Enhanced sulfate formation during China's severe winter haze episode in January 2013 missing from current models

Abstract

A regional haze with daily PM 2.5 (fine particulate matters with diameters less than 2.5 μm)exceeding 500 μg/m 3 lasted for several days in January 2013 over North China, offering an opportunity to evaluate models. Observations show that inorganic aerosols (sulfate, nitrate, and ammonium) are the largest contributor to PM 2.5 during the haze period, while sulfate shows the largest enhancement ratio of 5.4 from the clean to haze period. The nested-grid GEOS-Chem model reproduces the distribution of PM 2.5 and simulates up to 364 μg/m 3 of daily maximum PM 2.5 . Yet on average, the model is a factor of 3 and 4 lower in PM 2.5 and fails to capture the large sulfate enhancement from the clean to haze period. A doubling of SO 2 emissions over North China, along with daily meteorology corrections, would be required to reconcile model results with surface SO 2 observations, but it is not sufficient to explain the model discrepancy in sulfate. Heterogeneous uptake of SO 2 on deliquesced aerosols is proposed as an additional source of sulfate under high-relative humidity conditions during the haze period. Parameterizing this process in the model improves the simulated spatial distribution and results in a 70% increase of sulfate enhancement ratio and a 120% increase in sulfate fraction in PM 2.5 . Combined adjustments in emissions, meteorology, and sulfate chemistry lead to higher sulfate by a factor of 3 and 50% higher PM 2.5 , significantly reducing the model's low bias during the haze.

Persistent sulfate formation from London Fog to Chinese haze

“…The γ-values derived from the Beijing measurements are (2.1 ± 1.6) × 10 −5 and (4.5 ± 1.1) × 10 −5 during the transition (41% RH) and polluted (56% RH) periods, respectively, compared with (8.3 ± 5.7) × 10 −5 at 65% RH and (3.9 ± 1.2) × 10 −4 at 70% RH derived from the laboratory measurements. Hence, our laboratory experiments reproduce the rapid sulfate production measured under polluted ambient conditions, and these kinetic data are applicable for quantifying sulfate formation in atmospheric models (1,13,14).…”

“…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.…”

“…Atmospheric sulfur chemistry has remained an open problem (1,13,14). The formation of the 1952 London "Killer" Fog is still mysterious in terms of the detailed chemical mechanism for SO 2 conversion to sulfate (1,45).…”

“…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.…”

Isotopic constraints on heterogeneous sulfate production in Beijing haze

“…However, due to the generally low solar radiation and cloud liquid water content during haze (Zheng et al, 2015a;Wang et al, 2014), conventional sulfate formation via OH oxidation in the gas phase and from aqueous-phase SO 2 (referred to as S(IV) = SO 2 q H 2 O + HSO (McArdle and Hoffmann, 1983), O 3 (Hoffmann and Calvert, 1985), and O 2 via a radical chain mechanism initiated by transition metal ions (TMIs) in clouds (Ibusuki and Takeuchi, 1987;Alexander et al, 2009;Harris et al, 2013) cannot explain the observed high sulfate concentrations (Zheng et al, 2015a). To explain the observed high sulfate concentrations during haze in Beijing and the NCP, recent studies have suggested that heterogeneous reactions on/in aerosols/aerosol water are potentially important Hung and Hoffmann, 2015;Cheng et al, 2016;Wang et al, 2016Wang et al, , 2014Zheng et al, 2015a, b). In particular, Zheng et al (2015a) largely improved the underestimate of modeled sulfate concentrations in 2013 Beijing haze by using a relative-humidity-dependent uptake coefficient (γ ) of SO 2 on aerosols, without knowing the specific mechanisms of heterogeneous oxidation of SO 2 .…”