Purdue University: Shepson Atmospheric Chemistry Group: Halogen Studies

Halogen Studies

First, a little background on why we care about halogens:

In the mid-latitudes regions where humidity is relatively high, the hydroxyl radical (OH) serves as the main oxidizer of the atmosphere. OH breaks down VOCs and other common species that are present in the tropospheric boundary layer.  The polar regions are home to unique chemistry, partially due to the very low levels of water vapor in the air.  The scarcity of OH in this environment results in atomic halogen species controlling the oxidizing capacity of the atmosphere after polar sunrise.  Chlorine, in particular, is a great oxidizer of a wide variety of VOCs.

Bromine is also a good oxidizer in this environment, though not quite with the same potency as chlorine.  It is also more discriminating, typically only reacting with short, olefinic VOCs such as acetylene and propene.  More importantly, however, it reacts well with both ozone and gaseous elemental mercury (GEM), leading to the episodic destruction of both these species in what are termed "ozone depletion events" (ODEs) and "atmospheric mercury depletion events" (AMDEs) throughout polar spring.

ODEs have been widely studied since the mid-1980s when this phenomenon was first recognized. ODEs are characterized by tropospheric ozone levels dropping from background mole fractions of 30-40 ppbv to near zero within a time frame that can range between a couple of hours to a day.  These events only occur following polar sunrise and cease at the beginning of the ice melt.  Based on these observations, it was hypothesized that the conditions necessary for ozone depletion include low temperatures, available solar radiation, frozen salty surfaces, and a stable stratified boundary layer.  Though ODEs have since been observed over a wide range of temperatures, most evidence corroborates the remaining conditions.

There is now much evidence to support the hypothesis that halogens could be sourced from hypersaline surfaces, such as sea salt aerosols, young sea ice.  A series of photochemical reactions have been proposed, which serve as an autocatalytic cycle that activates bromine and releases it into the gas phase.  This reaction sequence is commonly referred to as the "bromine explosion."

Br + O3 → BrO + O2  (1)
BrO + HO2 → HOBr(g) + O2 (2)
HOBr(g) → HOBr(aq) (3)
HOBr(aq) + Br- + H+ → Br2(aq) + H2O (4)
Br2(aq) → Br2(g) (5)
Br2(g) + hν → 2Br  (6)

Chlorine can also be activated in this manner, and it has been established that there is active chlorine chemistry during this same time.  However, chlorine has been generally considered only a minor player in ODE chemistry as it typically has a higher affinity for VOCs.  In spite of this, chlorine can still play an important role in influencing the bromine reactions via the cross-reaction of ClO with BrO, which can efficiently regenerate Br radicals to continue ozone destruction.  Moreover, Cl oxidation of VOCs produces HO2 as a by-product, which then feeds back into reaction 2 above, thereby helping drive the bromine explosion.

Studies of BrO in the Arctic have been conducted previously using differential optical absorption spectroscopy (DOAS).  While this method has had much success in detecting BrO, there are also drawbacks.  Long-path DOAS uses path-lengths of several kilometers, possibly spanning heterogeneous air masses.  The passive MAX-DOAS cannot make measurements during night time.  Moreover, ClO cannot be measured unambiguously using DOAS due to interferences at its absorbing wavelength, resulting in a poor limit of detection.  Therefore, it is necessary to have a selective and sensitive in-situ method of measuring BrO and ClO radicals.

Measurements of reactive halogens in the Arctic:

In the Shepson Lab, we have developed a flowing chemical reaction method, which utilizes a VOC radical trap to quantitatively convert halogen radicals to a stable product that can be measured using gas chromatography.  In order to eliminate background and achieve low limits of detection, we use a VOC that is not present in the ambient atmosphere.  Initial studies have been conducted using trifluoropropene (TFP) as a trap, via the reaction sequence below:

XO. + NO → X. + NO2 (7)
X. + CH2=CHCF3 (TFP) → XCH2C(.)HCF3 (8)
XCH2C(.)HCF3 + O2 → XCH2C(OO.)HCF3  (9)
XCH2C(OO.)HCF3 + NO→ XCH2C(O.)HCF3 + NO2 (10)
XCH2C(O.)HCF3 + O2 → XCH2C(=O)HCF3 + HO2 (11)

Former group member Phil Tackett deployed a flowtube in Spring 2008 aboard the Canadian icebreaker Amundsen as part of the Circumpolar Flaw Lead program.  There, he conducted measurements of BrOx over first-year sea ice and frost flowers.  In spring 2009, former group member Chelsea Stephens measured ClOx in Barrow, AK, during the OASIS 2009 field campaign.  Examples from these two field studies are shown below.  More information on the campaigns can be found on their respective pages.

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