Abstract
Abstract. NH3 is the most important alkaline gas in the atmosphere and one of the key species affecting the behaviors of atmospheric aerosols. However, the impact of NH3 on secondary organic aerosol (SOA) formation remains poorly understood, especially the dynamic evolution of chemical compositions in the SOA formation process. In this study, a series of chamber experiments were performed to probe the individual and common effects of NH3 and NOx on toluene SOA formation through OH photooxidation. The chemical compositions of toluene SOA were characterized using the Aerodyne high-resolution time-of-flight aerosol mass spectrometer (AMS). The SOA yield increased from 28.1 % in the absence of NH3 to 34.7 % in the presence of NH3 but decreased to 19.5 % in the presence of NOx. However, the highest SOA yield of 42.7 % and the lowest carbon oxidation state (OSC) occurred in the presence of both NH3 and NOx, indicating that the higher-volatility products that formed in the presence of NOx could partition into the particle phase when NH3 was added. This resulted in a synergetic effect on SOA formation when NH3 and NOx co-existed. The heterogeneous reaction was the main pathway by which NH3 participated in SOA formation in the photooxidation process. The synergetic effect of NH3 and NOx was also observed in SOA optical absorption. A peak at 280 nm, which is characteristic of organonitrogen imidazole compounds, was observed in the presence of NH3, and its intensity increased when NOx was added into the chamber. This work improves our understanding of how the synergistic interactions between NH3 and NOx influence SOA formation and offers new insights into mitigating haze pollution.
Highlights
Secondary organic aerosols (SOAs) are an important component of atmospheric particulate matter (Moise et al, 2015; Liu et al, 2017) and can significantly affect atmospheric visibility, air quality, and human health (Paciga et al, 2014; Yang et al, 2016; Liu et al, 2017)
We present the results of a study in which we characterized the mass concentrations, chemical compositions, and optical properties of secondary organic aerosol (SOA) formed from the photooxidation of toluene under different NH3 and NOx concentrations
When compared with the control experiment, the SOA mass concentration data showed that the formation of toluene-derived SOA was enhanced in the presence of NH3, through acid– base reactions between carboxyl groups or Maillard reactions with carbonyls, but inhibited in the presence of NOx
Summary
Secondary organic aerosols (SOAs) are an important component of atmospheric particulate matter (Moise et al, 2015; Liu et al, 2017) and can significantly affect atmospheric visibility, air quality, and human health (Paciga et al, 2014; Yang et al, 2016; Liu et al, 2017). Because of the complexity of their chemical components, oxidation processes, and environmental factors, SOA formation mechanisms are very complex, and the current understanding of SOA formation is incomplete. This limited understanding hampers the ability of models to predict the magnitudes, dynamics, and distributions of atmospheric aerosols from particulate and precursor emissions (Ortiz-Montalvo et al, 2014). Our understanding of SOA formation mechanisms has been constantly improving, there is still a gap between the simulated SOA concentration in large-scale atmospheric models and field observations (Volkamer et al, 2006; Yang et al, 2018).
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