Observations and Modeling of Gaseous Nitrated Phenols in Urban Beijing: Insights From Seasonal Comparison and Budget Analysis

Abstract

Nitrated phenols (NPs) are critical components of brown carbon with climate effects and threaten human health and ecosystems. Early studies mainly focus on particulate NPs, while the sources and impacts of gas-phase NPs remain less known. This study observed 17 kinds of gaseous NPs in urban Beijing during winter and summer by nitrate-based long-time-of-flight chemical ionization mass spectrometer (NO3−-LToF-CIMS). NPs have an apparent seasonal variation and various diurnal patterns among different species. Source apportionment indicates that secondary productions are the major sources of NPs, besides minor contributions from coal burning and regional industrial emissions. As shown by the observation and box modeling, NO3-initiated nocturnal oxidation and OH-driven photochemistry are the secondary sources of NPs. In both seasons, photolysis constitutes the dominating removal pathway of NPs. We determined the observation-constrained photolysis frequency of typical NPs (i.e., C6H5NO3, C7H7NO3, C6H4N2O5, and C7H6N2O5), ranging from 1.75% to 7.5% of jNO2. Budget analysis reveals that benzaldehyde produced by styrene oxidation is a critical but overlooked precursor of C6H5NO3, and xylene is an essential precursor for C7H7NO3. Modeling results further show that the photolysis of total NPs produces significant amounts of HONO in summer, making noticeable contributions to atmospheric oxidation capacity. This study contributes to a better understanding of the budget of gaseous NPs in the urban atmosphere and provides insights into mitigating NPs-induced secondary pollution.