Large contribution of in-cloud production of secondary organic aerosol from biomass burning emissions

Organic aerosols (OA) constitute a significant portion of submicron aerosols and are crucial for understanding radiative climate forcing and air quality. OA comprises primary organic aerosol (POA), directly emitted, and secondary organic aerosol (SOA), formed from volatile organic compounds (VOCs). SOA accounts for a substantial portion (up to 76%) of ambient OA. While gas-phase SOA formation is well-studied, aqueous-phase SOA (aqSOA) formation via in-cloud chemistry has received less attention. Water-soluble organic gases (WSOGs) react in aqueous cloud environments to produce aqSOA. High aqSOA concentrations have been observed during haze and fog events, though the specific contribution of biomass burning (BB) emissions remains unclear. BB emissions, a major source of non-methane organic gases (NMOGs), contain numerous compounds that could contribute to aqSOA formation. Previous studies have limitations in simulating in-cloud aqSOA formation due to short cloud lifetimes in laboratory cloud chambers and overestimation of liquid water content (LWC) in bulk solution experiments. Further, studies primarily focus on individual compounds, rather than complex mixtures from real-world BB emissions. This study aimed to overcome these limitations using a WFR to simulate aqSOA formation from residential wood burning emissions under atmospherically relevant conditions.

Existing literature highlights the significant contribution of SOA to atmospheric aerosols and the need for better understanding of its formation pathways. While gas-phase oxidation of VOCs is well-established, the role of aqueous-phase chemistry in SOA formation, especially concerning biomass burning emissions, remains under-researched. Studies have shown the presence of high aqSOA concentrations during haze and fog events, linking it to biomass burning, but detailed mechanisms and quantifications are lacking. Prior work often focused on individual water-soluble compounds like levoglucosan and guaiacol, omitting the complexity of real-world biomass burning emissions. Technological constraints, such as short cloud lifetimes in laboratory settings and unrealistic LWC in bulk experiments, hampered comprehensive investigations. This study addresses these gaps by employing a novel experimental setup to better simulate atmospheric conditions and investigate aqSOA formation from a complex mixture of compounds.

The study utilized a wetted-wall flow reactor (WFR) coupled with a holding tank to simulate in-cloud aqSOA production from residential wood burning emissions. The WFR, previously used for studying isoprene cloud chemistry, maintained approximately 100% relative humidity (RH) while introducing organic gases from residential wood burning, OH radicals, and water vapor under kinetic conditions. The gas-phase chemical composition was comprehensively characterized using a suite of mass spectrometers, including PTR-MS, Vocus PTR-TOF-MS, and Dual-EESI. Water solubility of biomass burning vapors was determined by measuring their uptake into the water layer. Compounds formed in the WFR were nebulized to quantify the yield, bulk chemical composition, and molecular composition of the resulting aqSOA. An oxidation flow reactor (OFR) was used to produce gasSOA from the same emissions for comparison. The experimental setup consisted of a modern wood stove combusting pine and spruce wood logs. Emissions were sampled, diluted, and injected into a holding tank for consistent input to the WFR. CO, CO2, THC, NOx, and particle concentrations were monitored. Experiments included a pure uptake experiment (no OH radicals) and oxidation experiments (1–9 h) with OH radicals generated by a Xe-excimer laser. Aqueous solutions were analyzed by AMS and EESI-TOF-MS to characterize the aqSOA. Henry's law constants were estimated using EPI freeware, and a kinetic model (QEMRA) was used to describe partitioning of molecules with different solubilities. The aqSOA yield was calculated as the ratio of integrated aqSOA mass to the integrated mass of all NMOGs.

The molecular composition of gaseous organic compounds from residential wood burning was characterized using PTR-MS, Vocus PTR-TOF-MS, and Dual-EESI, identifying 680 elemental formulae. The CHO group was dominant (87.4% of the total mixing ratio). Water solubility analysis revealed that 1% of organic compounds were fully water-soluble, 19% moderately soluble in thick clouds, and 80% insoluble. The aqSOA exhibited a high oxygen-to-carbon (O/C) ratio (0.9 ± 0.1), consistent with field observations. Detailed molecular information from Particle-EESI showed an increase in the number of detected ions with oxidation time, with the CHO class dominating (80%). Aqueous-phase oxidation products of phenols were detected, including oligomers. Comparison of aqSOA and dissolved gases showed that aqSOA was significantly more oxygenated and contained larger carbon-containing species, indicating oligomerization played a crucial role. The aqSOA yield was 20% (after 9 h of aqueous-phase processing), substantially higher than the gas-phase SOA yield (0.17). The aqSOA yield decreased with processing time, likely due to the solubility of NMOGs. Comparison with gasSOA from the OFR indicated that aqSOA contains more highly oxygenated and carbon-containing species due to extensive oxidation and oligomerization in the aqueous phase.

The findings demonstrate the significant contribution of in-cloud chemistry to SOA formation from biomass burning, exceeding the contribution of gas-phase oxidation. The high aqSOA yield (20-43%) suggests that aqueous processing of biomass burning vapors is a major pathway for SOA formation, especially under conditions of high RH. This has important implications for climate forcing and air quality models, which currently underestimate the contribution of aqSOA. The study's focus on residential wood burning suggests that similar processes likely occur in wildfire plumes, with further research needed to quantify the contribution of wildfires to aqSOA. While the study used a higher LWC than typical in clouds, the non-equilibrium system provides valuable insights into atmospheric processes. The high O/C ratio and the prevalence of oligomers in aqSOA highlight the significant role of aqueous-phase oxidation and oligomerization in the formation of highly oxygenated and low volatility products. The comparison with gasSOA formed through gas-phase oxidation further reinforces the importance of considering aqueous-phase chemistry in predicting SOA formation.

This study demonstrates the substantial contribution of in-cloud aqueous-phase chemistry to secondary organic aerosol formation from biomass burning emissions, exceeding that of gas-phase oxidation. The high aqSOA yield highlights the need for improved representation of aqSOA formation in climate and air quality models. Future research should focus on expanding this work to include other biomass burning sources (wildfires), explore the effects of varying environmental conditions (pH, NOx, temperature, UVB light), and improve understanding of the solubility and reactivity of oxidized biomass smoke vapors. This research represents a crucial step towards a more comprehensive understanding of SOA formation and its impact on climate and air quality.

The study used a higher liquid water content (LWC) in the WFR compared to real-world clouds. While the non-equilibrium system provides valuable insights, it might overestimate the aqSOA formation to some extent. The study focused primarily on residential wood burning; the results might not be directly generalizable to other biomass burning sources like wildfires, which have different emission profiles and chemical compositions. The nebulization process used to analyze aqSOA might have introduced artifacts, affecting the observed oligomer distribution. Further research is needed to assess the potential formation of oligomers during the aerosol drying process. The study did not account for the influence of other atmospheric constituents such as NOx or UVB light, which could alter the oxidation pathways and aqSOA yields.