Trace amounts of Br and I are ubiquitous in non-sea-salt aerosol

Abstract:

Halogens from biogenic sources are presumed to make up a large fraction of the tropospheric halogen emissions. In the troposphere, halogens have been shown to reduce global tropospheric O₃ burdens by ~20%, which reduces global mean OH concentrations ~10% and increases CH₄ lifetime by ~1 year. On a per-atom basis, Br is >60 times more destructive to O₃ than Cl, and I is ~2 times more destructive than Br. Thus, sub-ppt levels of reactive Br and I can affect the oxidative capacity of the troposphere. The tropospheric Br_y and I_y budgets, however, are poorly constrained by observations, especially in the global remote troposphere. A hitherto missing piece of the tropospheric Br_y and I_y budgets are halogens that partition into non-sea-salt aerosol (nSSA).

To assess the global abundance of aerosol-bound Br and I, we flew a single-particle mass spectrometer during the NASA Atmospheric Tomography mission (ATom). ATom consists of four series of seasonal flights over the remote Pacific and Atlantic oceans with near pole-to-pole latitude coverage. During flight, near continuous altitude profiles were sampled from ~0.18 to 12 km, resulting in a global-scale aerosol composition dataset with single-particle resolution. Single-particle resolution allows us to infer the aerosol halogen sources, whether formation is primary or secondary, and gain insights into the physical and chemical factors that govern aerosol Br and I formation.

During Atom, we find that ~33% of nSSA contain Br and I. Globally, nSSA Br and I concentrations were found to ~0.2 and ~0.1 pptv, respectively. Currently, it is thought that oceanic emissions are the largest source of aerosol Br and I to the atmosphere; however, our measurements show that biomass burning may be an equal or greater source of Br and I to nSSA. Comparing Br and I concentrations in biomass burning and background particles suggest that nSSA Br and I are formed through secondary processing of biogenic halogens outside of biomass burning plumes. Finally, using an analysis of upper tropospheric/lower stratospheric nSSA, we corroborate that organic aerosol may play a role in binding Br and I in particles.