Observations of BrO Using Limb Scanning DOAS During the 2013 NOMADSS Campaign

Abstract:

Bromine monoxide (BrO) is an important atmospheric constituent, participating in the destruction of tropospheric and stratospheric ozone and the oxidation of gaseous elemental Hg (GEM) to form reactive Hg (RM). During the Nitrogen, Oxidants, Mercury and Aerosol Distributions, Sources and Sinks (NOMADSS) campaign in the summer of 2013, our group operated a limb-scanning Differential Optical Absorption Spectroscopy (DOAS) instrument onboard the NSF/NCAR C-130 aircraft with the goal of quantifying BrO, formaldehyde (HCHO), nitrogen dioxide (NO₂), and other trace gases. Here we present observations of Differential Slant Column Densities (DSCD) obtained in the limb derived using the DOAS analysis technique. A radiative transfer model (McArtim [Deutschmann et al., 2011]), constrained by in-situ measurement on the aircraft or from published literature, was utilized to simulate the DSCDs and to derive BrO mixing ratios at flight altitude.

BrO mixing ratios were typically below our 2σ detection limit (ranging from 0.5 – 1 ppt) with the exception of a high altitude (~7km asl) portion of a flight over Central and West Texas (RF 6), and a marine boundary layer (~0.2km asl) flight leg off the coast of South Carolina (RF 16). Around 1.9 ± 0.25 ppt of BrO was detected for about 1.5 hours in the high altitude flight, and around 1.0 ±0.5 ppt of BrO was detected in the marine boundary layer. Several features were present in the higher altitude air-mass with detectable BrO: O₃ and humidity were significantly depressed relative to other flight legs at the same altitude and large concentrations of RM were simultaneously observed. We will present possible sources for the anomalously high BrO mixing ratios in the sampled air mass and discuss the implications of our knowledge on free-tropospheric bromine chemistry and its effects on GEM oxidation.