Constraints on the Reactivity and Components of Nocturnal Nitrogen Oxides
MetadataShow full item record
NO and NO2 (NOx) are fundamentally important species to tropospheric chemistry. NOx abundances are tied to ozone production and thus determine the oxidizing capacity of the troposphere. Nocturnal reactions of NOx are often considered a major loss pathway for NOx and ozone. Recent measurements have shown that nitryl chloride (ClNO2) is produced at night by reactions of dinitrogen pentoxide (N2O5) on chloride containing particles. ClNO2 is photolyzed during the morning hours after sunrise to liberate highly reactive chlorine atoms. This chemistry takes place primarily in polluted environments where the concentrations of N2O5 precursors, NOx, and ozone, are high, though it can likely occur in remote regions at lower intensities. The following describes estimates and ambient measurements of the reactive processes central to ClNO2 formation and field measurements illustrating the potential importance of ClNO2 as a NOx reservoir and as a chlorine atom source. The nocturnal reactions of N2O5 to form ClNO2 were traditionally thought of as marine phenomena given the more obvious source of particle-phase chloride offered by sea spray emissions. However, long term chemical measurement databases and aerosol thermodynamic models are employed to show that this chemistry is likely widespread as is suggested by recent field measurements of ClNO2 in Boulder, CO, a site far removed from local sea salt aerosol sources. Direct measurements of N2O5 reaction probability on ambient aerosol particles were made in La Jolla, CA, using a custom flow reactor alongside measurements of aerosol particle size distributions and non-refractory composition. The largest apparent driver of day-to-day variability in the measured reaction probabilities at this site was the particle nitrate loading. The relative change as a function of particle nitrate illustrates the atmospheric importance of the so-called "nitrate effect" on N2O5 heterogeneous reactions that lead to the formation of ClNO2. The magnitude and sources of chlorine atoms in marine air remain highly uncertain but have potentially important consequences for air quality in polluted coastal regions. Continuous measurements of ambient nitryl chloride and molecular chlorine concentrations were made in southern California. In the Los Angeles region, ClNO2 was more ubiquitous than Cl2 during most nights of the study period. These observations are used to estimate the relative importance of chlorine atom sources in the polluted marine boundary layer. In contrast to the emphasis in previous studies, ClNO2 and hydrochloric acid are likely the dominant primary chlorine atom sources for the Los Angeles basin. As part of a wintertime field study in Weld County, CO, vertically resolved ClNO2 and Cl2 measurements taken on a 300 meter tall tower are reported. Gas and particle phase measurements aboard a moveable tower carriage allowed for a detailed description of the chemical state of the nocturnal atmosphere as a function of height. These observations show significant vertical structure in ClNO2 and Cl2 mixing ratios that undergo dynamic changes over the course of a night. From these measurements ClNO2 yields from N2O5-aerosol reactions are inferred. The derived yields in these plumes suggest efficient ClNO2 production within distinct combustion plumes originating from the Denver-Boulder urban corridor. Finally, the effects of ClNO2 production, photolysis, and subsequent chlorine atom reactions on chemical species relevant to air quality are examined. ClNO2 formation is incorporated into an existing Master Chemical Mechanism box model framework constrained by a large number of measurements taken during field studies in a polluted coastal environment. These results are compared to model runs excluding ClNO2 formation to assess the effects of ClNO2 on tropospheric oxidants, ozone, and nitrogen oxide partitioning.
- Chemistry