Significant role of biomass burning in heavy haze formation in a megacity: Molecular-level insights from intensive PM_2.5 sampling on winter hazy days

Abstract. Reports on molecular-level characterization of primary and secondary constituents in PM_2.5 at high-time resolution are limited to date, especially during haze events. The study explored molecular composition and source contributions of PM_2.5 with comprehensive analytical methods by conducting intensive sampling at roughly 2-hour intervals during hazy days in winter. Results show that organic matters were the predominant species, followed by NO₃^-. Biomass burning (BB) was the biggest contributor to organic carbon (OC), both in concentration and in proportion. Radiocarbon analysis of carbonaceous fractions reflects that fossil fuels dominate water-soluble organic carbon (WSOC) (61–82 %) likely resulting from increased coal combustion for residential cooking and heating and the coal-fired industry in cold times. Interestingly, the contribution of non-fossils instead of fossil fuels to WSOC enhanced with aggravating haze pollution, coinciding with significantly intensified BB during that time. Other non-fossil sources, including fungal spores and plant debris, showed a larger contribution to OC in light haze episodes. For secondary sources, naphthalene-derived secondary organic carbon (SOC) contributed more to OC in PM_2.5 (0.27–2.46 %) compared to biogenic emissions (0.05–1.10 %), suggesting fossil fuels may dominate SOC formation during urban haze events. SOC declined with rising haze pollution and presented high levels on days with high temperature and low relative humidity due to elevated photooxidation. Additionally, BB can raise secondary formation as well as the emissions of other sources, as demonstrated by the significant relationships between BB tracers and many other source tracers. These findings illustrate that BB likely plays a significant role in the heavy winter haze.