Halogen cycling and aerosol pH in the Hawaiian marine boundary layer A. A. P. Pszenny1,*, J. Moldanová2,3, W. C. Keene4, R. Sander2, J. R. Maben4, M. Martinez5, P. J. Crutzen2,6, D. Perner2, and R. G. Prinn1 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, Göteborg, 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-mm (primarily
sulphate) fractions. Inferred daytime pHs tended to be slightly lower than those at night, although
daytime median values did not differ statistically from nighttime medians. 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
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
Brt and Br - mixing ratios and enrichment factors (EFBr) were similar to those observed, with early
morning maxima and dusk minima. However, the diel cycles differed in detail among the various simulations. In low-salt simulations, halogen
cycling was less intense, Br - accumulated and Brt and
EFBr increased slowly overnight. In higher-salt simulations with more intense halogen cycling,
Br - and EFBr decreased and Brt increased rapidly after dusk. Cloud processing, which is not
considered in this version of MOCCA, may also affect these diel cycles (von Glasow et al., 2003). 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.
Citation: Pszenny, A. A. P., Moldanová, J., Keene, W. C., Sander, R., Maben, J. R., Martinez, M., Crutzen, P. J., Perner, D., and Prinn, R. G.: Halogen cycling and aerosol pH in the Hawaiian marine boundary layer, Atmos. Chem. Phys., 4, 147-168, doi:10.5194/acp-4-147-2004, 2004.