., 2015). Amount of studies have demonstrated that air humidity plays a key role in secondary aerosol formation, especially for inorganic species. In the field measurements, RH levels were generally utilized to speculate the possibility of occurrence of aqueous reaction. Simultaneously, elevated RH and inorganic ion mass concentrations during haze episodes resulted in an abundant aerosol liquid water content, the condensed water could serve as an efficient medium for multiphase reactions, thereby promoting the transformation of reactive gaseous pollutants into particles (Wu et al., 2018). However, the water vapor content (i.e absolute humidity) was not necessarily high in a high RH environment, lower temperature could give rise to the high RH due to the water vapor condensation (Fig. S1). Therefore, whether water vapor content and RH have distinguished impacts on secondary aerosol formation is worthy of exploration. Furthermore, the influence of water vapor content on secondary aerosol formation is rarely reported.

The gaseous oxidation and N₂O₅ hydrolysis are two typical pathways for nitrate formation (Seinfeld and Pandis, 2016). In the daytime, the pathway of OH + NO₂ dominates the formation of nitrate, whereas in the nighttime, N₂O₅ heterogeneous hydrolysis is an important source of particulate nitrate (Wang et al., 2017a, Wang et al., 2017b). Particularly, nitrates formation via N₂O₅ heterogeneous uptake was negligible at ground level due to high NO emissions, whereas fast particulate nitrate production via N₂O₅ uptake aloft throughout the night contributes to the surface particulate mass concentration (Wang et al., 2018). Recently, several works have demonstrated that photochemistry was active in winter polluted condition (Fu et al., 2020; Lu et al., 2019; Tan et al., 2018), O₃ and OH productions are sufficiently high to facilitate fast gas-phase and heterogeneous conversion of NO_x to nitrate. In the gaseous oxidation reaction, water vapor could contribute to OH formation after O₃ photolysis. In summer, especially in marine boundary layer, O₃ photolysis had a high contribution to OH production (Stone et al., 2012). However, the contribution of O₃ photolysis to OH production was thought as unimportant in polluted urban atmosphere in winter due to the lower water vapor content and O₃ concentration as well as weakened radiation, whereas HONO photolysis and ozonolysis of alkenes were more significant (Fu et al., 2020; Stone et al., 2012; Tan et al., 2017; Xue et al., 2016). Tianjin is near the Bohai Bay, the water vapor content is relatively higher than surrounding inland cities, moreover, the annual O_3-8h^90th concentration increased by 41% in the last 5 years (http://sthj.tj.gov.cn/). Therefore, the role of water vapor in the formation of OH and the subsequent promotion of NO_x to nitrates is worth exploring.

The consensus for secondary sulfate formation mainly includes gaseous oxidation by SO₂ + OH and SO₂ oxidation in cloud droplets by oxidants, including O₃, H₂O₂, NO₂, and TMI (Fe³⁺ and Mn²⁺) (Seinfeld and Pandis, 2016). The formation rate of sulfate through gaseous oxidation is typically much slower than that of nitrate, the dominate mechanism for sulfate formation is aqueous reaction. The rate of conversion of S(IV) to S(VI) is pH dependent, higher pH could enhance the oxidation rate. Cheng et al. (2016) and Wang et al. (2016) proposed that aqueous oxidation of SO₂ by NO₂ is very important to efficient sulfate formation in NCP and that high concentrations of NH₃ in the atmosphere can significantly enhance the sulfate aqueous phase formation when the aerosols are neutral. However, their work and other similar work with the same view were doubted as the aerosol pH at NCP showed a moderate aerosol acidity (Ding et al., 2019; Liu et al., 2017; Shi et al., 2017; Song et al., 2018). Recent studies about sulfate formation mainly focused on the underestimation of predicted sulfate in air quality models, and try to add missed mechanism in models. Liu et al. (2020) prosed that oxidation of SO₂ by H₂O₂ in the aerosol liquid water can contribute to the missing sulfate source during severe haze episodes. Heterogeneous production of hydroxymethanesulfonate (HMS) by SO₂ and HCHO has also been proposed to contribute to the unexplained sulfate by models (Moch et al., 2018; Song et al., 2019). However, due to the strict emission control of SO₂, sulfate in fine particles has been observed with a significant decrease in recent years, the importance of aqueous chemistry in sulfate formation is highly uncertain (Li et al., 2019).

In this work, in situ observations of aerosol species, meteorological parameters, and gaseous pollutants were conducted, the aims of the work are 1) to compare the pollution characteristics of haze episodes with different air humidity; 2) to explore the impact of air humidity, including RH and water vapor content on nitrate and sulfate formation, separately, which helps to provide a new sight for the exploration of the haze formation.