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We report measurements of bromine monoxide (BrO) and use an observationallyconstrained chemical box model to infer total gas-phase inorganic bromine(Bry) over the tropical western Pacific Ocean(tWPO) during the CONTRAST field campaign(January–February 2014). The observed BrO and inferred Bry profilespeak in the marine boundary layer (MBL), suggesting the need for a brominesource from sea-salt aerosol (SSA), in addition to organic bromine(CBry). Both profiles are found to be C-shaped with local maxima in theupper free troposphere (FT). The mediantropospheric BrO vertical column density (VCD) was measured as1.6×1013 molec cm−2, compared to model predictions of0.9×1013 molec cm−2 in GEOS-Chem (CBry but no SSAsource), 0.4×1013 molec cm−2 in CAM-Chem (CBry andSSA), and 2.1×1013 molec cm−2 in GEOS-Chem (CBry andSSA). Neither global modelfully captures the C-shape of the Bry profile. A local Bry maximumof 3.6 ppt (2.9–4.4 ppt; 95 % confidence interval, CI) is inferredbetween 9.5 and 13.5 km in air masses influenced by recent convectiveoutflow. Unlike BrO, which increases from the convective tropical tropopauselayer (TTL) to the aged TTL, gas-phase Bry decreases from the convectiveTTL to the aged TTL. Analysis of gas-phase Bry against multiple tracers(CFC-11, H2O ∕ O3 ratio, and potential temperature) reveals aBry 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 ofinorganic Bry (estimated from CFC-11 correlations), and is remarkablyinsensitive to assumptions about heterogeneous chemistry. Bry increasesto 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 Bry minimum in the aged TTL is qualitatively (but notquantitatively) captured by CAM-Chem, and suggests a more complexpartitioning of gas-phase and aerosol Bry species than previouslyrecognized. Our data provide corroborating evidence that inorganic brominesources (e.g., SSA-derived gas-phase Bry) are needed to explain thegas-phase Bry budget in the upper free troposphere and TTL. They arealso consistent with observations of significant bromide in UpperTroposphere–Lower Stratosphere aerosols. The total Bry budget in theTTL is currently not closed, because of the lack of concurrent quantitativemeasurements of gas-phase Bry species (i.e., BrO, HOBr, HBr, etc.) andaerosol bromide. Such simultaneous measurements are needed to (1) quantifySSA-derived Bry in the upper FT,(2) test Bry partitioning, and possibly explain the gas-phase Bryminimum in the aged TTL, (3) constrain heterogeneous reaction rates ofbromine, and (4) account for all of the sources of Bry to the lowerstratosphere

atmospheric chemistry and physics

BrO and inferred Br&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;y&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; profiles over the western Pacific: relevance of inorganic bromine sources and a Br&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;y&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; minimum in the aged tropical tropopause layer