Anal. Chem. 2010, 82, 770–776
Halogens in the Troposphere Barbara J. Finlayson-Pitts University of California Irvine
Oceans cover approximately two-thirds of the Earth’s surface and have unique emissions such as sea salt particles. Accordingly, they play a major role in the chemistry of the atmosphere, including that of local and regional air pollution and climate change. The major halide ion in seawater is the chloride ion, with bromide ions contributing to a smaller extent (see Supporting Information). Sea salt particles also often have organic coatings1 that are likely to play a role both in their chemistry and in the uptake and exchange of gases between the air and water interface of the particles.2,3 Other significant marine emissions include organohalogens and sulfur compounds such as dimethyl sulfide.4 In addition to these directly emitted compounds, a complex mixture of products is formed by their reactions in air.4 Halogen chemistry occurs not only in mid-latitude marine regions, but in the Arctic and Antarctic as well.5-10 Indeed, halogen chemistry is greatly enhanced under some conditions in these polar regions compared to mid-latitudes. Halogen chemistry has also been observed near alkaline dry lakes,11,12 in certain industrial areas,13 and in plumes from burning oil wells14 and volcanoes.15 Thus, though the oceans provide the largest source, halogen chemistry is clearly seen in other locations as well. The focus of this article is on measurement techniques for inorganic halogen gases important to tropospheric (lower atmosphere) chemistry; the reader is referred to other sources for a discussion of methods used in the stratosphere (upper atmosphere).4 Because of length restrictions, measurement methods for organohalogens, organosulfur, and particles are not included, nor are remote sensing techniques using satellites. 770
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LISA WINGEN
Although inorganic halogen gases are believed to play key roles in the chemistry of the lower atmosphere, many of them have not yet been detected or measured in ambient air. This article describes some of the current techniques and future needs for inorganic halogens in air. (To listen to a podcast about this feature, please go to the Analytical Chemistry multimedia page at pubs.acs.org/page/ ancham/audio/index.html.)
REACTIVE HALOGENS IN THE TROPOSPHERE For those gaseous inogranic halogens and their reaction products that have been measured in air, the mixing ratios typically range from low parts per trillion (ppt) to a few parts per billion (ppb). Mixing ratios are defined as the ratio of the number of molecules or moles to the total number of molecules or moles in air and are commonly used to express tropospheric levels of gases; for reference, there are 2.5 × 1019 molecules per cm-3 at 1 atm pressure and 298 K. Although these ppt-ppb levels may seem miniscule, they can have significant impacts on tropospheric chemistry. For example, Figure 1 shows a dramatic loss of surface level O3 measured in the Arctic during springtime, when the atmosphere is first exposed to sunlight after months of darkness; this event is called the “polar sunrise”. A clear anticorrelation of ozone with filterable bromide is evident.16 This destruction of tropospheric ozone is now known to be caused by ppt levels of bromine-containing gases17 such as Br2, BrCl, and HOBr that originate from bromide in sea salt particles. The role of halogens in the chemistry of the troposphere is summarized in the Supporting Information. Whereas chlorine atoms in the troposphere typically lead to ozone formation, bromine atoms in the troposphere lead to ozone destruction (Figure 1). Unlike chlorine atoms, bromine atoms only react with a few organics such as HCHO, but the reaction with O3 is 10.1021/ac901478p 2010 American Chemical Society Published on Web 12/30/2009
Figure 3. Some pathways for conversion of chloride ions to photochemically active chlorine atom precursors in the troposphere.
Figure 1. Loss of surface-level ozone and total bromine on a filter at Alert, Canada in April 1986. Adapted with permission from ref. 16.
Given the much greater amount of chloride versus bromide ions available in seawater (see Supporting Information), one might also expect chlorine to play a significant role in tropospheric chemistry. It is now known that a variety of inorganic chlorinecontaining gases can be formed from reactions of sea salt via chemistry shown schematically in Figure 3. Briefly, OH radicals, ClONO2, N2O5, and NO2 all react with chloride ionssfor example, those in sea salt or salt from dry lakessto generate halogen gases such as Cl2, ClNO2, and ClNO. These gases photolyze at wavelengths >290 nm that are available at the Earth’s surface (the “actinic region”)4 to generate chlorine atoms.6,10 Reactions can occur in the bulk aqueous phase of the particle or in some cases, at the air-water interface, but the kinetics and mechanisms of the two may be different. For example, the OH radical reaction with Cl- in the bulk requires acid to generate chlorine, whereas reaction at the air-water interface does not require an acid and is much faster.24,25 Uptake and reaction of HNO3 with chloride in sea salt particles generates gaseous HCl, which also leads to chlorine atoms via reaction with OH radicals. Hypochlorous acid (HOCl) is another expected product in air, Cl + O3 f ClO + O2
(1)
Figure 2. Overview of chemistry that converts bromide ions to photochemically active bromine gases. Adapted with permission from ref. 8.
ClO + HO2 f HOCl + O2
(2)
relatively fast. As shown schematically in Figure 2,8 there is a complex cycling of bromine compounds between the gas and condensed phases. Chemistry in the latter plays a major role in the continuous conversion of sea salt bromide into bromine atoms in the gas phase, leading to what has been termed a “bromine explosion”.18 These multiphase interactions combined with increases in the bromide to chloride ratio in the remaining brine as seawater freezes19 contribute to the disproportionate role played by bromine in tropospheric ozone destruction in the polar regions. Another factor is the enhancement of bromide ions at the surface of aqueous solutions20,21 and presumably also in the quasi-liquid layer of the snowpack and frost flowers, which increases the availability of Br- for reaction with gases such as O3 and HOBr. Because of this complex chemistry, bromine-containing gases that are of atmospheric interest include HBr, Br2, BrNO, BrNO2, BrONO2, and HOBr as well as the free radicals Br, BrO, and OBrO. Ozone destruction also occurs in mid-latitudes,22,23 but the different meteorology and lack of long dark periods in which Br2 can build up results in less dramatic ozone loss.
and it too can photolyze to generate chlorine atoms (Cl). In short, Cl2, ClNO, ClNO2, ClONO2, and HOCl, as well as free radicals such as Cl, ClO, and OClO are of tropospheric interest. As discussed below, HCl, Cl2, ClNO2, ClO, and OClO have been measured in air, and there is indirect evidence for peak chlorine concentrations in the 104-105 atoms per cm-3 range in mid-latitudes26 as well as in polar regions27 derived from measurement of the relative loss rates of a group of selected alkanes. Iodine chemistry has been observed over the ocean and in coastal regions, but its primary source is not sea salt (in which the molar ratio of Cl-/I- is ∼106:1), but rather organoiodine compounds produced by marine algae.28 However, iodine chemistry has also been observed in areas such as the Dead Sea Valley that do not have significant amounts of algae.29 Iodoorganic compounds absorb light in the actinic region, generating atomic iodine. Iodine atoms react very rapidly with O3 to generate a number of higher oxides (I2Ox up to x ) 5)30 that are believed to contribute to new particle formation in coastal regions.30,31 Algae also generate I2,32 which photolyzes Analytical Chemistry, Vol. 82, No. 3, February 1, 2010
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Figure 4. Overview of iodine chemistry in the troposphere. Adapted from ref. 28.
rapidly to form I atoms. As in the case for bromine and chlorine, there are interhalogen reactions involving iodine, chlorine, and bromine. Figure 4 is a schematic diagram summarizing the major features of iodine chemistry in the troposphere.28 METHODS FOR MEASURING HALOGEN GASES Though a great deal is known about the chemistry associated with halogens in the troposphere, the ability to specifically identify and measure many of the important species limits our understanding of the chemistry and its impact on local, regional, and global scales. In the following, we summarize some of the methods that have been used to measure inorganic halogen gases and highlight some particular analytical needs for future technique development. There is a critical need for methods that are both specific and sensitive for the challenging task of detecting the small concentrations of halogen compounds that exist in the complex mixture of gases and particles found in the lower atmosphere. Mist Chamber Techniques. Pszenny, Keene, and coworkers developed a mist chamber technique for measuring inorganic halogen gases.33,34 Different categories of inorganic chlorine gases such as HCl with potential contributions from ClNO + ClNO2 + ClONO2 (designated HCl*) and Cl2 with a potential contribution from HOCl (designated Cl2*) were separated and collected using sequential chambers containing solutions of different pH. The first chamber was acidic and absorbed HCl*, and the second was alkaline to take up Cl2*. Ion chromatography was used to measure the Cl- in the absorbing solutions. Typical levels of HCl* and Cl2* in the marine boundary layer were measured to be up to ∼8 ppb and ∼0.4 ppb, respectively.34,35 At dawn, the concentration of the Cl2* component did not fall to zero, suggesting that components such as HOCl that photolyze less rapidly than Cl2 might be contributing significantly to the 772
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measured levels.33,34 Unfortunately, to date there are no specific measurements of HOCl in air. Atmospheric Pressure Ionization Mass Spectrometry (API-MS). API-MS36 was first used by Spicer and coworkers to identify and measure Cl2, Br2, and BrCl in the troposphere.17,37 The sample is drawn into the discharge region, where air forms ions such as O2-, which has an electron affinity (EA) of only 0.45 eV.36 Species such as Cl2 (EA ) 2.38 eV), Br2 (EA ) 2.55 eV), and I2 (EA ) 2.55 eV) that have higher electron affinities accept an electron from O2-, thus generating the molecular ion of interest. Specificity is provided by use of MS/MS: a selected ion such as that at m/z ) 70 (derived from 35Cl2) is collisionally dissociated, and the mass spectrum of the fragments is scanned. Detection limits are of the order of a ppt. For example, this method was used for the first specific measurements at Long Island, N.Y., where up to 150 ppt Cl2 was observed at night.37 The characteristic isotope ratios for chlorine combined with the MS/MS capability provide an added measure of specificity. API-MS was also used for the first measurements of Br2 and BrCl in the Arctic at polar sunrise. Levels of these bromine gases up to 35 ppt were measured.17 As was the case for chlorine, the isotope ratios combined with the MS/MS capability were very useful in providing specificity. Detection limits were 0.2 ppt for Br2 and 2 ppt for BrCl.17 However, many unanswered questions remain. For example, Shepson and coworkers made indirect measurements of halogen gases in the Arctic at polar sunrise by photolyzing air samples in the presence of propene.38-40 The alkene traps Cl and Br atoms, forming chloro- and bromoacetone, respectively. As a result, this method measures the sum of all photolytic precursors to Cl or Br atoms. Detection limits were estimated to be 4 ppt for Br2 and
9 ppt for Cl2.41 Precursors of Cl atoms were reported to be up to 100 ppt prior to polar sunrise, with much smaller levels after sunrise.38 Similarly, the ratio of Br/Cl was smaller before polar sunrise than after.42 As described above, no Cl2 (
