Atmospheric gaseous hydrochloric and hydrobromic acid in urban Beijing, China: detection, source identification and potential atmospheric impacts

Abstract

Abstract. Gaseous hydrochloric (HCl) and hydrobromic acid (HBr) are vital halogen species that play essential roles in tropospheric physicochemical processes. Yet, the majority of the current studies on these halogen species were conducted in marine or coastal areas. Detection and source identification of HCl and HBr in inland urban areas remain scarce, thus limiting the full understanding of halogen chemistry and potential atmospheric impacts in the environments with limited influence from the marine sources. Here, both gaseous HCl and HBr were concurrently measured in urban Beijing, China, during winter and early spring of 2019. We observed significant HCl and HBr concentrations ranging from a minimum value at 1 × 108 molecules cm−3 (4 ppt) and 4 × 107 molecules cm−3 (1 ppt) up to 6 × 109 molecules cm−3 (222 ppt) and 1 × 109 molecules cm−3 (37 ppt), respectively. The HCl and HBr concentrations are enhanced along with the increase of atmospheric temperature, UVB and levels of gaseous HNO3. Based on the air mass analysis and high correlations of HCl and HBr with the burning indicators (HCN and HCNO), gaseous HCl and HBr are found to be related to anthropogenic burning aerosols. The gas–particle partitioning may also play a dominant role in the elevated daytime HCl and HBr. During the daytime, the reactions of HCl and HBr with OH radicals lead to significant production of atomic Cl and Br, up to 2 × 104 molecules cm−3 s−1 and 8 × 104 molecules cm−3 s−1, respectively. The production rate of atomic Br (via HBr + OH) is 2–3 times higher than that of atomic Cl (via HCl + OH), highlighting the potential importance of bromine chemistry in the urban area. On polluted days, the production rates of atomic Cl and Br are faster than those on clean days. Furthermore, our observations of elevated HCl and HBr may suggest an important recycling pathway of halogen species in inland megacities and may provide a plausible explanation for the widespread halogen chemistry, which could affect the atmospheric oxidation in China.

Halogens Enhance Haze Pollution in China

“…As to Br y , BrCl is also the most abundant species which contributes ∼60%, whereas HOBr and Br 2 take up ∼20%. Our simulated halogen species are in line with the available observational reports of reactive bromine species in China by Peng et al and Fan et al and also within the ranges in previous studies of chlorine species in China. ,, …”

“…In the past few years, observational studies have reported elevated levels of halogen species in China. The majority of these reports focused on ClNO 2 − and a few on Cl 2 , and bromine species (BrCl, HOBr, Br 2 , and HBr). , A few modeling studies evaluated the impacts of chlorine chemistry on O 3 and PM 2.5 . , However, the overall impact of anthropogenic halogens (particularly bromine) on aerosols and haze pollution in China remains poorly understood.…”

Midlatitude Ozone Depletion and Air Quality Impacts from Industrial Halogen Emissions in the Great Salt Lake Basin

“…There, reaction with NO x and VOCs plays a more substantial role, as in the following simplified mechanism XO + NO 2 + normalM → XONO 2 + normalM XONO 2 + normalH 2 normalO ( aq ) → HOX or X 2 + HNO 3 normalX + RH → O 2 RO 2 + HX where X represents a halogen atom. One prevalent mechanism in the midlatitudes is the conversion of N 2 O 5 to ClNO 2 on the surface of chloride-containing aerosol. − Transport of halogen reservoirs such as ClNO 2 , which may also be formed by the reaction of Cl radicals with NO 2 , has implications for the downwind oxidizing capacity of the troposphere, , and several recent studies have demonstrated the impact of gas-phase halogen species on urban air quality − via reactions with NO x and VOCs. Halogens also convert elemental mercury to oxidized mercury , and dimethyl sulfide to sulfate .…”

Isoprene–Chlorine Oxidation in the Presence of NOx and Implications for Urban Atmospheric Chemistry

“…In addition to oxidation chemistry driven by ozone (O 3 ), hydroxyl radicals (OH), and nitrate radicals (NO 3 ), chlorine radical (Cl) chemistry has been shown to substantially contribute to the atmospheric degradation of anthropogenic emissions, , inorganic aerosol formation, and secondary organic aerosol formation (SOA) . These impacts have been the strongest during wintertime in polluted urban environments, ,− likely due to increased combustion activities, increased biomass burning, and lowered boundary layer heights contributing to increased concentrations. Under typical ambient conditions, Cl concentrations are thought to range from 10 2 to 10 5 molecules cm –3 , one or several orders of magnitude lower than concentrations of the OH radical, − 10 6 molecules cm –3 .…”