JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. D13, PAGES 15,999-16,004,JULY 20, 1997 Measurement techniquefor the determination of photolyzable chlorine and bromine in the atmosphere G. A. Impey,P. B. Shepson,• D. R. Hastie,L. A. Bartie• Departmentof Chemistryand Centre for AtmosphericChemistry, York University,Toronto, Ontario, Canada Abstract. A techniquehas been developed to enablemeasurement of photolyzablechlorine and bromineat tracelevels in the troposphere.In thismethod, ambient air is drawnt•ough a cylindricalflow cell, whichis irradiatedwith a Xe arc lamp. In the reactionvessel of the photoactivehalogen detector (PHD), photolyrically active molecules Clp (including C12, HOC1, C1NO,C1NO2, and C1ONO2) and Brp (including Br2, HOBr, BrNO, BrNO2, and BrONO2) are photolyzed,and the halogenatoms produced react with properieto form stablehalogenated products.These products are thensampled and subsequently separated and detected by gas chromatography.The systemis calibratedusing low concentrationmixtures of C12and Br2 in air from commerciallyavailable permeation sources. We obtaineddetection limits of 4 pptv and 9 pptv as Br2 andC12, respectively, for 36 L samples. 1. Introduction (or C12)in the Arctic, largely as a result of the lack of suitable analyticalmethodologies. This paperreports the developmentof The episodicdestruction of groundlevel ozonein the Arctic at a measurementtechnique for the determinationof rapidly sunriseis a phenomenonthat hasbeen observed for many years. photolyzingchlorine (referred to hereas Clp) and bromine (Brp) With the onsetof polar sunrise,ozone levels are often observed speciesat pansper trillion by volume(pptv) mixingratios in the to drop from a backgroundconcentration of •40 ppbv to almost atmosphere.Impey et al. [this issue]discuss the resultsobserved zero on a timescaleof a day or less [Barrie et al., 1988] for from a field studyconducted in the Canadianhigh Arctic at Alert, periodsof 1-10 days. It has beensuggested that the extremely NorthwestTerritories, from mid-Februarythrough mid-April as rapid destructionof near-surfaceozone is associatedwith part of the Polar SunriseExperiment (PSE) 1995. bromine and chlorine atom chemistry [Barrie et al., 1988; McConnell et al., 1992; Jobsonet al., 1994]. While the primary sourceof the halogenatoms is still under investigation,current 2. Methodology information has led to the hypothesisthat the heterogeneous releaseof photochemicallyactive forms of chlorineand bromine The principleof the photoactivehalogen detector is to rapidly from sea salt aerosol, and/or recycling of adsorbedHBr and photolyzethe ambienthalogen-containing compound to releasea HOBr [Fan and ,lacob, 1992] may be responsible.Some of the halogen atom(s). The halogen atom(s) then reacts with a processesthought to be responsible,particularly as sourcesfor scavengerhydrocarbon molecule to producea stablehalogenated bromine atoms, are as follows: photolysisof BrNO or BrNO2 productthat can be collectedand measured.A schematicof the formed by reactionof NO2 or N205, respectively,with sea salt systemis shownin Figure 1. The sampleair is drawnthrough a aerosol [Finlayson-Pitts et al., 1989, !990]; photochemical lengthof-•7 m, 0.635 cm O.D. heavywalled TFE-Teflon tubing release of Br2 from Br' in the snow pack [McConnell et al., at 1 L/min to an in-line filter (Zitex 47 mm Teflon extra-fine, 1992]; or the recyclingprocess of HBr, HOBr, and BrONO2 on containedin a PFA-Teflon filter holder)that removesfine aerosol acidicaerosol to releaseBr2 [Fan and,lacob, 1992;Abbatt, 1994] from the air stream. Testsindicate that >94% of bothCI: and Br: along with BrC1 [Abbatt, 1994; Vogt et al., 1996]. These passthrough the inlet line assembly.Small flows (20 mL/min) of mechanismscan be testedwith appropriatemeasurements of the propene(C3H6) and nitric oxide(NO) from 100 ppm mixturesin precursorsand/or the halogenatedproducts. nitrogen are added to the inlet air stream resulting in To improve our understandingof which processesmay be concentrationsof ~0.5 ppm for both within the photoactive important with respect to halogen atom chemistry in the halogendetector (PHD) reactionvessel. The sampleair passes troposphere,reliable measurementmethods for photolyrically througha 2 L volumecylindrical pyrex vessel, 52 cm in length active sources of chlorine atoms (e.g., C12 and HOC1) and (sampleoutlet 17 cm from the end of the cell) with an outer bromine atoms (e.g., Br2 and HOBO are clearly needed. diameterof 7.0cm and a 0.159 cm thick borosilicateglass Althougha methodhas beenreported that enablesmeasurement window secured on either end. This allows transmission of the of C12[Keene et al., 1993; Pszennyet al., 1993], this is not the full rangeof visible light, decreasingto a transmissionof ! 0% at casefor Br2. There have, as yet, been no measurementsof Br2 280nm. Light from a 150 W xenon arc lamp (A1010 lamp housing,LPS-220 power supply, PTI Inc.), situatedat one end of •Nowat Departmentsof Chemistry and Earth and Atmospheric the cylindrical cell, is collimated by a fused silica lens Sciences,Purdue University, West Lafayette, Indiana. (01LQT0! ! PCC, D=50 mm, FL=-200 mm) and passedthrough 2Alsoat Atmospheric Environment Service, Toronto, Ontario, Canada. the cell. Photolyzablechlorine and bromine compoundswill yield free C1 and Br atomswhich reactrapidly with the added Copyright1997 by theAmerican Geophysical Union. propene,in the presenceof nitric oxide,to producechloroacetone Papernumber 97JD00850. and bromoacetoneas stableproducts. C1 andBr atomscan react 0148-0227/97/97JD-00850509.00 via threepossible pathways, as shownbelow: 15,999 16,000 IMPEY ET AL.' PHOTOLYZABLE HALOGEN MEASUREMENT TECHNIQUE Ambient Pump Air NO "--I • - To Pump MF• Zero Air Dump Permeation o•n: Cdilxmion SOUrCeS Closed:Ambienl Samplin• Mirror -•-'•'•' Zero Zero Air Air Figure 1. Schematicdiagram of thephotoactive halogen detector (PHD). (MFC, massflow controller). (R1a) X + CH2 = CHCH3 ---> XCH2CHCH3 chloroacetone and bromoacetone. The two tubes in series ensure that this is the caseand enabletesting of "breakthrough"of the (Rlb) X + CH2 = CHCH3 -4 CH2CH(X)CH3 analytes. After samplecollection, the tubes are extractedwith 100 [tL of glass-distilledbenzene (Caledon Lab). Analysisis (Rlc) X + CH2 = CHCH3 -4 HX + CH2 = CHCH2 typicallydone immediately after extraction,but testshave shown both chloroacetone and bromoacetone to be stable in solution for The haloacetoneproduct is thenproduced as follows. at least several days. The products chloroacetoneand (R2) XCH2CHCH3 + 02 -4 XCH2CH(O2)CH3 bromoacetone were found to be stable on the PoraPak tubes for at least 19 hours. Samplingrates are controlledby mass flow (R3) XCH2CH(O2)CH3 + NO -4 NO2 + XCH2CH(O)CH3 controllers,and Gast diaphragmpumps are used to pump air throughthe photoactivehalogen detector and sampletubes. The (R4) XCH2CH(O)CH 3 + 02 -4 HO2 + XCH2C(O)CH3 extractsare analyzed for chloroacetoneand bromoacetone using a HewlettPackard 5890 seriesII gaschromatograph equipped with For chlorine atoms, approximately10-15% abstracta hydrogen a SupelcoVOCOL 60 m x 0.25 mm x 1.5 [tm film thickness from carbon3 to form HC1 [Lee et al., 1977], while the rest add columnand an electroncapture detector. A 1 [tL extractsample is to either carbon 1 or carbon2 to form, amongother products, injectedusing a 2 [tL syringeand a splitlessmode of operation chloroacetoneand 2-chloropropanal,respectively. It has been because of the extremely low solution concentrations.The shown that the yield of chloroacetoneis of the order of 40% column is temperatureprogrammed from 50øC to 100øC at [Kleindienstet al., 1989]. The mechanismis similarfor bromine 1øC/min, then at 30øC/min to 240øC. A mixture of 5% methane atoms,although abstraction does not occur. The bromoacetone and 95% argon(P5) is usedas the carriergas. Calibration of the product yield is unknown to date but is currently being gaschromatograph is doneon a regularbasis with diluteliquid determinedin our laboratory. However,detailed knowledge of standardsof chloroacetone(Aldrich) and bromoacetone (prepared the reactionyields is not necessaryif the photoactivehalogen accordingto Blatt [1950]) in benzene. The bromoacetoneis detectoris calibratedwith known concentrationsof C12and Br2. preparedby adding equimolar concentrationsof acetoneand In this system,calibration gases can be input at the reactioncell bromineto a round bottom flask containingwater and glacial inlet followedby determinationof the mixingratio of the acetic acid. The oil which separatesis collected, dried, and h/fioacetoneproduct. It is onlynecessary that the product yields fractionated. A pure product is obtained with subsequent are invariantto input sampleconditions (e.g., humidi.ty). Air distillation. from the reaction cell is sampledby pumping the air, at C12and Br2 usedfor the PHD calibrationare generatedfrom 600 mL/min,through two tubesin seriespacked with 30 mg of VICI Metronics 140-662-002 (25+50% ng/min) and 110-010- PoraPak Q (copolymer of styrene and divinylbenzene; 0003 (37+10% ng/min) permeationcells, respectively.The cells Chrompak)to trapthe chloroacetone and bromoacetone products. are containedin specially designedTeflon tubes, which are Extensiveexperiments have indicated that for 30 mg tubesheld at containedtogether in a stainlesssteel holder. Heat tape -0øC, 36 L of air can be sampledwith completetrapping of surroundsthe unit, and the outputsof the permeationsource IMPEY ET AL.' PHOTOLYZABLEHALOGEN MEASUREMENT TECHNIQUE 16,001 devicesare held constantat 30øC using a Love