Person:
Holmes, Christopher

Profile Picture

Email Address

AA Acceptance Date

Birth Date

Research Projects

Organizational Units

Job Title

Last Name

Holmes

First Name

Christopher

Name

Holmes, Christopher
Author's Publications: search.filters.isAuthorOfPublication.df58c875-eca0-4d71-af64-8be846f6c717

Search Results

Now showing 1 - 2 of 2
  • Thumbnail Image
    Publication
    Global Lifetime of Elemental Mercury Against Oxidation by Atomic Bromine in the Free Troposphere
    (American Geophysical Union, 2006) Holmes, Christopher; Jacob, Daniel; Yang, Xin
    We calculate the global mean atmospheric lifetime of elemental mercury (Hg0) against oxidation by atomic bromine (Br) in the troposphere by combining recent kinetic data for the Hg-Br system with modeled global concentrations of tropospheric Br. We obtain a lifetime of 0.5–1.7 years based on the range of kinetic data, implying that oxidation of Hg0 by Br is a major, and possibly dominant, global sink for Hg0. Most of the oxidation takes place in the middle and upper troposphere, where Br concentrations are high and where cold temperatures suppress thermal decomposition of the HgBr intermediate. This oxidation mechanism is consistent with mercury observations, including in particular high gaseous Hg(II) concentrations in Antarctic summer. Better free-tropospheric measurements of bromine radicals and further kinetic study of the Hg-Br system are essential to more accurately assess the global importance of Br as an oxidant of atmospheric Hg0.
  • Thumbnail Image
    Publication
    Sources and Deposition of Reactive Gaseous Mercury in the Marine Atmosphere
    (Elsevier, 2009) Holmes, Christopher; Jacob, Daniel; Mason, Robert P.; Jaffe, Daniel A.
    Observations of reactive gaseous mercury (RGM) in marine air show a consistent diurnal cycle with minimum at night, rapid increase at sunrise, maximum at midday, and rapid decline in afternoon. We use a box model for the marine boundary layer (MBL) to interpret these observations in terms of RGM sources and sinks. The morning rise and midday maximum are consistent with oxidation of elemental mercury (Hg0) by Br atoms, requiring <2 ppt BrO in most conditions. Oxidation of Hg0 by Br accounts for 35–60% of the RGM source in our model MBL, with most of the remainder contributed by oxidation of Hg0 by ozone (5–20%) and entrainment of RGM-rich air from the free troposphere (25–40%). Oxidation of Hg0 by Cl is minor (3–7%), and oxidation by OH cannot reproduce the observed RGM diurnal cycle, suggesting that it is unimportant. Fitting the RGM observations could be achieved in the model without oxidation of Hg0 by ozone (leaving Br as the only significant oxidant) by increasing the entrainment flux from the free troposphere. The large relative diurnal amplitude of RGM concentrations implies rapid loss with a lifetime of only a few hours. We show that this can be quantitatively explained by rapid, mass-transfer-limited uptake of RGM into sea-salt aerosols as HgCl3− and HgCl42−. Our results suggest that 80–95% of HgII in the MBL should be present in sea-salt aerosol rather than gas-phase, and that deposition of sea-salt aerosols is the major pathway delivering HgII to the ocean.