Unexpected high production of sulfate and SOA after emissions mitigation and potential causes

The wintertime haze episode often occurs with high yields of sulfate and secondary organic aerosols (SOAs) in China, although the air quality improves a lot benefited from the strict implementation of “China Blue Air Actions”. The underestimation of observed high sulfate and SOA by current air quality models is exacerbated with increasing aerosol liquid water content (LWC), suggesting the presence of unidentified aqueous-phase processes involving aerosol water that dominate the formation of secondary aerosols. Several studies have demonstrated significant catalytic role of Element Caron (EC) in the conversion of SO₂ to sulfate and highlighted a substantial contribution of fossil fuel-derived carbon to aqueous-phase SOA. Those findings imply a crucial involvement of fossil fuel combustion in the high production of wintertime sulfate and SOA. However, further evidence is still required to elucidate the underlying chemical processes and driving factors.

What we do to elucidate the underlying chemical processes and key factors?

To comprehensively characterize the chemical components at both bulk and single particle levels, as well as gas precursors, we conducted a winter field study in northern China employing online analytic techniques, including ion chromatography (IC) systems, single particle aerosol mass spectrometer (SPAMS), and differential optical absorption spectroscopy (DOAS). Our investigation identified the single particles containing EC emitted from fuel combustion sources, distinguished between fresh and aged particle types, differentiated primary organic aerosols (POA) derived from fossil fuel combustion versus biomass burning, as well as identified SOAs. Subsequently, we explored the potential role of these characteristic particles and aerosol water in facilitating rapid sulfate formation and SOA production.

Drivers for secondary formation: Aerosol high liquid content and fossil fuel emissions

We revealed two crucial factors for the observed high production of sulfate and SOA, i.e., precursors (including gaseous pollutants, EC, POA, etc.) emitted from fuel combustion, and elevated aerosol water triggered by high humidity. Our findings demonstrate ubiquitous presence of particles containing EC, particularly under haze conditions characterized by higher levels of aged EC particles and SOA (represented by ECOC particles mixed with sulfate and nitrate), highlighting the significant impact of fuel combustion activities on winter haze pollution. 

The rapid production of sulfate at high humidity was attributed to the simultaneous generation of both inorganic sulfate (predominantly) and hydroxymethanesulfonate, which acts as an aqueous SOA but is often misidentified as inorganic sulfate when measured using ion chromatography with alkaline eluent. We propose that the oxidation of dissolved SO₂ by in-particle N(III), which likely generated through the heterogeneous reaction of NO₂ on surfaces of aged EC-containing, as well as the reaction between nitrate and Fe(Ⅱ) promoted by photolysis of Fe-oxalate particles, dominated the rapid formation of sulfate at an aerosol pH range of 4.0–4.5. For SOA formation, we demonstrate that POAs originating from fossil fuel combustion potentially serve as precursors by acting as carriers for condensation and transformation of secondary organic species or reactants for aqueous-phase oxidation under high humidity conditions. In the presence of high LWC, these fossil fuel-derived POAs can readily undergo transformation into SOAs, contributing up to 70% of SOAs at the single particle level when LWC exceeded ~50 μg m⁻³. 

Figure 1. Conceptual diagram depicting sulfate and SOA formation in deliquesced particles. High aerosol water triggered by extremely high RH drives the secondary transformation of primary emissions emitted from the combustion of fuels, particularly fossil fuels.

Implications for atmospheric chemistry and mitigation strategies

This study highlights the significant contribution of fuel combustion, particularly fossil fuel combustion, to the aqueous formation of sulfate and SOA. Fuel combustion releases complex mixtures of gaseous pollutants (i.e., SO₂, NOx, CO, NH₃ and VOCs), primary carbonaceous particles (i.e., EC and POAs), and metal minerals into the atmosphere., These primary emissions can undergo atmospheric chemistry processes, particularly aqueous-phase process under high humidity conditions, and ultimately contributes to or transform into secondary aerosols. Furthermore, it should be noted that the proposed pathways for sulfate formation involving aged EC and Fe-oxalate particles represent potential sources of the observed elevated HONO, which serves as a primary precursor to OH radicals, thereby potentially enhancing atmospheric oxidation capacity and accelerating secondary formation in winter. 

Giving these findings, we emphasize that effective mitigation of secondary aerosol formation in winter haze of northern China relies on reducing primary emissions, including OAs, EC, transition metals, and reactive gases, from the combustion of fuels, particularly that of fossil fuels. These findings are of great significance for winter haze pollution mitigation in areas with solid fuels combusted for heating, as well as in urban areas with high humidity.