Regional transport is a key source of carbonaceous aerosol in many Chinese megacities including Beijing. The sources of carbonaceous aerosol in urban areas have been studied extensively but are poorly known in upwind background areas. Here, we carried out positive matrix factorization (PMF) analysis of organics from online high resolution-time of flight-aerosol mass spectrometer (HR-TOF-AMS) observations and mass balance analysis of organic and elemental carbon (OC and EC) from radiocarbon (¹⁴C) measurements to quantify the contribution of fossil fuel and non-fossil emissions to carbonaceous aerosols at a rural background site in North China Plain. Coal-combustion related organic aerosol (CCOA+ Oxidized CCOA) and oxygenated organic aerosol (OOA) contribute 48% and 30% of organic matter in mass concentration, respectively. About 65% of the OC and EC are from fossil-fuel combustion. These results support the emission inventory-based study whih modelled the significant contribution of solid fuel consumption, particularly coal, on ambient PM_2.5 levels in NCP. It also provided concrete evidence of the organic aerosols emitted from the residential sector by using single particle analyses. What is new in this study is that we provided quantitative mass-based data on the relative contribution of different sources to OAs.
By combining AMS-PMF and ¹⁴C data, we found that primary and secondary OC from fossil fuel contribute 35% and 22% to total carbon (TC), and coal combustion emission dominates the fossil fuel sources. Biomass burning accounted for 21% of total carbon. Higher concentrations of organic aerosols at the upwind location provide strong evidence that rural areas are important source of aerosols to Beijing. The lag between the peak concentration of AMS-PMF resolved organic aerosols (coal combustion and biomass burning) in Gucheng and Beijing is approximately 1 day, which is consistent with the travel of slow-moving air masses during stagnant conditions. These results confirmed the importance of regional transport of organic aerosols to Beijing from the upwind rural areas.
In summary, combustion of solid fuel such as coal and biomass in rural areas in NCP was a major source of air pollution in winter 2016. They not only cause air pollution in the rural areas but also contribute to heavy haze pollution in urban Beijing. This result suggests that regional pollution controlling strategies, particularly on the residential sector, should be strengthened to manage air quality in megacities. There is already evidence that reducing solid fuel combustion from the residential sector is an effective measure to improve air quality in Beijing. Further transition to cleaner energy in the residential sector will not only improve air quality in surrounding megacities, but also deliver climate benefits, by reducing the emission of short-term climate forcers such as soot and brown carbon.
Our results confirm that solid fuel combustion for heating and cooking was the dominant source of PM₁ during heavy pollution events at the rural background site in winter 2016, highlighting the need for a continuous energy transition from solid fuels to cleaner sources.
 

fossil-fuel emission, carbon-14, AMS-PMF, urban, rural