Tropospheric budget of reactive chlorine

Abstract

Reactive chlorine in the lower atmosphere (as distinguished from chlorofluorocarbon‐derived chlorine in the stratosphere) is important to considerations of precipitation acidity, corrosion, foliar damage, and chemistry of the marine boundary layer. Many of the chlorine‐containing gases are difficult to measure, and natural sources appear to dominate anthropogenic sources for some chemical species. As a consequence, no satisfactory budget for reactive chlorine in the lower atmosphere is available. We have reviewed information on sources; source strengths; measurements in gas, aqueous, and aerosol phases; and chemical processes and from those data derive global budgets for nine reactive chlorine species and for reactive chlorine as a whole. The typical background abundance of reactive chlorine in the lower tropospheric is about 1.5 ppbv. The nine species, CH3 Cl, CH3 CCl3, HCl, CHClF2, Cl2* (thought to be HOCl and/or Cl2), CCl2 = CCl2, CH2 Cl2 , COCl2 , and CHCl3, each contribute at least a few percent to that total. The tropospheric reactive chlorine burden of approximately 8.3 Tg Cl is dominated by CH3 Cl (≈45 %) and CH3 CCl3 (≈25 %) and appears to be increasing by several percent per year. By far the most vigorous chlorine cycling appears to occur among seasalt aerosol, HCl, and Cl2*. The principal sources of reactive chlorine are volatilization from seasalt (enhanced by anthropogenically generated reactants), marine algae, volcanoes, and coal combustion (natural sources being thus quite important to the budget). It is anticipated that the concentrations of tropospheric reactive chlorine will continue to increase in the next several decades, particularly near urban areas in the rapidly developing countries.

Hemispheric average Cl atom concentration from <sup>13</sup>C/<sup>12</sup>C ratios in atmospheric methane

“…This is within the broad range of literature values derived from indirect measurements by e.g. relative rates of degradation of different hydrocarbons (also known as "hydrocarbon clock" observations), which give widely varying results ranging from 10 4 to 10 6 cm −3 in the MBL (Graedel and Keene, 1995), around or below 10 4 cm −3 in the global atmosphere (Singh et al, 1996), or 1.6·10 4 cm −3 (taking 1 / 4 of the noontime Cl concentration of 6.4·10 4 cm −3 derived from Lagrangian experiments in the North Atlantic conducted by Wingenter et al, 1996). Within the MBL, these concentrations would result in an effective KIE of −33 to −41‰.…”

Chemical characteristics of inorganic ammonium salts in PM<sub>2.5</sub> in the atmosphere of Beijing (China)

“…Emission fluxes of 50 Tg Cl yr −1 , 6 Tg Cl yr −1 , 4.6 Tg Cl yr −1 , and 2 Tg Cl yr −1 for HCl are emitted globally to the atmosphere from dechlorination of sea salt aerosols (Graedel and Keene, 1995;Keene et al, 1999), biomass burning , coal combustion and waste burning , respectively. As Beijing is ∼150 km from the sea, it has been demonstrated that the contribution to the aerosols from the sea could be ignored here (Yuan et al, 2004).…”

Inland Concentrations of Cl2 and ClNO2 in Southeast Texas Suggest Chlorine Chemistry Significantly Contributes to Atmospheric Reactivity

“…Research over the past several decades has shown that the presence of reactive gas phase chlorine species, particularly chlorine radicals (Cl • ), can contribute significantly to atmospheric reactivity [1][2][3][4][5][6][7]. High concentrations of Cl • can lead to increased rates of Volatile Organic Compound (VOC) oxidation [8][9][10][11][12][13] and enhanced rates of O3 production in the troposphere [1,3,7,[14][15][16].…”