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As brown carbon (BrC) consists of a collection of light-absorbing organic compounds, there is unlikely an individual substance that can be used as a proxy of all BrC constituents. Hence, available methods for distinguishing BrC from black carbon (BC) are often flawed. Moreover, the use of household solid fuels for winter rural heating is a common practice in northern China that significantly contributes to air pollution; however, it is unclear whether the intensive consumption of solid fuels during the heating season influences the yearly BrC emissions and the seasonal features of ambient BrC in northern China, which is a concern for future BrC control. Thus, a reliable method is necessary for determining the concentrations and optical properties of ambient BrC in different seasons. In this study, four typical months were selected to represent the four seasons of the year in Beijing. Daily atmospheric samples were collected using quartz filters for the analysis of BrC and BC. The integrating sphere (IS) approach was used for the first time to separate the light absorption of BrC from that of BC in ambient PM 2.5. We found that the yearly average abundance of BrC was 0.82 ± 0.44 μg m−3, peaking at 2.31 ± 0.44 μg m−3 in winter, followed by autumn, spring, and summer. Meanwhile, the solar energy absorption by BrC relative to that by BrC + BC across 350–850 nm (F BrC) was the highest in winter (21.78 ± 3.56%), followed by autumn (11.53 ± 5.29%), spring (6.38 ± 4.06%), and summer (4.26 ± 2.57%). The seasonal variations in BrC abundance and absorption share were consistent with the use of household heating practices (highest in winter and minor uses in late autumn and early spring), with biomass and/or coal being the dominant energy types. As low-efficiency burning of solid biomass fuels and fossil fuels is considered an important source of BrC, the aforementioned correlation suggests that rural residential heating plays a dominant role in controlling the seasonal features of BrC and should therefore be a main focus for BrC control in northern China. Hence, the ongoing pursuit of clean heating (switching from coal to gas or electricity) in northern China will help to mitigate the climate-warming effects of both BrC and BC and offset some of the adverse impacts of clean air efforts on climate. Our findings also have wider implications for pollution mitigation in other global regions that use solid fuels. • Brown and black carbon of Beijing PM 2.5 samples optically separated by integrating sphere method. • The seasonality of BrC parameters suggests taking household solid fuels as control focus. • The results also senseful even in other regions of the world using solid fuels. [ABSTRACT FROM AUTHOR]

The mass absorption efficiency (MAE) and mass scattering efficiency (MSE) of aerosols are critical to their light extinction capacity. In winter in northern China, episodes of clear air and heavy haze alternately occur due to the high primary emission and secondary formation of aerosols, coupled with changing weather conditions. These occurrences facilitate detailed investigation of how significant changes in the pollution level impact the MAE and MSE, they also enable the determination of whether current air quality improvement measures are also beneficial to the mitigation of climate change, to avoid climatic side effects of attempts to clean the air. In the present study, a one-month observation campaign was conducted in Beijing, China, during the 2016–2017 winter season. A photoacoustic extinctiometer, which is an in situ measurement instrument, was used to investigate the aerosol absorption coefficient (σ a) and scattering coefficient (σ s) at an infra-red wavelength of 870 nm. The MAE and MSE were found to vary differently with increasing pollution level. A positive correlation between the particulate matter concentration (PM 2.5) and the MAE was observed for PM 2.5 < 100 μg m−3, with the increase in the MAE declining for PM 2.5 = 100–200 μg m−3, and the MAE subsequently stabilizing. In contrast, the MSE continuously increased with increasing PM 2.5. The single scattering albedo (SSA), which governs the climatic effect of aerosols, was observed to be a function of both σ a and σ s , increasing from 0.827 ± 0.018 (average ± standard deviation) for a bottom particulate matter concentration group of (0, 25] μg m−3 to 0.924 ± 0.007 for a top particulate matter concentration group of (300, 400] μg m−3, indicating a positive correlation between the SSA and PM 2.5. These observations suggest a risk of current clean air measures weakening or eliminating the cooling effect of ambient aerosols in northern China. It seems that no previous study has involved the simultaneous acquisition of σ a and σ s at the same wavelength with the purpose of determining how increasing air pollution differently impacts the MAE and MSE. Practical measures that can be used to decrease the black carbon-to-PM 2.5 ratio include phasing out or reducing black carbon-rich sources and promoting technologies that decrease the black carbon-to-organic carbon ratio. Image 1 • MAE and MSE were found to show different responses to escalating pollution levels. • A risk of weakened cooling effect of ambient aerosol with the clean air process. • Practical solution was proposed to against the risks in China's clean air process. [ABSTRACT FROM AUTHOR]