Abstract
The global ocean/atmosphere flux of methyl bromide has been estimated from shipboard measurements of the saturation anomaly. When such data are extrapolated globally on the basis of constant saturation anomaly, the ocean is a net sink for methyl bromide [Lobert et al, 1995]. The same data can also be extrapolated on the basis of steady‐state production rate of methyl bromide in the water column, allowing regional and seasonal variations in temperature to affect the saturation anomaly. We have carried out this type of extrapolation, and we found that the oceans are a strong net source of methyl bromide to the atmosphere. The difference arises mainly due to slow degradation rates in water of higher latitudes. A reduction of the applied production rate by more than 35% is needed in order to switch the ocean from a source to a sink of methyl bromide. These results demonstrate the sensitivity of current estimates of oceanic flux to assumptions about methyl bromide production and destruction in the water column.
Highlights
AnomalyW. e havecarriedoutthistypeof extrapolationa,ndwe Model Description foundthatthe oceansare a strongnet sourceof methylbromide to the atmosphereT. he differencearisesmainly due to slow In this effortwe haveusedthe approachof Butler [1994] to degradatiornatesin waterof higherlatitudesA. reductionof the model the methyl bromide concentrationin discrete2ø x 2ø appliedproductionrateby morethan35% is neededin orderto regionsof the surfacewind-mixedareaof the ocean.The basic switchthe oceanfrom a sourceto a sink of methylbromide. equationdescribingthe evolutionof the concentratioonf methyl Theseresultsdemonstrattehe sensitivityof currentestimatesof bromidein eachcell extendingfrom the air-oceaninterfaceto oceanicflux to assumptionasboutmethylbromideproduction thebottomof thesurfacelayeris: and destruction in the water column
The implicit assumptionis that reducing is the eddy diffusionrate constantfor the removalof methyl anthropogeniecmissionsof methyl bromidewill lead to a bromiddeownwarfrdomthesurfacleayer(myr') andz isthe reductionin its concentratioinn the atnospherein, a mmmer depthof thesurfacelayer(m)
The methyl bromidesolubility,H, was calculatedby upwellingregionsT. heyextrapolatetdheseobservationgslobally applyinga salting out coefficient[De Bnyn and Saltzman, and concludedthat the oceansact as a net sink for methyl unpublishedata]to thepurewaterdatareportedby Wilhelmet bromide
Summary
Methyl bromide(CH3Br)is consideredto be a majorsourceof wherecoisthemethyblromidceoncentrat(imonolm'3)p, oisthe bromineto the stratospheraendan importantcontributotro the productionof methylbromidein the grid cell, ka is the aquatic destructioonf stratospheroiczone[WMO,1994].Thisissuehas degradatiroanteconsta(nytr'),Kwistheair-seeaxchangceoefled to attemptsto regulatethe antlu:opogenuicse of methyl ficien(tmyr4)p, gisthepartiapl ressuorefmethyblromidoever bromidethroughthe Montreal Protocoland the Clean Air Act thegrid(atm)H, istheHenryL'sawconsta(nmt3attomol')k, ,a [USEPA, 1993].
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