Enhanced biomass burning as a source of aerosol ammonium over cities in central China in autumn

Atmospheric ambient gaseous ammonia (NH₃), the most abundant alkaline gas, affects public health and climate change through its key role in the formation of secondary aerosols via reactions with acidic gases. Estimation of the contributions of ammonia sources is very challenging in the urban atmosphere. Stable nitrogen isotope ratio (δ¹⁵N) measurements have shown that urban aerosol NH₄⁺ and gaseous NH₃ are derived from fossil fuel combustion-related (FF) sources, such as coal combustion, NH₃ slip, and vehicle exhaust, and volatilization-related sources, such as agriculture and urban water volatilization. Biomass burning (BB) sources, especially residential biofuel, can produce vast quantities of NH₃ and other pollutants and may greatly influence air quality and contribute to increased urban NH₃ emissions. In the present study, we continually collected PM_2.5 samples at three urban sites in Central China during autumn and analyzed the major water-soluble ions and δ¹⁵N values of aerosol NH₄⁺. The concentrations of NH₄⁺ increased as the temperature decreased close to winter, whereas the δ¹⁵N values did not show this pattern. According to the Bayesian model after isotope fractionation correction, FF sources contributed to 56.4 ± 17.1%, 46.4 ± 18.2%, and 51.8 ± 14.9% of aerosol NH₄⁺ in Nanchang, Wuhan, and Changsha, respectively, throughout autumn. The contributions from BB sources were 34.5 ± 20.4%, 46.4 ± 21.4%, and 40.4 ± 17.4% for Nanchang, Wuhan, and Changsha, respectively. We also found the fraction of aerosol NH₄⁺ from BB increased in all three cities from September to November 2017, which was likely caused by increased heating demands with the decrease in temperature during the season. Furthermore, BB was responsible for a severe haze event (maximum PM_2.5 of 205.69 μg/m³) in Nanchang. These findings suggest government controls to improve air quality should include BB sources in addition to FF sources.