Atmospheric test of the J(BrONO<sub>2</sub>)/<i>k</i><sub>BrO+NO<sub>2</sub></sub> ratio: implications for total stratospheric Br<sub>y</sub> and bromine-mediated ozone loss

Abstract

Abstract. We report on time-dependent O3, NO2 and BrO profiles measured by limb observations of scattered skylight in the stratosphere over Kiruna (67.9° N, 22.1° E) on 7 and 8 September 2009 during the autumn circulation turn-over. The observations are complemented by simultaneous direct solar occultation measurements around sunset and sunrise performed aboard the same stratospheric balloon payload. Supporting radiative transfer and photochemical modelling indicate that the measurements can be used to constrain the ratio J(BrONO2)/kBrO+NO2, for which at T = 220 ± 5 K an overall 1.7 (+0.4 −0.2) larger ratio is found than recommended by the most recent Jet Propulsion Laboratory (JPL) compilation (Sander et al., 2011). Sensitivity studies reveal the major reasons are likely to be (1) a larger BrONO2 absorption cross-section σBrONO2, primarily for wavelengths larger than 300 nm, and (2) a smaller kBrO+NO2 at 220 K than given by Sander et al. (2011). Other factors, e.g. the actinic flux and quantum yield for the dissociation of BrONO2, can be ruled out. The observations also have consequences for total inorganic stratospheric bromine (Bry) estimated from stratospheric BrO measurements at high NOx loadings, since the ratio J(BrONO2)/kBrO+NO2 largely determines the stratospheric BrO/Bry ratio during daylight. Using the revised J(BrONO2)/kBrO+NO2 ratio, total stratospheric Bry is likely to be 1.4 ppt smaller than previously estimated from BrO profile measurements at high NOx loadings. This would bring estimates of Bry inferred from organic source gas measurements (e.g. CH3Br, the halons, CH2Br2, CHBr3, etc.) into closer agreement with estimates based on BrO observations (inorganic method). The consequences for stratospheric ozone due to the revised J(BrONO2)/kBrO+NO2 ratio are small (maximum −0.8%), since at high NOx (for which most Bry assessments are made) the enhanced ozone loss by overestimating Bry is compensated for by the suppressed ozone loss due to the underestimation of BrO/Bry with a smaller J(BrONO2)/kBrO+NO2 ratio.

Stratospheric Injection of Brominated Very Short‐Lived Substances: Aircraft Observations in the Western Pacific and Representation in Global Models

“…The mean value of the best estimate for Br y VSLS from these studies is 6 ppt, with a range of 3 to 8 ppt (Carpenter et al, 2014). If the modification to the kinetics that govern the BrONO 2 to Br y ratio put forth by Kreycy et al (2013) based on atmospheric observations is adopted, these values fall to a best estimate of 5 ppt, with a range of 2 to 8 ppt. This proposed kinetic revision would decrease our Br y VSLS estimate by 0.5 ppt and is considered in our overall uncertainty (Text S3).…”

“…The combined observed SGI VSLS and modeled PGI value generally agrees with estimates of Br y VSLS that are based on analysis of slant column measurements of BrO in the middle to upper stratosphere (e.g., Dorf et al, 2008;Parrella et al, 2013;Schofield et al, 2004). However, there are significant uncertainties concerning the modeled estimates of PGI (e.g., Liang et al, 2014;Sinnhuber & Folkins, 2006;Tegtmeier et al, 2012) as well as the inference of Br y from measurements of BrO for photochemically aged stratospheric air (Kreycy et al, 2013;Sioris et al, 2006).…”

“…Within the stratosphere, the contribution of BrONO 2 to the Br y budget increases as the mixing ratio of O 3 exceeds 200 ppb (grey shaded portion of Figure 7b) and NO 2 exceeds 50 ppt ( Figure S2a). As discussed below, Br y inferred for this flight segment is sensitive to the accuracy of the kinetic information used to define the rates of formation and loss of BrONO 2 , which currently has considerable uncertainty (Kreycy et al, 2013;Sioris et al, 2006). Most importantly, our estimate of PGI is weighted toward flight segments for which BrO constitutes about half of Br y .…”

“…They also ascribed the same uncertainty to both Br y VSLS and PGI (i.e., ±2.5 ppt), since organic bromine species are able to be measured with much better accuracy and precision than can be achieved for BrO. The need to infer total inorganic bromine from measured BrO adds an additional, important source of uncertainty (e.g., Kreycy et al, 2013) to estimates of both PGI and Br y VSLS .…”

BrO and inferred Br<sub><i>y</i></sub> profiles over the western Pacific: relevance of inorganic bromine sources and a Br<sub><i>y</i></sub> minimum in the aged tropical tropopause layer

“…The wavelength calibration for each spectrum is precisely determined by measuring the mercury atomic emission at 404.7 nm. This slit function was further refined by fitting two wavelength-dependent degrees of freedom, using the Kurucz spectrum as reference (Chance and Kurucz, 2010;Kurucz et al, 1984). The slit function was fixed for final calibration of the instrument wavelength mapping.…”