Although the flowtube method did not produce comprehensible data during the
BROMEX field campaign, the method still has the potential to capture the BrOxand ClOx
profiles. However, during the course of this work we were able to develop a CIMS method to detect BrO and ClO in real time. The CIMS instrument will be discussed in detail in Chapter 3. Here the ion chemistry that occurs within the CIMS that allows us to
detect ClO and BrO will be described. The CIMS uses CH3I to produce the reagent ion,
I-, which ionizes the halogen species, as shown in reaction 2.6-2.7
I- + ClO → IClO- (2.7) ClO and BrO measurements conducted during BROMEX 2012 are shown in Figure 2.14. The CIMS method has several advantages over the flowtube method such as real time measurements and ease of deployment.
2.12 References
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CHAPTER 3 IN-SNOWPACK AND VERTICAL PROFILE EXPERIMENTS CONDUCTED IN BARROW, ALASKA DURING THE WINTER OF 2014
3.1 Introduction
Halogen chemistry has been widely studied throughout the Arctic over the past several decades (Domine et al., 2008;Grannas et al., 2007;Simpson et al., 2007).
Bromine, in particular, has been linked with ozone depletion events (ODEs), where ozone drops from background levels of 40 ppb to below 5 ppb (Barrie et al., 1988;Oltmans et al., 1989). This has been shown to occur sporadically during the Arctic springtime (Oltmans et al., 2012). As most of these measurements took place at coastal locations, sea salt was proposed as one of the first sources of atmospheric molecular halogens (McConnell et al., 1992). It has been hypothesized that aerosols provided the initial
release and a surface for heterogeneous recycling of atmospheric Br2 to occur during
ODEs (Fan and Jacob, 1992;McConnell et al., 1992). The acidic environment of
sulphuric aerosols provide the low pH conditions that are necessary to produce Br2 in the
aqueous phase, which can later be released into the gas-phase (Fan and Jacob, 1992). However, heterogeneous recycling of HOBr on the surface snowpack was shown in a
modeling study to be necessary in order to match simulated O3 depletion rates to
observations (Michalowski et al., 2000). This provided evidence that heterogeneous recycling of halogens in the Arctic occurred on multiple ice surfaces. This furthered the
idea that not only is the snowpack needed for heterogeneous recycling but that it could be a direct source of molecular halogens. It was later discovered that the snowpack surface is open to deposition of sea salt, acidic aerosols, and stable bromine species (HOBr and
BrONO2) which can lead to the production of molecular halogens (Pratt et al., 2013;Wren
et al., 2013;Cao et al., 2014). Pratt et al. conducted snow chamber experiments that showed the surface snowpack can produce molecular bromine in the presence of UV
radiation and this production is enhanced by an increase in O3 concentration due to
heterogeneous chemistry as shown in reactions 3.1-3.6.
X2 + hv → 2X (3.1)
X + O3 → XO + O2 (3.2)
XO + HO2 → HOX + O2 (3.3)
HOX(g) → HOX(aq) (3.4)
HOX(aq) + H+ + X/Y- → X2(aq)/XY(aq) + H2O (3.5)
X2(aq)/XY(aq) → X2(g)/XY(g) (3.6)
However, Cl2 production was not observed during those snow chamber experiments,
although concentrations up to 400 ppt have been observed in the Artic boundary layer during the spring time (Liao et al., 2014). Current Arctic models are inhibited by the lack of understanding of the source of and emission rates of atmospheric molecular halogens,
especially for Cl2. This information is crucial towards properly simulating future Arctic
atmospheric composition, especially as the surrounding sea ice changes. To further
investigate the source of Cl2 and BrCl, we deployed a CIMS in Barrow, Alaska during
January and February of 2014 to perform real time measurements of gas-phase inorganic halogens within the interstitial snowpack air. We attempted to determine if the tundra
snowpack could produce Cl2 and BrCl. Additionally, vertical profile experiments were
conducted above the surface snowpack in order to determine the flux rate of Br2 and Cl2
from the snowpack.