BrO and Bry profiles over the Western Pacific: Relevance of Inorganic Bromine Sources and a Bry 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 (Bry) over the tropical Western Pacific Ocean (tWPO) during the CONTRAST field 40 campaign (January – February 2014). The median tropospheric BrO Vertical Column Density (VCD) over the tWPO was measured as 1.6×1013 molec. cm˗2, compared to model predictions of 0.4×1013 in CAM-Chem, 0.9×1013 in GEOS-Chem, and 2.1×1013 in GEOS-Chem with a sea-salt aerosol (SSA) bromine source. The observed BrO and inferred Bry profiles is found to be C-shaped in the troposphere, with local maxima in the marine boundary layer (MBL) and in the upper free troposphere. Neither global model fully captures this profile shape. Between 6 and 13.5 km, the inferred Bry is highly sensitive to 5 assumptions about the rate of heterogeneous bromine recycling (depends on the surface area of ice/aerosols), and the inclusion of a SSA bromine source. A local Bry maximum of 3.6 ppt (2.3-11.1 ppt, 95% CI) is observed between 9.5 and 13.5 km in air masses influenced by recent convective outflow. Unlike BrO, which increases from the convective TTL to the aged TTL, gas phase Bry decreases from the convective TTL to the aged TTL. Analysis of gas phase Bry against multiple tracers (CFC-11, H2O/O3 ratio, and θ) reveals a Bry minimum of 2.7 ppt (2.4-3.0 ppt, 95% CI) in the aged TTL, which is remarkably insensitive 10 to assumptions about heterogeneous chemistry. Bry increases to 6.3 ppt (5.9-6.7 ppt, 95% CI) in the stratospheric middleworld, and 6.9 ppt (6.7-7.1 ppt, 95% CI) in the stratospheric overworld. The local Bry 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 Bry species than previously recognized. Our data provide corroborating evidence that inorganic bromine sources (e.g., SSA derived gas phase Bry) are needed to explain the gas phase Bry budget in the TTL. They are also consistent with observations of significant 15 bromide in UTLS aerosols. The total Bry budget in the TTL is currently not closed, because of the lack of concurrent quantitative measurements of gas phase Bry species (i.e., BrO, HOBr, HBr, etc.) and aerosol bromide. These simultaneous measurements are needed 1) to quantify SSA derived Bry aloft, 2) to test Bry partitioning, and explain the gas phase Bry minimum in the aged TTL, 3) to constrain heterogeneous reaction rates of bromine, and 4) to account for all of the sources of Bry to the lower stratosphere.