Inorganic Bromine and Iodine over the Tropical Eastern and Western Pacific Ocean: Insights from Recent Aircraft Campaigns

Tropospheric bromine and iodine together are responsible for ~20% of the global tropospheric ozone loss. Halogens are emitted from oceans as biogenic very short lived organic species (VSLS: CHBr₃, CH₂Br₂, CH₃Br; CH₃I, CH₂I₂, etc.), and in inorganic forms (sea spray bromide; HOI, I₂, etc). While inorganic sources account for ~75% of the global halogen source, their contribution to the total gas-phase inorganic bromine (Br_y) and iodine (I_y) in the upper troposphere and lower stratosphere (UTLS) remains poorly understood. A complication arises since carbon loss by VSLS (due to oxidation) forms products that are indistinguishable from the Br_y and I_y species that are quickly derived from inorganic sources, and may reach the UTLS as a result of deep convective transport. Measurements to constrain gas-phase Br_y (= Br + BrO + HOBr + Br₂ + BrCl + HBr + BrNO₃ + BrNO₂) and I_y (= I + IO + OIO + HOI + I₂+ INO₃ + I₂O_x, x=2,3,4) are generally sparse. In the troposphere, knowledge about Br_y and I_y relies in large part on measurements of bromine oxide (BrO) and iodine oxide (IO) radicals, which are important components of BrO_x (=Br + BrO) and IO_x (= I + IO) that affect the ozone layer, and regulate tropospheric ozone.

This presentation summarizes results from two aircraft campaigns that measured BrO and IO profiles in the open atmosphere over remote oceans (0-15km altitude) by means of the University of Colorado Airborne MAX-DOAS instrument (CU AMAX-DOAS) aboard the NSF/NCAR GV aircraft over the tropical Eastern Pacific Ocean, tEPO (Volkamer et al., 2015, doi: 10.5194/amt-8-2121-2015; Wang et al., 2015, doi: 10.1073/pnas.1505142112) and the tropical Western Pacific Ocean, tWPO (Koenig et al., 2017, doi: 10.5194/acp-2017-572) as part of the TORERO (Tropical Ocean tRoposphere Exchange of Reactive Halogen Species and Oxygenated Hydrocarbons, Jan-Feb 2012) and CONTRAST (Convective Transport of reactions species in the troposphere, Jan-Feb 2014) field campaigns. These measurements are used in conjunction with an experimentally constrained box model to infer inorganic Br_y and I_y profiles in recent convective, aged tropospheric, and lower stratospheric air masses. The data sets reveal a suprising and consistent structure with local Br_y (and I_y) maxima in convective outflow, and local minima in the aged UTLS, and provide a challenging test for models of our understanding of the halogen sources, gas-/particle partitioning, and the processes that influence the O₃ chemistry in this climate sensitive region of the atmosphere.