NSFGEO-NERC:BLEACH

The abundance of reactive halogens in the troposphere may have a profound influence on the oxidation capacity of the atmosphere both directly as oxidants of hydrocarbons and other reduced trace gases and indirectly through their influence on hydroxyl radical and ozone abundances. However, our understanding of the abundance and the spatial and temporal variability of tropospheric reactive halogens remains highly uncertain. Only recently have global chemical transport models begun to include comprehensive reactive halogen chemistry. Recent modeling studies suggest that reactive halogens significantly reduce the oxidation capacity of the atmosphere [Schmidt et al., 2016], and that reactive halogens have increased since the preindustrial due to anthropogenic emissions [Sherwen et al., 2017]. Modeling studies also suggest positive interactions between different reactive halogen species (Cl, Br, I), with increases in one reactive halogen species increasing the others [Wang et al., 2019]. However, assessment of these model results is hindered by a dearth of observational constraints. Additionally, observations of reactive halogen species (e.g., BrO) using different measurement techniques can yield very different results, further complicating model assessment.
We propose to measure an unprecedented set of reactive gaseous and particulate halogen abundances at the BIOS Tudor Hill Marine Atmospheric Observatory during two seasons (winter and summer) with three different instruments.
Observations will include what we expect to be among the most abundant gaseous bromine, chlorine and iodine species (HCl, BrO, ClNO2, IO, among several others) and their aerosol-phase counterparts (Br-, Cl-, I-) from which the gas-phase species are derived. For gas-phase measurements, we will three different observational techniques (CIMS, LP-DOAS, TILDAS) allowing for comparison of the measurements of some key reactive halogens while also expanding the suite of measured halogen species. Since heterogeneous reactions on sea salt aerosol (SSA) and the ocean surface are the largest source of tropospheric reactive halogens, a tropical island is an ideal location. Additionally, Bermuda experiences a seasonality in wind direction due to the summertime presence of the Bermuda high-pressure system, resulting in transport of pollution from North America in winter and clean, marine conditions during summer, allowing us to test our previous model results suggesting that anthropogenic activity increases tropospheric reactive halogen abundances, with implications for the oxidizing capacity of the atmosphere.
Model simulations using the GEOS-Chem global chemical transport model will be integrated into the research plan, and the model's chemical mechanism will be further developed as part of this project.
The proposed field campaign will provide currently missing observational constraints for the model, allowing for model assessment and improvement of the model's chemical mechanism for reactive halogen chemistry. The model will also be used to help interpret the observations, which will be accomplished through model sensitivity simulations coupled with comparison of the observations with the model. Measurements proposed here will provide a quantitative observational constraint for the dependence of reactive halogen abundances on pollution levels, as well as allow us to assess the model's representation of the abundance and speciation of reactive halogens at a tropical, marine location and their interactions with one another.