Iron(III)-Induced Activation of Chloride and Bromide from Modeled Salt Pans

Abstract

The photochemistry of halides in sea spray aerosol, on salt pans, and on other salty surfaces leads to a formation of reactive halogen species. We investigated the photochemical formation of atomic chlorine (Cl) and bromine (Br) in the gas phase in the presence of laboratory-modeled salt pans consisting of sodium chloride doped with iron(III) chloride hexahydrate (0.5 and 2 wt %). The samples were spread on a Teflon sheet and exposed to simulated sunlight in a Teflon smog chamber in purified, humidified air in the presence of a test mixture of hydrocarbons at the ppb level to determine Cl, Br, and OH formation by the radical clock method. Driven by the photolytic reduction of Fe(III) to Fe(II), the production rates of the Fe(III)-doped NaCl salt samples (up to10(7) atoms cm(-3) s(-1)) exceeded the release of Cl above a pure NaCl sample by more than an order of magnitude in an initially O3-free environment at low NOX. In bromide-doped samples (0.5 wt % NaBr), a part of the Cl release was replaced by Br when Fe(III) was present. Additions of sodium sulfate, sodium oxalate, oxalic acid, and catechol to NaCl/FeCl3 samples were found to restrain the activation of chloride.

Climate engineering by mimicking natural dust climate control: the iron salt aerosol method

“…The ISA-induced increase in q Br concentration at seasalt-containing tropospheric conditions has been confirmed (Wittmer et al, 2015b). This establishes ISA as part of an ozone-depleting reaction cycle and additional cooling stage.…”

“…Dorfman et al (2015) found recent loess dust sedimentation rates in the Alaskan Arctic Burial Lake of 0.15 mm yr −1 . According to research results from artificial iron-doped salt pans (Wittmer et al, 2015b), iron-salt-doped sea-salt aerosols (Wittmer et al, 2015a) or sea-salt-doped iron oxide aerosols or pure iron oxide aerosols in contact with gaseous HCl chloride and bromide in sunlit surfaces are oxidized to q Cl and q Br by photoreduced Fe(III) if the pH of the reaction media is 3.5 or lower.…”

“…These salt environments gain iron from precipitating dust or from iron-containing brines they have precipitated from. As soon as these environments become acidic, they oxidize CH 4 by iron photolysis-induced q Cl (Wittmer et al, 2015b).…”

Iron(III)-induced activation of chloride from artificial sea-salt aerosol

“…In particular, hypersaline environments, such as the Dead Sea or Australian salt lakes, [14] offer conditions that promote iron-induced halogen release, as recently investigated by Wittmer et al for modelled salt surfaces. [15] The present study, however, concentrates on the effect of Fe 3þ in saline aerosols that can also be found in these regions and when mineral aerosol particles mix with sea salt. In particular, chloride (Cl À ) enhances the dissolution of iron.…”

“…A more detailed summary and description of Fe III -induced Cl 2 formation can be found in Lim et al [24] or Wittmer et al [15] In addition to the photosensitive iron-induced halogen release mechanism, further trace gas species, such as O 3 , NO x and sulfur dioxide (SO 2 ), possibly influence Cl 2 release, mainly when dissolved in the aqueous phase where they can change the pH, [25] coordinate with Fe 2þ or Fe 3þ , [26] scavenge Cl [22] or additionally activate halides by the known heterogeneous mechanisms. [13] Concerning O 3 , Sadanaga et al [27] observed an enhancement of the O 3 uptake rate and Cl 2 release in the presence of water-soluble Fe 3þ and O 3 in the dark.…”