Insight into wet scavenging effects on sulfur and nitrogen containing organic compounds in urban Beijing

Winter haze pollution in the North China Plain (NCP), characterized by high submicron aerosol (PM1) concentrations, significantly impacts air quality. Organic aerosols (OA), a major component of PM1 (31–59% during winter haze), remain incompletely understood at the molecular level, especially regarding secondary organic aerosol (SOA) formation (43–78% of OA in Asian sites). SOA forms via atmospheric oxidation of volatile organic compounds (VOCs) and oxidation of primary OA (POA) on wet particles or fog/cloud droplets. Photochemical oxidation contributes significantly to less-oxidized oxygenated OA (LO-OOA), while aqueous-phase processing is important for more-oxidized oxygenated OA (MO-OOA). Studies suggest that LO-OOA can evolve into MO-OOA via photochemical oxidation and aqueous-phase oxidation, emphasizing the need to analyze SOA formation with detailed precursors and meteorological factors. SOCs and NOCs are ubiquitous in OA and serve as important SOA tracers. While some understanding exists regarding SOCs (e.g., glycolic acid sulfate formation from glyoxal uptake on sulfate aerosols) and NOCs (gas-phase NOCs measured by NO3-ToF-CIMS, contributing to OA via gas-to-particle conversion), a detailed molecular-level understanding of their atmospheric formation and evolution remains lacking. Precipitation's wet scavenging effect removes gas and aerosol, impacting SOCs and NOCs formation and evolution.

Extensive research has focused on the chemical composition, sources, formation pathways, and aging processes of PM1 in severe haze episodes in the NCP, particularly in Beijing. Studies using real-time field observations have characterized the chemical composition of PM1, revealing the dominance of organic aerosols. However, the molecular-level chemical composition of OA and its formation and aging processes, especially for SOA, remain unclear. Previous research has identified various pathways for SOA formation, including atmospheric oxidation of VOCs and aqueous-phase processing within wet particles or fog/cloud droplets. The relative importance of photochemical oxidation (for LO-OOA) and aqueous-phase processing (for MO-OOA) in SOA formation has been investigated. Existing literature highlights the significant contribution of SOCs and NOCs to OA mass loading and the need for a better understanding of their formation and evolution mechanisms. Studies have characterized some SOCs, such as glycolic acid sulfate and organosulfates derived from anthropogenic and biogenic sources. For NOCs, gas-phase species have been identified, but information on particulate NOCs remains limited at the molecular level. The lack of molecular-level studies on SOCs and NOCs necessitates this research to better understand atmospheric formation and evolution processes and the influence of wet scavenging.

High-resolution real-time measurements of submicron aerosol species and related gaseous precursors were conducted in downtown Beijing from October 1st to November 12th, 2021. The observation site was located in an area with significant traffic and residential density. Non-refractory PM1 (NR-PM1) was measured using an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-TOF-AMS). VOCs (isoprene, benzene, toluene, naphthalene, and monoterpene) were measured using a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS). Gaseous pollutants (SO2, NO2, NOx, CO, O3, HCHO, and NH3) and meteorological parameters (RH, temperature, wind speed, wind direction, and UV radiation) were also monitored using various instruments. Air mass transport was analyzed using backward trajectories from the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model and Global Data Assimilation System (GDAS). AMS data analysis involved using the high-resolution data analysis packages (PIKA V1.25 G) in Igor Pro to obtain NR-PM1 species and elemental ratios of OA. Source apportionment of OA was performed using PMF (Positive Matrix Factorization) analysis. SOCs and NOCs mass concentrations were estimated using specific equations based on measured fragment ratios. The estimation of aerosol liquid water content (ALWC) involved calculating water associated with inorganic and organic components using the ISORROPIA II thermodynamic model and the k-Köhler theory.

Two wet scavenging events (rain and snow) significantly impacted aerosol composition. Wet scavenging dramatically reduced NR-PM1 (>90% efficiency). SOCs and NOCs also exhibited high scavenging efficiencies (SOCs: 99%; NOCs: 85–95%). OA composition transitioned from SOA-dominated (aq-OOA and OOA) before precipitation to POA-dominated (COA and HOA) afterward, due to efficient removal of inorganic components and a decrease in favorable meteorological conditions for SOA formation. Before precipitation, SOCs had a higher proportion than NOCs in OA. However, the opposite was observed after precipitation, suggesting an altered dominance. This change is attributed to the higher scavenging rates of SOCs compared to NOCs and favorable conditions (UV radiation, NOx, and O3) for NOCs generation after precipitation. Analysis of SOCs indicated glycolic acid sulfate as the dominant species, formed via aqueous-phase processing, particularly before rain and snow. Aromatic- and monoterpene-derived SOCs were also present before precipitation, formed photochemically. For NOCs, highly oxidized amides and amino acids were dominant, primarily generated photochemically, both before and after precipitation. Analysis of correlations between SOCs/NOCs and related chemical species confirmed the importance of aqueous-phase processing (for SOCs) and photochemical pathways (for both SOCs and NOCs). The relative importance of different pathways varied before and after the precipitation events depending on the availability of precursors and related meteorological parameters.

The study's findings demonstrate the significant impact of wet scavenging on the composition and evolution of SOCs and NOCs in urban Beijing's OA. The shift from SOCs to NOCs dominance after precipitation highlights the importance of considering wet scavenging processes in atmospheric models. The identification of key formation pathways for SOCs (aqueous-phase for glycolic acid sulfate, photochemical for aromatic and monoterpene derivatives) and NOCs (photochemical for highly oxidized amides and amino acids) improves our understanding of OA evolution. The results emphasize the interactive role of aqueous-phase and photochemical processes in influencing the composition and abundance of SOCs and NOCs in the atmosphere. These findings have implications for air quality assessments and the development of more accurate atmospheric models that incorporate the complex interplay between wet scavenging and chemical processes.

This study provides a comprehensive insight into the impact of wet scavenging on SOCs and NOCs in urban Beijing's OA. The distinct formation pathways of these compounds, influenced by both aqueous-phase and photochemical processes, and their contrasting removal efficiencies during precipitation events, significantly alter OA composition. Future research should focus on exploring the long-term effects of wet scavenging on atmospheric aerosol composition and the development of more sophisticated atmospheric models that incorporate these findings.

The study's findings are based on observations from a single site in downtown Beijing during a specific period. The generalizability of these findings to other locations or seasons requires further investigation. The estimation of SOCs and NOCs concentrations relies on specific assumptions and calculation methods, which might introduce uncertainties. The complex nature of atmospheric processes may involve other factors not considered fully in this study.