Vapor isotopic evidence for the worsening of winter air quality by anthropogenic combustion-derived water

Abstract

Anthropogenic combustion-derived water (CDW) may accumulate in an airshed due to stagnant air, which may further enhance the formation of secondary aerosols and worsen air quality. Here we collected three-winter-season, hourly resolution, water-vapor stable H and O isotope compositions together with atmospheric physical and chemical data from the city of Xi’an, located in the Guanzhong Basin (GZB) in northwestern China, to elucidate the role of CDW in particulate pollution. Based on our experimentally determined water vapor isotope composition of the CDW for individual and weighted fuels in the basin, we found that CDW constitutes 6.2% of the atmospheric moisture on average and its fraction is positively correlated with [PM2.5] (concentration of particulate matter with an aerodynamic diameter less than 2.5 μm) as well as relative humidity during the periods of rising [PM2.5]. Our modeling results showed that CDW added additional average 4.6 μg m−3 PM2.5 during severely polluted conditions in the GZB, which corresponded to an average 5.1% of local anthropogenic [PM2.5] (average at ∼91.0 μg m−3). Our result is consistent with the proposed positive feedback between the relative humidity and a moisture sensitive air-pollution condition, alerting to the nontrivial role of CDW when considering change of energy structure such as a massive coal-to-gas switch in household heating in winter.

The Political Regulation Wave

“…Researchers and practitioners are gradually gaining new knowledge about contributors to PM 2.5 . For instance, a recent study finds that water vapor from combustion can enhance the formation of secondary aerosols (Xing et al 2020). At a State Council meeting led by Premier Li Keqiang in September 2020, experts also identified the overuse of ammonia in treating NO x emissions as a cause for haze.…”

Reply to Hopke and Dai: The correlation between PM 2.5 and combustion-derived water is unlikely driven by local residential coal combustion

“…To sum up our simulation result of the heavy haze episode, we found that the primary and secondary sulfate contributed 3.1% and 9.4%, respectively, to the total PM 2.5 in Xi'an (2). There is an additional 42.0% of the PM 2.5 coming from nitrate, ammonium, and SOA (2). Many of the secondary PM 2.5 formations are known to be sensitive to relative humidity (9).…”

“…When measuring atmospheric water vapor isotope composition in our field campaign both industrial and residential contributions of CDW were included. It should be mentioned that Xing et al (2) do not attribute the cause of the observed correlation to the CDW fraction or any individual factors. In fact, we highlight the potential positive feedback between haze events and the increase of CDW fraction in air moisture.…”

“…Hopke and Dai (1) propose that the observed correlation between PM 2.5 (concentration of particulate matter with an aerodynamic diameter ≤2.5 μm) concentration and the fraction of anthropogenic combustion-derived water (CDW) by Xing et al (2) is likely due to local residential coal combustion (RCC). Here is our response.…”

“…This statement is incorrect for the Guanzhong Basin and the city of Xi'an during 2016 to 2018 heating seasons. Primary sulfate contributions, both industrial and residential, have been considered in our WRF-Chem model (3), and the average secondary sulfate accounts for 74.5% of the total sulfate during the simulated episode in Xi'an (2). This percentage is consistent with the estimated 37% primary sulfate over the total sulfate PM 2.5 in Huang et al ( 4) considering that RCC has had a dramatic reduction over the last decade in Shaanxi Province (5).…”