OMI Measurements of BrO, HCHO, and CHOOMI Measurements of BrO, HCHO, and CHO--CHOCHOKelly Chance and Thomas P. Kurosu

Atomic and Molecular Physics Division, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USAkchance@cfa.harvard.edu; tkurosu@cfa.harvard.edu; http://cfa-www.harvard.edu/atmosphere/

ABSTRACT

We report progress in determining abundances of the trace gases BrO, OClO, and HCHO from the EOS-Aura Ozone Monitoring Instrument. Updates and improvements in the data analysis algorithm are described and examples of measurements given. OMI BrO includes releases from the shelf ice in polar spring. HCHO includes production from biogenic sources, biomass burning, agriculture and urban pollution.

60 across-track pixels2,600 km total swath width

13×24 km2 at nadir*

13×120 km2 at edges*

13×13 km2 spatial zoom mode

The OMI Instrument: Swath Dimensions

OMI Spectral Coverage and Resolution

The OMI Instrument

The Ozone Monitoring Instrument (OMI) is a nadir-viewing spaceborne imaging spectrograph. It was launched on board the EOS Aura satellite in July, 2004, into a 705 km sun-synchronous orbit with a 1:45 PM crossing time in the ascending node.

VIS

UV-2

UV-1

Channel

26×480.63 nm365-500 nm

13×240.45 nm310-365 nm

13×240.42 nm270-310 nm

Spatial Resolution @ Nadir (km2)

Spectral resolution (FWHM)

Spectral Range

The Retrieval Algorithm

Direct fitting (i.e. non-DOAS) by nonlinear least-squares:

I = (Ioe-Absorbers + Ring + Pol1) * Pol2

Fitting includes: Ring effect calculated from a high reference solar spectrum; dynamic wavelength calibration using cross-correlation with the Fraunhofer spectrum; calculated correction for spectral undersampling)

Major recent modification:

Improved dark current correction to L1 data; De-spiking -dynamical removal of bad pixels during L1-L2 fitting Less cross-track “striping” in the L2 product

Vertical columns are determined using air mass factors calculated using the GEOS-CHEM 3-D chemistry and transport model and LIDORT radiative transfer model

The Results Shown Here

• Include BrO (338-358 nm), HCHO (337-357 nm) and glyoxal (CHO-CHO, 430-460 nm).• OClO is near or below the OMI detection limit for the time period that OMI data are available in the South (we should see it soon from analysis of data in the North) and should see it in data recently acquired for the Antarctic winter.

• The results show residual “striping” and across-track effect, but these are much reduced from what they were earlier this year.

BrO

BrO (bromine oxide) is an element in the destruction cycle of stratospheric ozone – the large abundance of BrO makes the catalytic Br+O3 cycle about 40 —100 times more efficient than that of chlorine. BrO is distributed fairly uniformly over the globe, with a stratospheric minimum at the equator of 2-4×1013 mol-cm-2. Tropospheric sources are few and include volcanic eruptions, releases form the Dead Sea, and bromine explosions on the polar shelf ice. BrO is being retrieved in the fitting window 338-358 nm.

BrO is the most advanced of the OMI SAO products, with a validation release currently pending. Plots show a day of BrO vertical columns (15 OMI orbits 09/29—30/2004) over Antarctica. Bromine explosions along the edges of the shelf ice (c.f., the average sea ice concentration for 10/2004 from the National Snow and Ice Data Center) and over Hudson Bay show the resolving power of OMI observations. Fitting uncertainties (1σ level) range between ~0.5-1.5××1013 mol-cm-2, translating into relative uncertainties as low as 4% in regions of elevated BrO and up to 40% towards the equator.

Sea ice concentration Oct 2004(National Snow and Ice Data Center)

OMI BrO Total Column (11 March 2005)

HCHO

HCHO (formaldehyde) is a volatile organic compound (VOC) mainly produced from methane oxidation and isoprene emissions. Its average lifetime is ~4 hrs, with the major sinks being photolysis and reaction with OH. HCHO has been routinely retrieved from GOME by SAO/Harvard, the optimum fitting window being 337—356 nm. Presently, a monthly average formaldehyde product from OMI is starting to emerge, but data quality is inferior to GOME, mainly due to calibration issues in the OMI L1b data product.

Plots show average formaldehyde geometric vertical column for the period of 09/24—10/19/2004. Elevated HCHO columns are present in regions of biomass burning (eastern Amazon, Central Africa, SE Asia), evident from the MODIS fire maps; isoprene emissions (western Amazon); and anthropogenic activity (Jakarta, Red Basin).

OMI MODIS Fire Map

Jakarta

Chongqing(Red Basin)

CHO-CHO

CHOCHO (glyoxal) is a volatile organic compound (VOC) recently observed in Mexico city [Volkamer et al., 2005]. It is produced from oxidation of a large number of other VOCs, and its main sinks are photolysis and reaction with OH. Since CHOCHO concentrations, unlike those of HCHO, are not affected directly by vehicle emissions, CHOCHO is a better indicator for VOC oxidation, the main source of photochemical smog. The average lifetime of glyoxal is ~1.3 hrs, which makes it challenging to be retrieved form satellite observation due tothe absence of significant accumulation and transport into the free troposphere; average loading is ~1.5×1015 mol/cm2, or about 8% of NO2.

CHOCHO retrievals from OMI are performed in the spectral window 430-460 nm. The images show the average CHOCHO geometric vertical columns for July 2005. Besides the obvious noise over the region of the South Atlantic Anomaly, two regions of enhanced CHOCHO are discernible: The area of northern Angola and the south-western part of the Democratic Republic of the Congo, and the Hong Kong/Guanghzou region.

Volkamer et al., DOAS measurements of glyoxal as an indicator for fast VOC chemistry in urban air, GRL 32L08806, doi:10.1029/2005GL022616, 2005

Acknowledgements

Special thanks to Rainer Volkamer for alerting us to the possibility of glyoxal measurements, and for providing reference spectra. This research is supported by NASA