Strong Impacts of Legitimate Open Burning on Brown Carbon Aerosol in Northeast China

The massive agricultural sector in Northeast China results in a huge amount of crop residues, which have long been disposed mainly through “illegal” open burning. A transition of local policy occurred in 2018, with a window of approximately 3 months issued for agricultural fires. Here, we investigated the responses of brown carbon (BrC) to the new policy. Persistently high levels of BrC mass (estimated as methanol-soluble organic carbon) and absorption coefficient were observed during the “legitimate burning” period. Agricultural fires, the major driver responsible for the elevated BrC levels, were found to be distinctive with relatively low combustion efficiencies and resulted in BrC with less light absorptivity, compared to residential burning of crop residues. The agricultural fire emissions also led to overestimation of elemental carbon (EC) mass, by a factor of up to 1.6. This in turn resulted in substantial underestimation off_BrC/EC, the fraction of solar energy absorbed by BrC relative to EC. This study indicates that the new open burning policy could not be considered successful with respect to either air quality improvement or climate change mitigation.