Tropospheric Halogen Chemistry

Halogens are very reactive chemicals that are known to play an important role in anthropogenic stratospheric ozone depletion chemistry, first recognized by Molina and Rowland (1974). However, they also affect the chemistry of the troposphere. They are of special interest because they are involved in many reaction cycles that can affect the oxidation power of the atmosphere indirectly by influencing the main oxidants O3 and its photolysis product OH and directly, e.g., by reactions of the Cl radical with hydrocarbons (e.g., CH4).Already by the middle of the nineteenth century, Marchand (1852) reported the presence of bromine and iodine in rain and other natural waters. He also mentions the benefits of iodine in drinking water through the prevention of goitres and cretinism. In a prophetic monograph "Air and Rain: The Beginnings of a Chemical Climatology," Smith (1872) describes measurements of chloride in rain water, which he states to originate partly from the oceans by a process that he compares with the bursting of "soap bubbles" which produces "small vehicles" that transfer small spray droplets of seawater to the air. From deviations of the sulfate-to-chloride ratio in coastal rain compared to seawater, Smith concluded that chemical processes occur once the particles are airborne.For almost a century thereafter, however, atmospheric halogens received little attention. One exception was the work by Cauer (1939), who reported that iodine pollution has been significant in Western and Central Europe due to the inefficient burning of seaweed, causing mean gas phase atmospheric concentrations as high as or greater than 0.5 μg m-3. In his classical textbook Air Chemistry and Radioactivity, Junge (1963) devoted less than three pages to halogen gas phase chemistry, discussing chlorine and iodine. As reviewed by Eriksson (1959a, b), the main atmospheric source of halogens is sea salt, derived from the bursting of bubbles of air which are produced by ocean waves and other processes. Early work by Cauer (1951) had shown that Cl/Na and Cl/Mg ratios were lower in air than in seawater, indicating loss of chlorine by "acid displacement" from sea salt by the strong acids, H2SO4 (Eriksson (1959a, b) and HNO3 (Robbins et al., 1959). Already the first measurements of bromine in aerosols by Duce et al. (1963) showed that bromine, like chlorine, was lost from the sea salt particles, whereas iodine was strongly enriched ( Duce et al., 1965). Research since the early 1980s has shown that photochemical processes are actively involved.Interest in the chemistry of atmospheric halogens took a steep upward surge after it was postulated that the release of industrially produced halocarbons, in particular the chlorofluorocarbons (CFCs), CFCl3, and CF2Cl2, could cause severe depletions in stratospheric ozone (Molina and Rowland, 1974) by the reactions involving the CFC photolytic product radicals, Cl and ClO, as catalysts. The first stratospheric measurements of ClO did indeed show its presence in significant quantities in the stratosphere so that by the end of the 1970s USA, Canada, and the Scandinavian countries issued laws against the use of CFC gases as propellants in spray cans. In the mid-1980s the springtime stratospheric ozone hole over Antarctica was discovered by Farman et al. (1985), involving heterogeneous reactions on polar stratospheric clouds that lead to chlorine activation ( Solomon et al., 1986). Ten years later, in 1996, a complete phaseout ofthe production of the CFCs and a number of other chlorine- or bromine-containing chemicals came into effect for all nations in the developed world. In this contribution we will, however, concentrate on the impact of reactive chlorine, bromine, and iodine on tropospheric ozone chemistry.Halogens have the potential to be important in many facets of tropospheric chemistry. A multitude of gas phase reactions and gas-particle interactions occur that include coupling with the sulfur cycle and reactions with hydrocarbons. Loss of ozone by catalytic reactions involving halogen radicals lowers the concentrations of the hydroxyl radical OH and thus the oxidation power of the atmosphere. Figure 1 shows these and other relevant halogen-related processes schematically. The sum of particulate and gaseous halogen concentrations maximize in the marine troposphere. Important for our climate - via feedback with cloud microphysics mainly in the large regions of marine stratocumulus - are links between halogen chemistry and the sulfur cycle. HOBraq and HOClaq can increase the liquid phase oxidation of S(IV) to S(VI), while BrO can decrease the most important in situ source for SO2 in the marine troposphere, namely, the oxidation of DMS to SO2 by reaction with OH by providing an alternate pathway (BrO+DMS) that reduces the yield of SO2 from DMS oxidation. Thus, the presence of bromine and chlorine in the troposphere lowers gas phase SO2 concentrations and thus the formation of new sulfate particles via the reaction sequence SO2+OH→H2SO4. (17K)Figure 1. Schematic depiction of the most important halogen-related processes in the troposphere. High mixing ratios of iodine oxide at a coastal site indicate a potentially significant role of iodine for the destruction of O3 and new particle embryo formation (Alicke et al., 1999; O'Dowd et al., 1998). Almost 20 years earlier, Chameides and Davis (1980) suggested that open ocean iodine chemistry would be initiated by the photolysis of CH3I. This was based on the measurements of Lovelock et al. (1973) and Singh et al. (1979), who found volume mixing ratios of CH3I of 1-5 pmol mol-1 over the ocean.The potentially strong involvement of halogens in tropospheric chemistry was first observed in the Arctic, where strong ozone depletion events were found to coincide with high levels of bromine (Barrie et al., 1988).The first mid-latitude demonstration of reactive halogen chemistry in the troposphere was made downwind of salt pans in the Dead Sea area, where the so far highest atmospheric mixing ratios of BrO were measured (Hebestreit et al., 1999). Volcanoes are sources of halogens as well, mainly in the form of HCl. Biomass burning releases halogens as do industrial processes.So far we have only mentioned chlorine, bromine, and iodine. This is justified because chemistry of fluorine is of no consequence, as very unreactive HF is efficiently formed in the atmosphere, e.g., via the reaction F+H2O→HF+OH. However, several fluorine-containing gases of anthropogenic origin are potentially powerful greenhouse gases, because they absorb strongly in the infrared atmospheric window region near 10 μm. Fully fluorinated gases - such as SF6, CF4, and C2F6 - have atmospheric lifetimes of the order of thousands of years and thus possess very high global warming potential (GWP). Although their abundance in the atmosphere has not yet grown large enough to be of concern for Earth's climate, their production must ultimately be curtailed in the future. The most abundant fully fluorinated gas, CF4, had an atmospheric volume mixing ratio of ∼75 pmol mol-1 in 1995 (Warneck, 1999). Because of their higher concentrations in the atmosphere, about 270 pmol mol-1 and 530 pmol mol-1, respectively, the CFC gases, CFCl3 and CF2Cl2, already exert a significant radiative greenhouse forcing (Ramanathan, 1975) on Earth's climate. For further discussion about atmospheric fluorine, the reader is referred to a thorough review article by Harnisch (1999).Several overview articles have been published on tropospheric halogen chemistry since the early 1980s, starting with Cicerone (1981). Wayne et al. (1995) list in great detail reaction paths, laboratory data, and atmospheric implications of halogen oxides. A good overview on laboratory measurements was also given by de Haan et al. (1999). Reaction cycles involved in tropospheric halogen chemistry and measurements are also thoroughly discussed by Platt (2000) and Platt and Hönninger (2003). Important compilations of laboratory studies that were made to elucidate chemical reaction paths are given by, e.g., DeMore et al. (1997), Sander et al. (2000), and Atkinson et al. (1999, 2000). Emission inventories for chlorine were compiled by Graedel and Keene (1995) and Keene et al. (1999).In Section 4.02.2 of this overview we will first describe the main halogen reaction mechanisms and then discuss, in Section 4.02.3, the springtime surface ozone depletion events in high latitudes that were first observed in the Arctic. Another main part of this chapter is concerned with halogens in the marine boundary layer ( Section 4.02.4). In Section 4.02.5 we describe interactions of halogens with some other elements of atmospheric importance. A very recently discovered environment where halogen chemistry plays a large role are salt lakes ( Section 4.02.6). There the chemistry bears similarity to that of the high-latitude ozone depletion events. This is followed in Section 4.02.7 by a discussion of halogen chemistry in the free troposphere and in Section 4.02.8 by other sources of halogens such as industry and biomass burning.