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

Abstract

Abstract. We report measurements of bromine monoxide (BrO) and use an observationally constrained chemical box model to infer total gas-phase inorganic bromine (Br y ) over the tropical western Pacific Ocean (tWPO) during the CON-TRAST field campaign (January-February 2014). The observed BrO and inferred Br y profiles peak in the marine boundary layer (MBL), suggesting the need for a bromine source from sea-salt aerosol (SSA), in addition to organic bromine (CBr y ). Both profiles are found to be C-shaped with local maxima in the upper free troposphere (FT). The median tropospheric BrO vertical column density (VCD) was measured as 1.6×10 13 molec cm −2 , compared to model predictions of 0.9 × 10 13 molec cm −2 in GEOS-Chem (CBr y but no SSA source), 0.4 × 10 13 molec cm −2 in CAM-Chem (CBr y and SSA), and 2.1×10 13 molec cm −2 in GEOS-Chem (CBr y and SSA). Neither global model fully captures the Cshape of the Br y profile. A local Br y maximum of 3.6 ppt (2.9-4.4 ppt; 95 % confidence interval, CI) is inferred between 9.5 and 13.5 km in air masses influenced by recent convective outflow. Unlike BrO, which increases from the convective tropical tropopause layer (TTL) to the aged TTL, gas-phase Br y decreases from the convective TTL to the aged TTL. Analysis of gas-phase Br y against multiple tracers (CFC-11, H 2 O / O 3 ratio, and potential temperature) reveals a Br y minimum of 2.7 ppt (2.3-3.1 ppt; 95 % CI) in the aged TTL, which agrees closely with a stratospheric injection of 2.6 ± 0.6 ppt of inorganic Br y (estimated from CFC-11 correlations), and is remarkably insensitive to assumptions about heterogeneous chemistry. Br y increases to 6.3 ppt (5.6-7.0 ppt; 95 % CI) in the stratospheric "middleworld" and 6.9 ppt (6.5-7.3 ppt; 95 % CI) in the stratospheric "overworld". The local Br y minimum in the aged TTL is qualitatively (but not quantitatively) captured by CAM-Chem, and suggests a more complex partitioning of gas-phase and aerosol Br y species than previously recognized. Our data provide corroborating evidence that inorganic bromine sources (e.g., SSA-derived gas-phase Br y ) are needed to explain the gas-phase Br y budget in the upper free troposphere and TTL. They are also consistent with observations of significant bromide in Upper Troposphere-Lower Stratosphere aerosols. The total Br y budget in the TTL is currently not closed, because of the lack of concurrent quantitative measurements of gas-phase Br y species (i.e., BrO, HOBr, HBr, etc.) and aerosol bromide. Such simultaneous measurements are needed to (1) quantify SSA-derived Br y in the upper FT, (2) test Br y partitioning, and possibly explain the gas-phase Br y minimum in the aged TTL, (3) constrain heterogeneous reaction rates of bromine, and (4) account for all of the sources of Br y to the lower stratosphere.

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

“…While many other chemical and physical measurements acquired by the GV HIAPER payload during RF06 and RF15 are used in our study, the time series central to our analysis are shown in Figure 3. Previous CONTRAST bromine studies (Chen et al, 2016;Koenig et al, 2017) have used the relation between mixing ratios of O 3 , CO, and H 2 O collected during RF15 to chemically define both the stratosphere as well as a transition layer, where there is a mixture of tropospheric and stratospheric air. While our study primarily uses the thermal tropopause, we also make use of a chemical tropopause (described below) to facilitate comparison to these two studies.…”

“…We rely on measurements of CH 3 Br, halons, and a suite of organic VSLS provided by the Whole Air Sampler (WAS) instruments (Navarro et al, 2015) during both ATTREX and CONTRAST. We also utilize measurements of BrO obtained by two instruments during CONTRAST: in situ observations from a Chemical Ionization Mass Spectrometer (CIMS; Chen et al, 2016) and remote observations from an Airborne Multi AXis Differential Optical Absorption Spectroscopy (AMAX-DOAS) instrument (Koenig et al, 2017). A limb-scanning mini-Differential Optical Absorption Spectroscopy (mini-DOAS) instrument measured BrO during ATTREX Werner et al, 2017).…”

“…Navarro et al (2015) presented an estimate of SGI VSLS based on WAS observations of CBr y compounds acquired during ATTREX and PGI based solely on a model estimate of gas phase Br y . Both Chen et al (2016) and Koenig et al (2017) estimated PGI using observations of BrO at the base of the stratosphere acquired during CONTRAST but did not incorporate measurements of CBr y . In this study, we use empirical bromocarbon-tracer relations to combine all three sets of measurements (i.e., WAS organics plus CIMS and AMAX-DOAS BrO) to develop comprehensive, observation-based estimates of SGI VSLS , PGI, and overall Br y VSLS .…”

“…We have made no attempt to quantify the amount of PGI that results from the oxidation of VSLS versus other sources, such as sea-salt aerosol. We refer the interested reader to Koenig et al (2017) for analysis concerning the possible contributions of sea-salt aerosol as well as stratospheric injection of brominated particles to PGI.…”

Atmospheric Acetaldehyde: Importance of Air‐Sea Exchange and a Missing Source in the Remote Troposphere

“…It is a major precursor of peroxyacetyl nitrate (PAN; Fischer et al, 2014), which affects the longrange transport of NO x (NO + NO 2 ; Singh et al, 1990). It also affects hydrogen oxide radicals (Moxim et al, 1996;Seinfeld & Pandis, 2012) and reactive halogen chemistry (Hornbrook et al, 2016;Koenig et al, 2017). High levels of CH 3 CHO have been reported in the remote marine boundary layer (MBL; Read et al, 2012;Singh et al, 2003) and the free troposphere (FT; Singh et al, 2001Singh et al, , 2003.…”