Probing the subtropical lowermost stratosphere, tropical upper troposphere, and tropopause layer for inorganic bromine
Authors: Werner B. , Stutz J. , Spolaor M. , Scalone L. , Raecke R. , Festa J. , Colosimo F. , Cheung R. , Tsai C. , Hossaini R. , Chipperfield M. P. , Taverna G. S. , Feng W. , Elkins J. W. , Fahey D. W. , Gao Ru-Shan , Hintsa E. J. , Thornberry T. D. , Moore F. L. , Navarro M. A. , Atlas E. , Daube B. , Pittman J. , Wofsy S. , Pfeilsticker K. 
Published: September 2016

Abstract. We report on measurements of CH4, O3, NO2, BrO and some key brominated source gases within the subtropical lowermost stratosphere (LS), tropical upper troposphere (UT) and tropopause layer (TTL) (14–18.5 km). The measurements were performed within the framework of the NASA-ATTREX (National Aeronautics and Space Administration - Airborne Tropical Tropopause Experiment) project from aboard the Global Hawk (GH) during 6 deployments over the Eastern Pacific in early 2013. O3, NO2, and BrO were remotely monitored by analyzing limb scattered skylight in the UV and visible spectral ranges using the observations of the mini-DOAS (Differential Optical Absorption Spectroscopy) instrument (Stutz et al., 2016). CH4 was measured in-situ by the Harvard HUPCRS instrument and by the NOAA-UCATS instrument. O3 was recorded at high precision by the NOAA dual-beam UV photometer, and some key brominated source gases were analyzed in whole air samples of the GWAS (Global Hawk Whole Air Sampler) instrument. All of these measurements are used for comparison 10 with TOMCAT/SLIMCAT 3-D model simulations, aiming at improvements of our understanding of the bromine budget and photochemistry in the LS, UT, and TTL. Potential changes in local O3 (and NO2 and BrO) due to transport processes are separated from photochemical processes in inter-comparisons of measured and modeled CH4 and O3. After accounting for some minor deficiencies in the details of the modeled vertical transport, excellent agreement is achieved among measured and simulated CH4 and O3, indicating that in the subtropical LS and TTL O3 concentrations mostly vary due to dynamical rather than photochemical processes. The TOMCAT/SLIMCAT simulations are further used for the interpretation of the measured NO2 and BrO. In excellent agreement with the model predictions, NO2 concentrations are found to range between 70–170 ppt in the subtropical LS, and in the TTL they are close to, or below the detection limit of 15 ppt in daytime. The measured BrO concentrations range between 3–9 ppt in the subtropical LS, and in the TTL they reach 0.5 &amp;pm; 0.5 ppt at the bottom of the TTL (150 hPa/355 K/14 km) and up to about 5 ppt at the top of the TTL (70 hPa/425 K/18.5 km, for the TTL definition see Fueglistaler et al. (2009)), in overall good agreement with the model simulation, and the expectation based on the destruction of brominated source gases. The TOMCAT/SLIMCAT simulations tend to slightly under-predict measured BrO depending on the photochemical regime studied, even when constrained to the measured O3 and NO2, and adjusted to match the observed concentrations of some key brominated source gases. The measured BrO and modeled BrO / Brinorgy ratio is further used to calculate inorganic bromine, Brinorgy. For the TTL (i.e. when [CH4] &amp;pm; 1390 ppb), Brinorgy is found to increase from a mean of 2.63 &amp;pm; 1.04 ppt for &amp;pm; in the range of 350–360 K to 5.11 &amp;pm; 1.57 ppt for θ = 390 to 400 K, whereas in the subtropical LS (i.e. when [CH4] &amp;pm; 1390 ppb),it reaches 7.66 &amp;pm; 2.95 ppt for theta in the range of 390–400 K. Finally, the TOMCAT/SLIMCAT simulations indicate a net destruction of ozone of −0.5 ppbv/day at the base of the TTL (θ = 355 K) and a net production of +1.8 ppbv/day at its top (θ = 383 K).