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Bromine Monoxide enhancements over the North Pole

Aura's Ozone Monitoring Instrument (OMI), with its wide swath, is the only instrument that can detect Bromine Monoxide (BrO) at the pole on every orbit. Here, stratospheric BrO has been carefully separated from the total column (using a chemical and dynamical model) to produce estimates of the tropospheric BrO column. Validation was accomplished with aircraft in situ measurements during the NASA ARCTAS field campaign, showing that bromine chemical activation takes place at the Arctic near the surface.

Why is BrO important in the Arctic?

BrO plays an important role in tropospheric ozone chemistry and the resulting oxidation capacity of the polar boundary layer

Bromine radicals catalytically destroy ozone, leading to nearly complete removal near the surface that is termed an ozone (O3) depletion event (ODE).

Br and BrO are thought to oxidize elemental mercury to more reactive (toxic) gaseous mercury that deposits to the polar ecosystem

MLS figure

Figure 1: Two flights through areas of enhanced tropospheric BrO (boxed regions) near the North pole. [hi-res]

  • 1st column: derived tropospheric BrO column from OMI.
  • 2nd column: sea level pressure from GMAO reanalysis.
  • 3rd column: wind speed near surface from GMAO.
  • 4th column: planetary boundary layer (PBL) height.
  • 5th column: temperature and bromine constituent profiles from in situ aircraft sensors. This analysis shows that BrO activation occurs in low pressure systems with high winds

Technical Description of Figure:
To derive the tropospheric column of BrO, the stratospheric column is first estimated using a chemical and dynamical model developed at the University of Maryland by coauthors Ross Salawitch and Tim Canty. The stratospheric column is subtracted from the total column that is produced by coauthors Kelly Chance and Thomas Kurosu at the Harvard Smithsonian Astronomical Observatory (SAO). The residual tropospheric column must be carefully computed by assuming a profile shape. Used here was a composite profile derived from ARCTAS field campaign measurements.

Scientific significance:
Before the ARCTAS and ARCPAC field campaigns, ground- and satellite-based data, primarily from remote sensing, indicated numerous areas with large enhancements of BrO, thought to be occurring near the surface where it is hypothesized that bromine is released from blowing snow. The enhancements were linked to events of severe ozone depletion also near the surface. One of the objectives of the ARCTAS campaign was to fly into these regions and analyze the chemical composition of the air. When the aircraft were guided into the BrO "hot-spot" areas using satellite data, researchers were very surprised to see neither BrO enhancements nor ozone depletions. Salawitch et al. (GRL, 2010) showed that much of the BrO seen by satellites in these hot spot areas, was in fact due to compressed BrO in the stratosphere. This study takes the next step in interpretation of data from the ARCTAS campaign.

This is the first study to quantitatively evaluate satellite-based estimates of tropospheric BrO in the Arctic with aircraft in situ measurements. This study showed for the first time a rapid activation of bromine chemistry at high temporal resolution, made possible by wide-swath OMI observations. Two flights crossed regions of high tropospheric BrO and both were validated by the aircraft sensors. The study therefore gives further weight to the hypothesis that there is a tropospheric source of bromine. By using high resolution meteorological from the NASA GMAO, the study shows that the source is related to high winds (blowing snow) associated with low pressure systems and/or a high planetary boundary layer where the catalytic bromine-ozone chemistry can be maintained. The study also emphasizes the importance of having an accurate estimate of the stratospheric contribution to the total BrO column. It also showed that tropospheric BrO can only be observed in the Arctic only under certain conditions (over bright surfaces - primarily ice), when thick clouds are not present, and not at very high solar zenith angles.

Data Sources: NASA Aura Ozone Monitoring Instrument (OMI), Chemical Ionization Mass Spectrometer (CIMS) BrO measurements from DC_8 and WP-3D aircraft, NASA Goddard Modeling and Assimilation Office (GMAO) MERRA reanalysis.

Reference: Choi et al. 2011, Analysis of satellite_derived Arctic tropospheric BrO columns in conjunction with aircraft measurements during ARCTAS and ARCPAC, Atmos. Chem. Phys. Disc., 11, 26173-26243, doi:10.5194/acpd-11-26173-2011, accepted pending revisions.