Abstract
Abstract. It is an open question how localized elevated emissions of bromoform (CHBr3) and other very short-lived halocarbons (VSLHs), found in coastal and upwelling regions, and low background emissions, typically found over the open ocean, impact the atmospheric VSLH distribution. In this study, we use the Lagrangian dispersion model FLEXPART to simulate atmospheric CHBr3 resulting from assumed uniform background emissions, and from elevated emissions consistent with those derived during three tropical cruise campaigns. The simulations demonstrate that the atmospheric CHBr3 distributions in the uniform background emissions scenario are highly variable with high mixing ratios appearing in regions of convergence or low wind speed. This relation holds on regional and global scales. The impact of localized elevated emissions on the atmospheric CHBr3 distribution varies significantly from campaign to campaign. The estimated impact depends on the strength of the emissions and the meteorological conditions. In the open waters of the western Pacific and Indian oceans, localized elevated emissions only slightly increase the background concentrations of atmospheric CHBr3, even when 1∘ wide source regions along the cruise tracks are assumed. Near the coast, elevated emissions, including hot spots up to 100 times larger than the uniform background emissions, can be strong enough to be distinguished from the atmospheric background. However, it is not necessarily the highest hot spot emission that produces the largest enhancement, since the tug-of-war between fast advective transport and local accumulation at the time of emission is also important. Our results demonstrate that transport variations in the atmosphere itself are sufficient to produce highly variable VSLH distributions, and elevated VSLHs in the atmosphere do not always reflect a strong localized source. Localized elevated emissions can be obliterated by the highly variable atmospheric background, even if they are orders of magnitude larger than the average open ocean emissions.
Highlights
Very short-lived halocarbons (VSLHs) with atmospheric lifetimes shorter than 6 months from natural oceanic sources are dominated by brominated and iodinated compounds (Carpenter and Liss, 2000; Quack et al, 2004; Law et al, 2006)
We focus on the VSLH bromoform (CHBr3) since most organic oceanic bromine is released into the atmosphere in this form
For the top-down method, the emissions of VSLHs are constrained by the measured abundances at the “top” so that model simulations based on the constrained global emission estimates reproduce the observed atmospheric concentrations
Summary
Very short-lived halocarbons (VSLHs) with atmospheric lifetimes shorter than 6 months from natural oceanic sources are dominated by brominated and iodinated compounds (Carpenter and Liss, 2000; Quack et al, 2004; Law et al, 2006). For the top-down method, the emissions of VSLHs are constrained by the measured abundances at the “top” (atmosphere) so that model simulations based on the constrained global emission estimates reproduce the observed atmospheric concentrations. We use the Lagrangian particle dispersion model FLEXPART to investigate the transport and atmospheric distribution of VSLHs. Taking bromoform as an example, we compare the atmospheric signals estimated from the elevated and hot spot emissions measured during the ship campaigns to the distribution derived from only uniform background emissions. We present the observed hot spots of CHBr3 emissions, and compare the simulated atmospheric mixing ratios resulting from elevated emissions during three campaigns with the background values in Sect.
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