Article open access publication

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

Atmospheric Chemistry and Physics, Copernicus Publications, ISSN 1680-7324

Volume 17, 24, 2017

DOI:10.5194/acp-17-15245-2017, Dimensions: pub.1099903690,

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  1. (1) University of Colorado Boulder, grid.266190.a
  2. (2) Cooperative Institute for Research in Environmental Sciences, grid.464551.7
  3. (3) National Oceanic and Atmospheric Administration, grid.3532.7
  4. (4) University of Michigan, grid.214458.e
  5. (5) National Center for Atmospheric Research, grid.57828.30
  6. (6) Drexel University, grid.166341.7
  7. (7) University of Maryland, College Park, grid.164295.d
  8. (8) Institute of Physical Chemistry "Rocasolano", grid.429036.a
  9. (9) Argentine National Research Council (CONICET), FCEN-UNCuyo, UNT-FRM, Mendoza, Argentina
  10. (10) University of York, grid.5685.e
  11. (11) Harvard University, grid.38142.3c
  12. (12) University of Copenhagen, grid.5254.6, KU
  13. (13) University of Miami, grid.26790.3a
  14. (14) University of California, Irvine, grid.266093.8
  15. (15) Carnegie Mellon University, grid.147455.6
  16. (16) Georgia Institute of Technology, grid.213917.f
  17. (17) Goddard Space Flight Center, grid.133275.1
  18. (18) Joint Center for Earth Systems Technology, University of Maryland, Baltimore County, Baltimore, MD, USA

Description

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 campaign (January–February 2014). The observed BrO and inferred Bry 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 (CBry). 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 (CBry but no SSA source), 0.4×1013 molec cm−2 in CAM-Chem (CBry and SSA), and 2.1×1013 molec cm−2 in GEOS-Chem (CBry and SSA). Neither global model fully captures the C-shape of the Bry profile. A local Bry 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 Bry decreases from the convective TTL to the aged TTL. Analysis of gas-phase Bry against multiple tracers (CFC-11, H2O ∕ O3 ratio, and potential temperature) reveals a Bry 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 Bry (estimated from CFC-11 correlations), and is remarkably insensitive to assumptions about heterogeneous chemistry. Bry 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 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 upper free troposphere and TTL. They are also consistent with observations of significant bromide in Upper Troposphere–Lower Stratosphere 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. Such simultaneous measurements are needed to (1) quantify SSA-derived Bry in the upper FT, (2) test Bry partitioning, and possibly explain the gas-phase Bry minimum in the aged TTL, (3) constrain heterogeneous reaction rates of bromine, and (4) account for all of the sources of Bry to the lower stratosphere.

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Times Cited: 11

Field Citation Ratio (FCR): 2.67

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