BrO and inferred Br_y profiles over the western Pacific: relevance of inorganic bromine sources and a Br_y minimum in the aged tropical tropopause layer

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 CONTRAST 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 1.6×10¹³ molec cm⁻², compared to model predictions of 0.9×10¹³ molec cm⁻² in GEOS-Chem (CBr_y but no SSA source), 0.4×10¹³ molec cm⁻² in CAM-Chem (CBr_y and SSA), and 2.1×10¹³ molec cm⁻² in GEOS-Chem (CBr_y and SSA). Neither global model fully captures the C-shape 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, H₂O ∕ O₃ 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.