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Abstract
Abstract. Multiple axis differential absorption spectroscopy (MAX-DOAS) measurements of bromine monoxide (BrO) probed the vertical structure of halogen activation events during March–May 2012 at Barrow, Alaska. An analysis of the BrO averaging kernels and degrees of freedom obtained by optimal-estimation-based inversions from raw MAX-DOAS measurements reveals the information is best represented by reducing the retrieved BrO profile to two quantities: the integrated column from the surface through 200 m (VCD200 m), and the lower tropospheric vertical column density (LT-VCD), which represents the integrated column of BrO from the surface through 2 km. The percentage of lower tropospheric BrO in the lowest 200 m was found to be highly variable ranging from shallow layer events, where BrO is present primarily in the lowest 200 m, to distributed column events where BrO is observed at higher altitudes. The highest observed LT-VCD events occurred when BrO was distributed throughout the lower troposphere, rather than concentrated near the surface. Atmospheric stability in the lowest 200 m influenced the percentage of LT-VCD that is in the lowest 200 m, with inverted temperature structures having a first-to-third quartile range (Q1–Q3) of VCD200 m/LT-VCD from 15–39%, while near-neutral-temperature structures had a Q1–Q3 range of 7–13%. Data from this campaign show no clear influence of wind speed on either lower tropospheric bromine activation (LT-VCD) or the vertical distribution of BrO, while examination of seasonal trends and the temperature dependence of the vertical distribution supported the conclusion that the atmospheric stability affects the vertical distribution of BrO.
Highlights
The seasonal return of sunlight during late winter in the polar regions is associated with production of reactive halogens (e.g., Br, bromine monoxide (BrO), Cl) from saline ice surfaces (Abbatt et al, 2012; Saiz-Lopez and von Glasow, 2012)
We found that the LT-vertical column density (VCD) was not heavily influenced by changes in the aerosol profile, with an estimated error of 4.6 % (1σ ), which is lower than error introduced by uncertainties in the BrO differential slant column density (dSCD) measurements
The methods described in this paper outline the reduction of vertical profiles retrieved from MAX-DOAS observations using optimal estimation to produce two quantities – the lower tropospheric vertical column density (LT-VCD), and the near-surface vertical column density in the lowest 200 m (VCD200 m) – that appropriately reflect the information content of ground-based MAX-DOAS measurements for time series analysis
Summary
The seasonal return of sunlight during late winter in the polar regions is associated with production of reactive halogens (e.g., Br, BrO, Cl) from saline ice surfaces (Abbatt et al, 2012; Saiz-Lopez and von Glasow, 2012). These halogen species influence boundary layer chemistry through phenomena such as boundary layer ozone depletion events (ODEs) (Barrie et al, 1988; Simpson et al, 2007b) and mercury deposition events (MDEs) (Schroeder et al, 1998; Steffen et al, 2008). Peterson et al.: Vertical structure of BrO (e.g., Foster et al, 2001; Spicer et al, 2002; Toom-Sauntry and Barrie, 2002; Krnavek et al, 2012; Pratt et al, 2013; Liao et al, 2014) point to heterogeneous chemistry involving salts present on ice surfaces (e.g., NaBr, NaCl) as important sources of reactive halogens
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