Secondary organic aerosol (SOA) constitute a large fraction of atmospheric fine particle mass (PM_2.5), with implications for air-quality degradation and climate forcing. An increasing number of studies point toward aqueous chemical reactions occurring in cloud droplets and wet aerosols as an important missing pathway for SOA formation. This family of processes, representing thousands of chemical reactions, products, and intermediates, including chemistry in both atmospheric gas and aqueous phases, is collectively known as aqueous SOA (aqSOA) formation. Because the biogenic precursors (such as plant volatile and biomass burning) are noticed to be more polar and more hydrophilic than fossil precursors (such as coal combustion and vehicle emission), most studies have investigated the formation of aqSOA from biogenic/biomass precursors. The contributions of fossil fuel-derived carbon to the aqSOA are still unrecognized. In this study, we directly measured the compound-specific δ¹³C-Δ¹⁴C isotope fingerprints of aqSOA molecules, and provide ambient isotopic observational evidence for a large aqSOA formation from anthropogenic fossil emissions.

Find aqSOA molecules

Oxalic acid, likely represents the highest oxidation state of organic aerosol (O/C ratios = 2), are among the most abundant SOA components and are key end product in the aqueous-phase photochemical oxidation of various volatile organic compounds (VOCs) in clouds or wet aerosols. Therefore, oxalic acid can serve as a signature compound to trace the aqSOA formation pathways.

In our study, we measured oxalic acid and related polar compounds during two contrasting phases of the East Asia monsoon system, i.e., continental outflow vis-à-vis coastal background air masses. Oxalic acid and its aqueous-phase intermediate product (pyruvic acid and glyoxylic acid) are significantly depleted in δ¹³C compositions during continental outflow compared to the coastal background, which stands for the kinetic isotope effects (KIE) during aqueous-phase chemical pathway. By combining the δ¹³C values and aerosol chemical compositions, we further provided more evidence for the extensive aqueous-phase processing of continental outflow aerosols.

Carbon sources of aqSOA

Biogenically derived carbon is expected to reflect the atmospheric level of ¹⁴C due to the photosynthetic uptake of atmospheric CO₂, and is referred to as radiocarbon “modern”. Conversely, fossil fuel-derived carbon contains no detectable ¹⁴C due to radioactive decay (¹⁴C half-life is 5730 yr). Fossil carbon is labeled as radiocarbon “dead” or “old”. With a known Δ¹⁴C composition of oxalic acid, the proportion of oxalic acid derived from fossil sources and biogenic sources can be estimated.

Contrary to the paradigm that water-soluble organic aerosols are largely biogenic, the compound-specific Δ¹⁴C results indicate that fossil carbon produced 55-70% of the aqSOA molecules (i.e., oxalic acid, glyoxylic acid, and methylglyoxal) in continental outflow samples. In contrast, the fossil oxalic acid only accounts for 30% of total oxalic acid in the coastal background samples. Therefore, the dominance of fossil fuel sources to their aqSOA formation in the continental outflow is obvious. The Δ¹⁴C characterization of aqSOA molecules are extended to five emission hotspot megacities of China (Beijing, Shanghai, Guangzhou, Chengdu, and Wuhan), which further confirm a ubiquitous contribution of aqueous-phase transformation of fossil precursors to organic aerosols.

Atmospheric implications

Our year-round observation witnesses a positive feedback loop among aerosol liquid water (ALW), inorganic particles (e.g., sulfate and nitrate), as well as aqSOA compounds. The inorganic particles induce ALW growth, facilitating the partitioning of gas-phase oxidation products into aqueous-phase medium, and promoting the formation of aqSOA. The formation of aqSOA in the organic aerosol fraction would decrease aerosol viscosity and increase ALW, as in turn promotes further gas-to-liquid transfer of water-soluble organic precursors. We provide ¹⁴C-based evidence that, besides inorganic species, substantial aqSOA compounds are also derived from anthropogenic-fossil sources. Therefore, a broad control of various anthropogenic emissions such as H₂O, SO₂, NO_x, and VOC precursors is vital for reducing the organic particulate pollution.

Nitrate is expected to be the dominant atmospheric hygroscopic particle in China due to the long-term regulatory plans aimed at reducing sulfate emissions, as well as increased emission of nitrate precursors. Furthermore, there has been a rapid translation from coal combustion to natural gas in China. However, natural gas combustion can produce more than three times as much water vapor as coal burning. The enhancing effects of particulate nitrate and combustion-derived water vapor on aqSOA formation should be addressed when changing energy structure in future climate and air quality scenarios.

Atmospheric aqueous-phase processes are expected to increase due to the enhanced evapotranspiration in a warmer world. An absence of accounting for such aqueous processing incurred by fossil precursors could lead to an underestimation of the anthropogenic contribution to organic aerosols. We hope that the new methodology and results would be enlightening to other high fossil-fuel consumption locations such as North America, Europe, and South Asia where the aqueous-phase chemistry pathway has been demonstrated significant.