Halogen cycling and aerosol pH in the Hawaiian marine boundary layer
1Center for Global Change Science, Massachusetts Institute of Technology, Cambridge, MA, USA
2Air Chemistry Division, Max Planck Institute for Chemistry, Mainz, Germany
3Swedish Environmental Research Institute, G¨otheborg, Sweden
4Department of Environmental Sciences, University of Virginia, Charlottesville, VA, USA
5Department of Meteorology, Pennsylvania State University, University Park, PA, USA; now at 2 (above)
6Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA, USA
*Now at: Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, and Mount Washington Observatory, North Conway, NH, USA
Abstract. Halogen species (HCl* (primarily HCl), Cl* (including Cl2 and HOCl), BrO, total gaseous inorganic Br and size-resolved particulate Cl− and Br −) and related chemical and physical parameters were measured in surface air at Oahu, Hawaii during September 1999. Aerosol pH as a function of particle size was inferred from phase partitioning and thermodynamic properties of HCl. Mixing ratios of halogen compounds and aerosol pHs were simulated with a new version of the photochemical box model MOCCA that considers multiple aerosol size bins.
Inferred aerosol pHs ranged from 4.5 to 5.4 (median 5.1, n=22) for super-mm (primarily sea-salt) size fractions and 2.6 to 5.3 (median 4.6) for sub-μm (primarily sulphate) fractions. Simulated pHs for most sea-salt size bins were within the range of inferred values. However, simulated pHs for the largest size fraction in the model were somewhat higher (oscillating around 5.9) due to the rapid turnover rates and relatively larger infusions of sea-salt alkalinity associated with fresh aerosols.
Measured mixing ratios of HCl* ranged from <30 to 250 pmol mol-1 and those for Cl* from <6 to 38 pmol mol-1. Simulated HCl and Cl* (Cl+ClO+HOCl+Cl2) mixing ratios ranged between 20 and 70 pmol mol−1 and 0.5 and 6 pmol mol−1, respectively. Afternoon HCl* maxima occurred on some days but consistent diel cycles for HCl* and Cl* were not observed. Simulated HCl did vary diurnally, peaking before dusk and reaching a minimum at dawn. While individual components of Cl* varied diurnally in the simulations, their sum did not, consistent with the lack of a diel cycle in observed Cl*.
Mixing ratios of total gaseous inorganic Br varied from <1.5 to 9 pmol mol−1 and particulate Br − deficits varied from 1 to 6 pmol mol−1 with values for both tending to be greater during daytime. Simulated mixing ratios of Br species were consistent with those of observed total gaseous inorganic Br, however the diel cycle was reversed with higher values predicted at night. This may be due to cloud processing, which is not considered in the current version of MOCCA. Measured BrO was never above detection limit (~2 pmol mol-1) during the experiment, however relative changes in the BrO signal during the 3-hour period ending at 11:00 local time were mostly negative, averaging −0.3 pmol mol−1. Both of these results are consistent with MOCCA simulations of BrO mixing ratios.
Increasing the sea-salt mixing ratio in MOCCA by ~25% (within observed range) led to a decrease in O3 of ~16%. The chemistry leading to this decrease is complex and is tied to NOx removal by heterogeneous reactions of BrNO3 and ClNO3. The sink of O3 due to the catalytic Cl-ClO and Br-BrO cycles was estimated at −1.0 to −1.5 nmol mol−1 day−1, a range similar to that due to O3 photolysis in the MOCCA simulations.