R. A. Cox

Sea salt aerosol production and bromine release: Role of snow on sea ice

[1] Snow lying on sea ice could be a potentially important source of sea salt aerosol, as small snow particles, rich in salts, can be easily lifted into the air though blowing-snow events. Using a measured distribution of snow salinity on Antarctic sea ice and a blowing snow sublimation parameterization, we derive a method for estimating sea salt aerosol production, and bromine release, during blowing-snow events. Compared with sea salt aerosol production rates from the open ocean, we find that the aerosol production rate from snow can be more than an order of magnitude larger per unit area under typical weather conditions. The large sea ice cover may thus enhance the supply of sea salt to the polar lower atmosphere. This is consistent with observations of sea salt aerosol seasonality and with the ice-core record. This large emission of sea salt from snow also implies an additional tropospheric bromine source in these regions.

Formaldehyde production in clean marine air

The atmospheric mixing ratio of formaldehyde (HCHO) was measured continuously in clean marine air at the Australian Baseline Station at Cape Grim between mid November and mid December 1993. A diurnal cycle in mixing ratio was observed, consistent in amplitude with the expected photochemical source of this species. However the absolute values of the HCHO mixing ratio were higher than expected if the major source of HCHO under clean, low-NOx conditions is photolysis of methyl hydroperoxide (CH3OOH) derived from oxidation of methane by OH radicals. Possible explanations for elevated HCHO levels are considered. One sufficient to explain the observed HCHO levels is that reaction between hydroperoxy (HO2) and methylhydroperoxy (CH3O2) radicals may not proceed with 100% efficiency to form CH3OOH, but may have an additional branch yielding HCHO in clean marine conditions. There is some evidence from laboratory studies consistent with this proposal.

OIO and the atmospheric cycle of iodine

IO and BrO radicals are intermediates in the atmospheric photo-oxidation of iodo- and bromocarbons and can act as catalysts for ozone loss. We have studied the kinetics and mechanisms of the reactions of IO with itself and with BrO to establish their role in the atmospheric chemistry of iodine. We have found that iodine dioxide, OIO, is produced in these reactions. The results of these and other experimental observations together with a recent computational study suggest an unexpectedly high photochemical stability for OIO. It is shown that OIO formation and its attachment to particles could account for the high enrichment of iodine in the small size fraction of marine aerosol, which is important for the transport of iodine from the sea to the continents. OIO may be a route to the formation of iodate, which is present in atmospheric precipitation. OIO formation also implies a reduced efficiency for iodine catalysed ozone loss.

An overview of the EASOE Campaign

The scientific planning of the EASOE campaign is outlined and the various constituent and meteorological data sets are described.

Activation of stratospheric chlorine by reactions in liquid sulphuric acid

Active chlorine release on H2SO4 aerosol particles via the reaction of HOCl and HCl in solution is discussed. Based on current laboratory data, the process is shown to have an appreciable rate at temperatures below about 195 K, when the aerosol particles become sufficiently dilute to allow both HCl dissolution and fast ClONO2 hydrolysis. Evolution of part-per-billion concentrations of Cl2 could be significant in cases in which volcanically enhanced aerosol remains liquid until the frost point (≈ 190 K) as observed on several occasions in the Arctic stratosphere.

UV-visible absorption cross sections of gaseous Br2O and HOBr

The absorption cross-section of gaseous HOBr was determined over the wavelength range 235 to 430 nm with a spectral resolution of 0.6 nm full width at half maximum (FWHM) using a diode array spectrometer. The spectrum of HOBr shows two main absorption bands with maxima near 282 nm (σ = (3.1 ± 0.4) × 10−19 cm2 molecule−1 and 350 nm (σ = 12.5 ± 1.6) × 10−20 cm2 molecule−1) extending out to 430 nm. The absorption cross-sections in the first absorption band are in good agreement with a recent determination; the cross-sections in the second band however, are approximately a factor of 2.5 larger than previously determined. In addition we provide evidence in support of a weak band in HOBr around 440 nm (σ ≈ 7.5 × 10−21 cm2 molecule−1) as observed by Barnes et al. [1996]. The absorption cross-section of Br2O, which was used to prepare HOBr, was determined over the wavelength range 230 to 750 nm. The spectrum shows four absorption bands with maxima at 314 nm (σ = (2.1 ± 0.3) × 10−18 cm2 molecule−1), 350 nm (σ = (1.9 ± 0.2) × 10−18 cm2 molecule−1), 520 nm (σ = (4.4 ± 0.5) × 10−20 cm2 molecule−1), and 665 nm (σ = (6.2 ± 0.9) × 10−20 cm2 molecule−1). The visible bands at 520 nm and 660 nm have not been observed previously. The equilibrium constant, for the reaction Br2O + H2O ⇔ 2HOBr was determined to be 0.037 ± 0.004 at 298 K. Measurement of the equilibrium constant as a function of temperature enabled values for ΔH298 K = (13.0 ± 0.5) kJ mol−1 and ΔS298 K = (16 ± 2) J mol−1 K−1 to be determined. The absorption cross-section data for HOBr have been used in a photochemical box model to investigate the significance of these results in the lower stratosphere. The model results are compared with observations during a recent Stratospheric Photochemistry, Aerosols and Dynamics Expedition (SPADE) and show that the revised HOBr cross-section, coupled to the rapid heterogeneous conversion of BrONO2 to HOBr, can account quantitatively for the abrupt morning rise in HOx.

Temperature-dependent absorption cross sections for HNO3 and N2O5

Absorption cross-sections for HNO3 and N2O5 have been measured in the wavelength region 220 - 450 nm, using a dual beam diode array spectrometer with a spectral resolution of 0.3 nm. The results for both compounds are in good agreement with recommended values at room temperature. Cross-sections of both HNO3 and N2O5 show a marked reduction with decreasing temperature in the range 295 - 233 K.