Steven S. Brown

A large atomic chlorine source inferred from mid-continental reactive nitrogen chemistry.

Halogen atoms and oxides are highly reactive and can profoundly affect atmospheric composition. Chlorine atoms can decrease the lifetimes of gaseous elemental mercury and hydrocarbons such as the greenhouse gas methane. Chlorine atoms also influence cycles that catalytically destroy or produce tropospheric ozone, a greenhouse gas potentially toxic to plant and animal life. Conversion of inorganic chloride into gaseous chlorine atom precursors within the troposphere is generally considered a coastal or marine air phenomenon. Here we report mid-continental observations of the chlorine atom precursor nitryl chloride at a distance of 1,400 km from the nearest coastline. We observe persistent and significant nitryl chloride production relative to the consumption of its nitrogen oxide precursors. Comparison of these findings to model predictions based on aerosol and precipitation composition data from long-term monitoring networks suggests nitryl chloride production in the contiguous USA alone is at a level similar to previous global estimates for coastal and marine regions. We also suggest that a significant fraction of tropospheric chlorine atoms may arise directly from anthropogenic pollutants.

Gasoline emissions dominate over diesel in formation of secondary organic aerosol mass

Although laboratory experiments have shown that organic compounds in both gasoline fuel and diesel engine exhaust can form secondary organic aerosol (SOA), the fractional contribution from gasoline and diesel exhaust emissions to ambient SOA in urban environments is poorly known. Here we use airborne and ground-based measurements of organic aerosol (OA) in the Los Angeles (LA) Basin, California made during May and June 2010 to assess the amount of SOA formed from diesel emissions. Diesel emissions in the LA Basin vary between weekdays and weekends, with 54% lower diesel emissions on weekends. Despite this difference in source contributions, in air masses with similar degrees of photochemical processing, formation of OA is the same on weekends and weekdays, within the measurement uncertainties. This result indicates that the contribution from diesel emissions to SOA formation is zero within our uncertainties. Therefore, substantial reductions of SOA mass on local to global scales will be achieved by reducing gasoline vehicle emissions.

Nitryl chloride and molecular chlorine in the coastal marine boundary layer.

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. We made continuous measurements of ambient ClNO(2) and Cl(2) concentrations from May 15 to June 8 aboard the Research Vessel Atlantis during the CalNex 2010 field study. In the Los Angeles region, ClNO(2) was more ubiquitous than Cl(2) during most nights of the study period. ClNO(2) and Cl(2) ranged from detection limits at midday to campaign maximum values at night reaching 2100 and 200 pptv, respectively. The maxima were observed in Santa Monica Bay when sampling the Los Angeles urban plume. Cl(2) at times appeared well correlated with ClNO(2), but at other times, there was little to no correlation implying distinct and varying sources. Well-confined Cl(2) plumes were observed, largely independent of ClNO(2), providing support for localized industrial emissions of reactive chlorine. Observations of ClNO(2), Cl(2), and HCl are used to constrain a simple box model that predicts their relative importance as chlorine atom sources in the polluted marine boundary layer. In contrast to the emphasis in previous studies, ClNO(2) and HCl are dominant primary chlorine atom sources for the Los Angeles basin.

Heterogeneous Atmospheric Chemistry, Ambient Measurements, and Model Calculations of N2O5: A Review

For several decades, dinitrogen pentoxide (N2O5) has been recognized as an important reactive intermediate in the atmospheric chemistry of nitrogen oxides and nitrate aerosol, especially during nighttime. However, due to the lack of ambient observations of N2O5, the nocturnal nitrogen oxide chemistry could not be quantified until recent years. The objective of the present article is to assess the current state-of-the-art knowledge of N2O5 dynamics within the tropospheric aerosol. An up-to-date summary of N2O5 chemistry and major loss mechanisms are provided. Furthermore, techniques for measuring ambient N2O5 and an overview of typical N2O5 levels in the troposphere are described. In addition, model representations of N2O5 chemistry are reviewed along with key features of N2O5 vertical profiles based on numerical simulations. Lastly, the article provides the outstanding uncertainties and needs for further research into the atmospheric chemistry of N2O5. These include the need for better characterization of N2O5 heterogeneous uptake under temperature conditions characteristic of mid- and high-latitude winter seasons; greater understanding of the influence of individual aerosol components on N2O5 uptake and representation of these components in atmospheric models; and comprehensive descriptions of nighttime vertical profiles of N2O5 and related pollutants.

Simultaneous in situ detection of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy

This article describes the application of cavity ring-down spectroscopy (CaRDS) to the simultaneous concentration measurement of nitrate radical, NO3, and dinitrogen pentoxide, N2O5, in the ambient atmosphere. The sensitivity for detection of both NO3 and N2O5 is 0.5 pptv (2σ) for a 5 s integration, comparable to or better than previous measurements of NO3 (e.g., via DOAS), but with significantly better time resolution. Furthermore, direct measurement of N2O5 represent a previously unavailable capability. Concentrations of both species are measured simultaneously in two separate flow systems and optical cavities pumped by the same pulsed dye laser at 662 nm. One of the flow systems remains at ambient temperature for detection of NO3, while the other is heated to 80 °C to induce thermal decomposition of N2O5 providing a measurement of the sum of the NO3 and N2O5 concentrations. This article outlines a series of laboratory and field tests of the instrument’s performance. Important considerations include sig...

Rate constants for the reaction OH+NO2+M → HNO3+M under atmospheric conditions

Abstract We present rate constants for the title reaction, k 1 , measured using a pulsed photolysis–laser induced fluorescence technique between 220 and 250 K in 20–250 Torr of N 2 and 20–50 Torr of O 2 . Our measured k 1 agree with literature values at low temperatures but show that the current recommendations for atmospheric modeling overestimate k 1 by 10–30% in the falloff region below 250 K. The revised values of k 1 help to better define the role of NO 2 in the stratosphere.

Tropospheric Halogen Chemistry:Sources, Cycling, and Impacts

In the past 40 years, atmospheric chemists have come to realize that halogens exert a powerful influence on the chemical composition of the troposphere and through that influence affect the fate of pollutants and may affect climate. Of particular note for climate is that halogen cycles affect methane, ozone, and particles, all of which are powerful climate forcing agents through direct and indirect radiative effects. This influencecomes from the high reactivity of atomic halogen radicals (e.g.,Cl, Br, I) and halogen oxides (e.g., ClO, BrO, IO, and higher oxides), known as reactive halogen species in this review. These reactive halogens are potent oxidizers for organic and inorganic compounds throughout the troposphere.

Vertically Resolved Measurements of Nighttime Radical Reservoirs in Los Angeles and Their Contribution to the Urban Radical Budget

Photolabile nighttime radical reservoirs, such as nitrous acid (HONO) and nitryl chloride (ClNO(2)), contribute to the oxidizing potential of the atmosphere, particularly in early morning. We present the first vertically resolved measurements of ClNO(2), together with vertically resolved measurements of HONO. These measurements were acquired during the California Nexus (CalNex) campaign in the Los Angeles basin in spring 2010. Average profiles of ClNO(2) exhibited no significant dependence on height within the boundary layer and residual layer, although individual vertical profiles did show variability. By contrast, nitrous acid was strongly enhanced near the ground surface with much smaller concentrations aloft. These observations are consistent with a ClNO(2) source from aerosol uptake of N(2)O(5) throughout the boundary layer and a HONO source from dry deposition of NO(2) to the ground surface and subsequent chemical conversion. At ground level, daytime radical formation calculated from nighttime-accumulated HONO and ClNO(2) was approximately equal. Incorporating the different vertical distributions by integrating through the boundary and residual layers demonstrated that nighttime-accumulated ClNO(2) produced nine times as many radicals as nighttime-accumulated HONO. A comprehensive radical budget at ground level demonstrated that nighttime radical reservoirs accounted for 8% of total radicals formed and that they were the dominant radical source between sunrise and 09:00 Pacific daylight time (PDT). These data show that vertical gradients of radical precursors should be taken into account in radical budgets, particularly with respect to HONO.

Aircraft instrument for simultaneous, in situ measurement of NO3 and N2O5 via pulsed cavity ring-down spectroscopy

This article describes a cavity ring-down spectrometer (CaRDS) specifically designed and constructed for installation on the NOAA WP-3D Orion (P-3) aircraft for sensitive, rapid in situ measurement of NO3 and N2O5. While similar to our previously described CaRDS instrument, this instrument has significant improvements in the signal-to-noise ratio, the time resolution, and in overall size and weight. Additionally, the instrument utilizes a custom-built, automated filter changer that was designed and constructed to meet the requirement for removal of particulate matter in the airflow while allowing fully autonomous instrument operation. The CaRDS instrument has a laboratory detection sensitivity of 4×10−11cm−1 in absorbance or 0.1pptv (pptv denotes parts per trillion volume) of NO3 in a 1s average, although the typical detection sensitivities encountered in the field were 0.5pptv for NO3 and 1pptv for N2O5. The instrument accuracy is 25% for NO3 and 20%–40% for N2O5, limited mainly by the uncertainty in the...

Design and Application of a Pulsed Cavity Ring-Down Aerosol Extinction Spectrometer for Field Measurements

This paper describes the design and application of a pulsed cavity ring-down aerosol extinction spectrometer (CRD-AES) for in-situ atmospheric measurement of the aerosol extinction coefficient and its relative humidity dependence. This CRD-AES measures the aerosol extinction coefficient (σ ep) at 355 nm, 532 nm, 683 nm, and 1064 nm with a minimal size dependent bias for particles with diameter less than 10 μm. The σ ep at 532 nm is measured with an accuracy of 1% when extinction is ≥ 10 Mm −1 . The precision is limited by statistical fluctuations within the small optical volume and the time resolution of extinction at 2% uncertainty for various air mass types is evaluated. The CRD-AES is configured with two separate cavity ring-down cells for measurement of the extinction coefficient at 532 nm. This allows the determination of the RH dependence of extinction at 532 nm through independent RH control of the sample for each measurement. Gas phase absorption and minimization of potential interferences is also considered.