[1] The analysis for BrO using the technique of differential optical absorption spectroscopy as applied to spectra of light scattered from the zenith sky has historically presented something of a challenge, leading to uncertainty about the accuracy of measurements. This has largely been due to the large sensitivity of the measurement to many analysis parameters and due to the small size of the absorption features being measured. BrO differential slant columns have been measured by six different groups taking part in an intercomparison exercise at Observatoire de Haute-Provence in France from 23 to 27 June 1996. The data are analyzed in a collaborative attempt to improve the overall analysis for BrO through investigation of a series of sources of errors in the instrumentation, calibration, input to the analysis, and the spectral analysis itself. The study included comprehensive sensitivity tests performed using both actual measurements and synthetic data. The latter proved invaluable for assessing several aspects of the spectral analysis without the limitations of spectral quality and instrument variability. The most significant sources of error are identified as the wavelength calibration of several of the absorption cross sections fitted and of the measured spectra themselves, the wavelength region of the fitting, the temperature dependence of the O 3 absorption cross sections, failure to adequately account for the so-called I 0 effect, inadequate offset correction, and inadequate measurement of the individual instrument slit functions. Recommendations for optimal analysis settings are presented, and comparing the results from the analysis of the campaign data shows BrO differential slant column observations from the various groups to be in agreement to within 4% on average between 87°and 90°s olar zenith angle, with a scatter of 16%.

[1] Ground-based zenith sky UV-visible measurements of stratospheric bromine monoxide (BrO) slant column densities are compared with simulations from the SLIMCAT three-dimensional chemical transport model. The observations have been obtained from a network of 11 sites, covering high and midlatitudes of both hemispheres. This data set gives for the first time a near-global picture of the distribution of stratospheric BrO from ground-based observations and is used to test our current understanding of stratospheric bromine chemistry. In order to allow a direct comparison between observations and model calculations, a radiative transfer model has been coupled to the chemical model to calculate simulated slant column densities. The model reproduces the observations in general very well. The absolute amount of the BrO slant columns is consistent with a total stratospheric bromine loading of 20 ± 4 ppt for the period 1998-2000, in agreement with previous estimates. The seasonal and latitudinal variations of BrO are well reproduced by the model. In particular, the good agreement between the observed and modeled diurnal variation provides strong evidence that the BrO-related bromine chemistry is correctly modeled. A discrepancy between observed and modeled BrO at high latitudes during events of chlorine activation can be resolved by increasing the rate constant for the reaction BrO + ClO ! BrCl + O 2 to the upper limit of current recommendations. However, other possible causes of the discrepancy at high latitudes cannot be ruled out.

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Measurement of the background levels and study of the chemistry of trace organic carbon species in the remote marine troposphere occurred during an April-July 1987 SAGA II cruise in remote regions of the Pacific and Indian Oceans. Measured compounds included carboxylic acids, formaldehyde, light hydrocarbons (C2-C4), and ozone. The results show seasonal, diel, and spatial dependencies for the organic acids. Distinct latitudinal gradients are seen for most sampled compounds. Formic acid is well correlated with suspected precursors, formaldehyde and light hydrocarbons. Acetic acid follows a similar pattern as formic acid, although its precursors are as yet undefined. Did patterns of low amplitude for the organic acids in the remote marine troposphere suggest a natural contribution to tropospheric photochemistry, and to the global carbon cycle as well. For the northern hemisphere Pacific Ocean, the mean formic acid mixing ratio was 0.80 + 0.30 ppbv, the mean acetic acid value was 0.78 + 0.32 ppbv. For the southern hemisphere Pacific Ocean, formic acid averaged 0.22 + 0.13 ppbv, for acetic acid, the mean was 0.28 +_ 0.18 ppbv. For the northem hemisphere Indian Ocean, the mean formic acid mixing ratio was 0.75 + 0.24 ppbv, and the mean acetic acid value was 0.69 + 0.27 ppbv. For the southern hemisphere Indian Ocean, the mean formic acid value was 0.19 + 0.17 ppbv, and the mean acetic acid value was 0.29 + 0.16 ppbv. Highest levels of organic acids were encountered near known anthropogenic source regions, in air masses of continental origin, or near regions of naturally produced alkenes (C 2, C3). The ozone-alkene oxidation scheme appears to play a major role in gas phase organic acid production in the remote marine troposphere. Nighttime gas phase deposition of the organic acids onto the ocean surface appears to be a major sink. 2Now at College of Sciences, University of Maine, Orono. in the gas phase. Analogous aqueous-phase reactions leading 3Now at NOAA, GMCC Samoa Observatory, Pago Pago, American Samoa. to the production of acetic acid are recognized as being 4Now at Joint Institute for the Study of the Atmosphere and Ocean, negligibly slow [Jacob and Wofsy, 1988a] and therefore not a University of Washington, Seatfie. significant source. Such reactions are also limited by the lower solubility of acetaldehyde. Predicted gas-phase formic acid mixing ratios calculated from aqueous phase reaction Copyright 1990 by the American Geophysical Union. mechanisms are between 35 and 65 pptv [Charneides and Paper number 90JD01223. Davis, 1983]. Since it is generally agreed that aqueous-phase 0148-0227/90/90JD-01223505.00 production of organic acids alone cannot account for the 16,391 16,392 ARLANDER ET AL.: GASEOUS OXYOENATED HYDROCARBONS FTIR spectroscopic evidence for the formation of CH2(OH)OOH in D.R. Cronn, College of Sciences, University of Maine, Orono, ME the gas phase reaction of HO e with CH:O, Chem. Phys. Lett., 75, 04469. 533, 1980b. J.C. Farmer, NOAA, GMCC Samoa Observatory, Pago Pago, Norton, R. B., Measurement ...