Bromine partitioning in the tropical tropopause layer: implications for stratospheric injection

Abstract

Abstract. Very short-lived (VSL) bromocarbons are produced at a prodigious rate by ocean biology and these source compounds (SGVSL), together with their inorganic degradation products (PGVSL), are lofted by vigorous convection to the tropical tropopause layer (TTL). Using a state-of-the-art photochemical mechanism within a global model, we calculate annual average stratospheric injection of total bromine due to VSL sources to be 5 pptv (parts per trillion by volume), with ~ 3 pptv entering the stratosphere as PGVSL and ~ 2 pptv as SGVSL. The geographic distribution and partitioning of VSL bromine within the TTL, and its consequent stratospheric injection, is highly dependent on the oceanic flux, the strength of convection and the occurrence of heterogeneous recycling reactions. Our calculations indicate atomic Br should be the dominant inorganic species in large regions of the TTL during daytime, due to the low ozone and cold conditions of this region. We propose the existence of a "tropical ring of atomic bromine" located approximately between 15 and 19 km and between 30° N and 30° S. Daytime Br / BrO ratios of up to ~ 4 are predicted within this inhomogeneous ring in regions of highly convective transport, such as the tropical Western Pacific. Therefore, we suggest that experimental programs designed to quantify the bromine budget of the TTL and the stratospheric injection of VSL biogenic bromocarbons should include a strategy for the measurement of atomic Br during daytime as well as HOBr and BrCl during nighttime.

Probing the subtropical lowermost stratosphere and the tropical upper troposphere and tropopause layer for inorganic bromine

“…In particular, for our daytime measurements, it is observed that (a) BrO increases with O 3 and available Br inorg y and thus altitude and (b) the predicted BrO / Br inorg y ratio decreases towards the bottom of the TTL, where (c) HBr and/or Br atoms may become comparable to BrO, but HOBr does not play a major role in the Br inorg y partitioning. While observation (a) is due to the increased destruction of primarily the short-lived Br org y species and the efficient reaction of the released Br atoms with increasing altitude and increasing ozone concentrations, observations (b) and (c) are due to reactions of the Br atoms with CH 2 O (and less H 2 O 2 ) into HBr, which is recycled back by reactions with OH and by variable amounts heterogeneously (depending on the available surface of aerosols and cloud particles) to Br atoms, as predicted by Fernandez et al (2014), and Saiz-Lopez and . The predicted minor role of HOBr eventually formed by reactions of OH radicals with heterogeneously produced Br 2 or by the reaction HO 2 + BrO and photolytic destruction of HOBr in the TTL is also noteworthy.…”

“…Elevated BrO concentrations are measured within the LS (range 3-9 ppt), and lower BrO concentrations are measured in the TTL (range 0.5-5 ppt), with the smallest BrO concentrations (0.5-1 ppt) occurring near the bottom of the TTL. Overall, this behavior is expected from arguments based on the amount and composition of the brominated organic and inorganic source gases, their lifetimes, atmospheric transport, and photochemistry (e.g., Fueglistaler et al, 2009;Aschmann et al, 2009;Hossaini et al, 2012b;Ashfold et al, 2012;WMO, 2014;Fernandez et al, 2014;Saiz-Lopez and Fernandez, 2016). In particular, for our daytime measurements, it is observed that (a) BrO increases with O 3 and available Br inorg y and thus altitude and (b) the predicted BrO / Br inorg y ratio decreases towards the bottom of the TTL, where (c) HBr and/or Br atoms may become comparable to BrO, but HOBr does not play a major role in the Br inorg y partitioning.…”

“…Past research has revealed that total stratospheric bromine (Br y ) has (in 2013) four major sources or contributions: (1) CH 3 Br which is emitted by natural and anthropogenic sources, with a present contribution of 6.9 ppt to Br y , (2) four major halons (CClBrF 2 or halon-1211; CBrF 3 or halon-1301; CBr 2 F 2 or halon-1202; and CBrF 2 CBrF 2 or halon-2402), all emitted from anthropogenic activities, with a present contribution of 8 ppt to Br y ; (3) socalled very short-lived species (VSLS); and (4) inorganic bromine transported into the upper troposphere, e.g., previously released from brominated VSLS and/or sea salt (e.g., Saiz-Lopez et al, 2004;Fernandez et al, 2014;Schmidt et al, 2016). This inorganic bromine is also partly transported into the stratosphere.…”

A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

“…5. Hg 0 oxidation in terms of mixing ratio features a secondary maximum in the tropical upper troposphere, where the Br / BrO ratio is high (Parrella et al, 2012;Fernandez et al, 2014).…”

NO<sub>2</sub> seasonal evolution in the north subtropical free troposphere

“…The model includes the tropospheric chemistry mechanism of MOZART-4, implementing also organic and inorganic halogen (chlorine, bromine and iodine) photochemistry mechanisms, taking into account natural and anthropogenic sources, heterogeneous recycling and dry and wet deposition Ordoñez et al, 2012;Fernandez et al, 2014). Anthropogenic emissions due to fossil fuel and biofuel combustions come from the POET (precursors of ozone and their effects in the troposphere) database for 2000.…”