Bromine Conundrum over Tropical Oceans: A Missing BrO Sink in the Remote Marine Boundary Layer

Tropospheric halogens are emitted from oceans into the atmosphere in organic form, and as sea spray aerosols. Heterogeneous chemistry on acidic aerosol surfaces lead to the release of bromide (and chloride) to the gas-phase, and leaves aerosols depleted in bromide (and chloride) relative to their sodium ratios in sea-water. Sea-spray is widely considered to be the largest source of tropospheric halogens globally. While state-of-the-art atmospheric models differ in their treatment of this halogen source, models consistently predict elevated BrO that is widespread over oceans. However, such elevated BrO is rarely observed by field observations, and this mismatch presents an unresolved conundrum in our understanding of halogen chemistry over remote oceans. The tendency of atmospheric models to over-predict BrO remains poorly tested at tropical latitudes, where ~75% of the global O₃ and CH₄destruction occurs.

This presentation investigates the bromine closure over the tropical Central and Eastern Pacific Ocean, including an area where the MECCA-CAM global chemistry transport model predicts a global hotspot of BrO. This hotspot remains untested by previous field observations of BrO. Simultaneous measurements of gas-phase BrO, and size resolved aerosol bromide (chloride, sodium etc) have been conducted during the cruise component of the Tropical Ocean tRoposphere Exchange of Reactive Halogens and OVOC (TORERO) field campaign aboard the NOAA RV Ka'imimoana. These observations are used to infer total gas-phase Br_y. Sea-salt bromide is found to be depleted in the aerosols relative to sea-water, consistent with the efficient bromine activation. However, very low gas-phase BrO observations below the detection limit (<<1 ppt) were observed, and are found to be inconsistent with the model. We conclude that a gas-phase process is missing that converts BrO_x (= Br + BrO) into gas-phase Br_y. Results from initial laboratory experiments to test this hypothesis are also presented. Collectively, the laboratory and field evidence point to existing bias in our perception of bromine sources, and provide a possible solution for the bromine conundrum over remote oceans.