Composite global emissions of reactive chlorine from anthropogenic and natural sources: Reactive Chlorine Emissions Inventory

Abstract

Emission inventories for major reactive tropospheric Cl species (particulate Cl, HCl, ClNO 2 , CH 3 Cl, CHCl 3 , CH 3 CCl 3 , C 2 Cl 4 , C 2 HCl 3 , CH 2 Cl 2 , and CHClF 2 ) were integrated across source types (terrestrial biogenic and oceanic emissions, sea‐salt production and dechlorination, biomass burning, industrial emissions, fossil‐fuel combustion, and incineration). Composite emissions were compared with known sinks to assess budget closure; relative contributions of natural and anthropogenic sources were differentiated. Model calculations suggest that conventional acid‐displacement reactions involving S (IV) + O 3 , (IV) + O 3 H 2 O 2 , and H 2 SO 4 and HNO 3 scavenging account for minor fractions of sea‐salt dechlorination globally. Other important chemical pathways involving sea‐salt aerosol apparently produce most volatile chlorine in the troposphere. The combined emissions of CH 3 Cl from known sources account for about half of the modeled sink, suggesting fluxes from known sources were underestimated, the OH sink was overestimated, or significant unidentified sources exist. Anthropogenic activities (primarily biomass burning) contribute about half the net CH 3 Cl emitted from known sources. Anthropogenic emissions account for only about 10% of the modeled CHCl 3 sink. Although poorly constrained, significant fractions of tropospheric CH 2 Cl 2 (25%), C 2 HCl 3 (10%), and C 2 Cl 4 (5%) are emitted from the surface ocean; the combined contributions of C 2 Cl 4 and C 2 HCl 3 from all natural sources may be substantially higher than the estimated oceanic flux.