High levels of molecular chlorine in the Arctic atmosphere

Transcription

1 High levels of molecular chlorine in the Arctic atmosphere Jin Liao 1,2,3, L. Gregory Huey 1,*, Zhen Liu 1,4, David J. Tanner 1, Chris A. Cantrell 5, John J. Orlando 5, Frank M. Flocke 5, Paul B. Shepson 6, Andrew J. Weinheimer 5, Samuel R. Hall 5, Kirk Ullman 5, Harry J. Beine 7, Yuhang Wang 1, Ellery D. Ingall 1, Chelsea R. Stephens 6, Rebecca S. Hornbrook 5, Eric C. Apel 5, Daniel Riemer 5, Alan Fried 5, Roy L. Mauldin III 5,8,9, James N. Smith 5, Ralf M. Staebler 10, J. Andrew Neuman 2,3, John B. Nowak 2,3 1 School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA. 2 Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado, USA. 3 Earth System Research Laboratory, NOAA, Boulder, Colorado, USA. 4 Combustion Research Facility, Sandia National Laboratories, Livermore, CA, 94551, USA. 5 National Center for Atmospheric Research, Boulder, Colorado, USA. 6 Department of Chemistry, Purdue University, West Lafayette, IN, USA. 7 Department of Land, Air and Water Resources, University of California, Davis, California, USA. 8 University of Helsinki: University of Helsinki, Department of Physics, FI Helsinki, Finland. 9 Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, Boulder, Colorado 80309, USA. 10 Air Quality Processes Section, Environment Canada, Toronto, Canada. NATURE GEOSCIENCE 1

2 * Correspondence to: L. Gregory Huey Supplementary Information Contents Supplementary Text 1. CIMS measurement method 2. Cl 2 effective mixing height, estimated fluxes, and potential sources 3. FLEXPART model and QuikSCAT sea ice back scattering coefficient 4. Chlorine Chemistry Model 5. Observed and modeled HCl 6. Impact of chlorine chemistry on HOBr 7. BrCl measurements 8. Test of the conversion efficiency of HOCl to Cl 2 on inlet surfaces Supplementary References Supplementary Tables Supplementary Figures 2 NATURE GEOSCIENCE

3 SUPPLEMENTARY INFORMATION 1. CIMS measurement method A chemical ionization mass spectrometer (CIMS) was used to measure speciated halogen compounds including Cl 2, HCl, BrCl, BrO, HOBr, and Br 2 at Barrow, AK near the coast of the Arctic Ocean in spring 2009 during the Ocean-Atmospheric-Sea Ice-Snowpack (OASIS) campaign. The details of the measurement site and the CIMS instrument are described in Liao et al. (18). The measurements of BrO, HOBr, and Br 2 are reported in Liao et al. (19). The reagent ion was used to measure Cl 2, BrO, HOBr, Br 2, and HCl before 18 March. The reagent ion was switched to hydrated I - to measure Cl 2, BrO, HOBr, Br 2, ClNO 2, and BrCl after March 18 because this ion was found to have better selectivity and sensitivity than. Cl 2 was monitored at mass 197 and 199 amu with hydrated I and at mass 70 and 72 amu with. HCl was monitored only at mass 164 amu because Br 2 can be detected as with at mass 162 amu. All of the masses monitored using I are provided in Table 1 in Liao et al. (18). Ion reactions of chlorine species with or hydrated I are shown below. Cl 2 + (R1S) Cl 2 + I(H 2 O) n + nh 2 O (R2S) HCl + SF 5 Cl + HF (R3S) BrCl + I(H 2 O) n IBrCl nh 2 O (R4S) NATURE GEOSCIENCE 3

4 The signals at mass 197 were plotted versus 199 amu (Figure 1S). The excellent correlation (R 2 = 0.99) between the signals at mass 197 and 199 amu and a slope of 0.65 indicate that compounds containing two chlorine atoms were detected at these masses. The concentrations of the detected species were determined by subtracting the background signals from the ambient signal and dividing the resulting net signals by the sensitivity. The background signals were determined by frequently scrubbing the ambient gas and particles with a glass wool filter. The background of Cl 2 and HCl were well determined while the BrCl background was less stable and BrCl concentrations were often below the detection limits. Cl 2 and Br 2 permeation tubes were used to calibrate the sensitivity of Cl 2 and other halogen species (18). A known amount of Br 2 (1.47 ppbv) or Cl 2 (5.18 ppbv) from the permeation tubes was added to the inlet every 2 hours to track the sensitivity of the observed halogen species. The emission rates of the permeation tubes were measured every few days by conversion to I 3 in aqueous solution, which was quantified by optical absorption at 352 nm. The emission rate of Cl 2 was found to be 183 ng min -1 with a standard deviation of 7% over the course of the campaign. The emission rates of the permeation tubes were also further confirmed by ion chromatography measurements. In the chemical ionization region, the reaction R2S converts gas phase Cl 2 to ICl - 2 which was detected in the mass spectrometer. The CIMS cannot detect chloride in aerosols as ICl - 2 unless it is converted into gas phase Cl 2. The most efficient way to convert aerosol chloride into Cl 2 is through reaction R5S (31). We have ruled out this potential 4 NATURE GEOSCIENCE

5 SUPPLEMENTARY INFORMATION interference by determining the conversion rate of HOCl on our inlet coated with NaCl crystals. HOCl + Cl + H Cl 2 + H 2 O (R5S) BrCl could also potentially react with chloride on the walls of our inlet to form Cl 2 (R6S). However, we expect this process to be less efficient than the HOCl reaction. In addition, if the observed Cl 2 were from BrCl conversion in the inlet of the CIMS this would imply very large daytime concentrations of BrCl which efficiently photodissociates to form bromine and chlorine atoms. However, we frequently observed high Cl 2 levels when low bromine levels were observed by both the CIMS and the LP-DOAS (18). BrCl + Cl Cl 2 + Br (R6S) 2. Cl 2 effective mixing height, estimated fluxes, and potential sources The effective mixing height is important to estimate the impact of the observed Cl 2. The noontime boundary layer height at Barrow during the campaign is estimated to be about 400 m from ozonesonde data. However, due to the short lifetime of Cl 2 (10 minutes) and a likely surface source, Cl 2 may not be vertically well mixed in the boundary layer. The effective mixing height of Cl 2 is estimated to be 2.5 to 11.1 m based on eddy diffusion (26) using measured eddy diffusivities of 100 to 2000 cm -2 s -1 and a photolytic lifetime of Cl 2 of 10 minutes. Vertical profiles of Cl 2 are clearly needed to determine the overall impact of chlorine chemistry in the boundary layer. NATURE GEOSCIENCE 5

6 The required Cl 2 flux is in a reasonable range compared to O 3 fluxes estimated with literature values of deposition velocities. A volumetric production rate of cm -3 s -1 (Fig. 2) corresponds to a surface flux of ppbv cm s -1 with a mixing height of 10 m or 0.67 ppbv cm s -1 with a mixing height of 100 m. Assuming an O 3 deposition velocity of 0.01 cm s -1 (32, 33) and an O 3 mixing ratios of 30 ppbv, the O 3 flux is estimated to be 0.3 ppbv cm s FLEXPART model and QuikSCAT sea ice back scattering coefficient The FLEXPART Lagrangian particle dispersion model (27) driven by windfields from the Weather Research and Forecasting (WRF version 3.3) model (34) is used to simulate the 2-day transport history of the air masses arriving at the measurement site (71 17' 26" N, ' 19" W Barrow, Alaska) (Figure 2S). The WRF (V3.3) model was constrained by the Climate Forecast System Reanalysis (CFSR) from the National Centers for Environmental Prediction (NCEP) ( The output windfield from the WRF (V3.3) model has a horizontal spatial resolution of 36 km and 46 layers from the surface to 10 hpa in the vertical. The FLEXPART model shares the same horizontal and vertical resolution with WRF, and was run in the backward mode to calculate retro-plumes for every hour (35,36,37) through the campaign. Each retro-plume consists of 5000 particles released from a 4 km (horizontal) 4 km (horizontal) 400 m (vertical) volume box located at the measurement site and was simulated for 2 days. In order to identify the source regions of the Cl 2 or its precursors, a 2-D horizontal field of 6 NATURE GEOSCIENCE

7 SUPPLEMENTARY INFORMATION vertically integrated residence time (seconds) from the surface to 400 m, defined as footprint residence time (FRT) (35,37), for each 2-day retro-plume is calculated. Nonzero FRT at a certain grid (location) indicates the air masses over pass the lowest 400 m during the past 2 days (35,36,37). QuikSCAT sea ice backscatter coefficient is used as a proxy of sea ice age (38,39) (Figure 2Sa). QuikSCAT sea ice backscatter coefficient data were downloaded from The data have a spatial resolution of 12.5 km 12.5 km and temporal resolution of 1 day, and range from 0 to 250. Higher backscatter coefficient represents a larger fraction of multi-year sea ice. In summary, we found no clear relation between the age of the sea ice and chlorine levels. We also found no clear directional preference. High chlorine levels were found to correspond to long residence times in the boundary layer within 500 km of Barrow independent of wind direction. 4. Chlorine Chemistry Model A time dependent box model constrained to Cl 2 was used to simulate the concentrations of other related chlorine compounds. The reactions included in the chlorine time dependent model are summarized in Table 1S. The input species and their typical levels NATURE GEOSCIENCE 7

8 are provided in Table 2S. Loss rates of HOCl and HCl to heterogeneous surfaces (k HOCl, k HCl, k Cl2 ) are assumed to be the condensation rates of the gas phase species to aerosol surfaces. Because the surface area of snow is not well known, the heterogeneous loss rates may be underestimated. The mass accommodation coefficients of HOCl, HCl, and Cl 2 are 0.3 (40), 0.18 (41), and 0.02 (42), respectively. The gas phase reaction rate constants are obtained from NASA Jet Propulsion Laboratory compilation (1) and IUPAC compilation (43). 5. Observed and modeled HCl HCl levels were predicted by constraining to observations of Cl 2 and other species. Cl atoms were calculated via Eqn. (1) using the steady-state assumption. The concentrations of Cl 2, ClO, NO, O 3, OH, HO 2, hydrocarbons (RH), and aldehyde (RCHO) are taken from observations. Because high levels of NO often observed were due to local pollution, NO measurements less than 20 pptv were used to calculate the average NO mixing ratio as the input in Eqn. (1). [ ] pred = [ ] [ ] [ ] [ ][ ] [ ][ ] [ ][ ] [ ] [ ] [ ] [ ] (1) [HCl] pred = ( [ ] [ ])[ ] (2) HCl is also assumed to be in steady-state, with an average lifetime of 4 hours due to loss on aerosol surfaces (see Eqn. (2)). k HCl is the condensation rate of HCl to aerosol surfaces. 8 NATURE GEOSCIENCE

9 SUPPLEMENTARY INFORMATION The accommodation coefficient of HCl is assumed to be 0.18 (42). The reaction rate constants are from the Jet Propulsion Laboratory compilation (1) The predicted HCl generally agreed (R 2 = 0.69) with the observed HCl (Figure 3S). In the remote marine boundary layer, significant HCl is unlikely produced from acid displacement because of the low levels of atmospheric acid available to interact with the sea salt (44). These indicate that the observed Cl 2 levels were generally consistent with the measured HCl concentrations. 6. Impact of chlorine chemistry on HOBr HOBr can be predicted by constraining to BrO and HO 2 measured values (19). In addition, HOBr levels can be predicted used a steady-state model to predict HO 2 values. A photochemical model was built upon a basic HO x model (45) by either adding bromine reactions or bromine and chlorine reactions (see Table 3S and Table 1S). The HOBr predictions have a better agreement with HOBr observations in both correlation and magnitude when HO 2 was predicted from the model incorporating chlorine chemistry (Figure 4S). 7. BrCl measurements NATURE GEOSCIENCE 9

10 Significant levels of bromine monochloride (BrCl) were detected during ozone depletion events (ODEs) in the Arctic polar sunrise (46). However, BrCl was below detection limits of 0.5 pptv and 2 pptv when clearly elevated Br 2 was observed in recent campaigns in Pacific marine boundary and Arctic marine boundary (47, 30). Lab experiments (48,49) found that BrCl can be a significant product of uptake of HOBr on frozen salt surface when [Br ]/[Cl ] ratio on the sea salt surfaces are smaller than ~ , which is much smaller than in the sea water of Moreover, lab experiment (50) found that BrCl can be produced by uptake of Cl 2 or HOCl on chloride/bromide ice surfaces. More direct measurements of BrCl are helpful for investigating the formation of BrCl and its importance as a halogen radical precursor in the Arctic environment. During the OASIS Campaign, much less BrCl than Cl 2 was observed with BrCl mixing ratios above detection limit only in the early morning or late afternoon. The BrCl had no clear correlation with other bromine species (BrO, HOBr, and Br 2 ) and the formation of BrCl through the reaction of ClO with BrO was calculated to be unimportant in this environment. On the other hand, BrCl was detected only when Cl 2 was observed. The BrCl measurements are plotted against Cl 2 measurements multiplied by the Cl 2 heterogeneous loss rate (assuming a loss to the snowpack with a lifetime of 2.5 hrs) and divided by J BrCl when J BrCl > s -1 (Figure 5S). BrCl measurements were well correlated with Cl 2 k snow /J BrCl (R 2 =0.70) with a slope of This indicates that BrCl is probably a product from heterogeneous reaction of chlorine compounds (e.g. Cl 2, HOCl, or HCl) and photolysis of BrCl is the dominant loss of BrCl in the daytime at Barrow. This implication is supported by the finding from Huff and Abbatt (50) that BrCl is 10 NATURE GEOSCIENCE

11 SUPPLEMENTARY INFORMATION formed from uptake of Cl 2 or HOCl on bromide-ice film. This also indicates that bromide in the aqueous phase can be activated through the formation of BrCl. 8. Test of the conversion efficiency of HOCl to Cl 2 on inlet surfaces HOCl was synthesized by the reaction of gas phase Cl 2 that was bubbled through an aqueous solution of AgNO 3, analogous to the method we have used to synthesize HOBr (19). A mixture of HOCl and Cl 2 was either directly delivered to the CIMS or through Teflon tubing 2 times longer in length than the inlet used in the field. The Teflon tubing was coated with NaCl crystals and held at the same temperature as the inlet surfaces in the field (30 o C). The flow was only 25% of the flow rate in the field and the humidity was adjusted to a value representative for Barrow (H 2 O mixing ratio = 0.08%). The decrease of the HOCl signal and the increase in the Cl 2 signal was monitored to measure the conversion of HOCl to Cl 2 on the coated test inlet. We found that 15% of the HOCl was lost but only 2% was converted to Cl 2 in the coated test inlet. However, we found no evidence of HOCl conversion to Cl 2 in an uncoated inlet at the same conditions. The test conditions for the coated inlet are clearly more favorable for Cl 2 production than in the field instrument as the wall interaction is much greater due to the longer length and slower flow rate. For this reason, we believe that 15% HOCl conversion is a conservative upper limit for the inlet used in the field and that the Cl 2 observed in Barrow is clearly not an artifact of HOCl conversion. NATURE GEOSCIENCE 11

12 Reference: 31 Finley, B. D. & Saltzman, E. S. Measurement of Cl 2 in coastal urban air. Geophys. Res. Lett., 33, 11, L11811 (2006). 32 Helmig, D., Ganzeveld, L., Butler, T. & Oltmans, S. J. The role of ozone atmosphere-snow gas exchange on polar, boundary-layer troposphere ozone a review and sensitivity analysis. Atmos. Chem. Phys., 7, (2007). 33 Helmig, D. et al. Ozone dynamics and snow-atmosphere exchanges during ozone depletion events at Barrow, Alaska. J. Geophys. Res.-Atmos. 117, D20303 (2012). 34 Fast, J. D. et al. Evolution of ozone, particulates, and aerosol direct radiative forcing in the vicinity of Houston using a fully coupled meteorology-chemistryaerosol model. J. Geophys. Res.-Atmos. 111, doi:d /2005jd (2006). 35 Cooper, O. R. et al. A springtime comparison of tropospheric ozone and transport pathways on the east and west coasts of the United States, J. Geophys. Res.- Atmos, 110, D05S90, doi: /2004jd (2005). 36 Cooper, O. R. et al. Increasing springtime ozone mixing ratios in the free troposphere over western North America, Nature, 463, , doi: /nature08708 (2010). 37 Stohl, A. et al. A backward modeling study of intercontinental pollution transport using aircraft measurements. J. Geophys. Res.-Atmos. 108, doi: /2002jd (2003). 38 Kwok, R., Annual cycles of multiyear sea ice coverage of the Arctic Ocean: J. Geophys. Res. 109, C11004, doi: /2003jc002238(2004). 12 NATURE GEOSCIENCE

13 SUPPLEMENTARY INFORMATION 39 Simpson, W. R., Carlson, D., Hönninger, G., Douglas, T. A., Sturm, M., Perovich, D. & Platt, U.: First-year sea-ice contact predicts bromine monoxide (BrO) levels at Barrow, Alaska better than potential frost flower contact. Atmos. Chem. Phys. 7, (2007). 40 Pratte, P. & Rossi, M. J. The heterogeneous kinetics of HOBr and HOCl on acidified sea salt and model aerosol at 40-90% relative humidity and ambient temperature. Physical Chemistry Chemical Physics 8, , doi: /b604321f (2006). 41 Fluckiger, B. & Rossi, M. J. Common precursor mechanism for the heterogeneous reaction of D2O, HCl, HBr, and HOBr with water ice in the range K: Mass accommodation coefficients on ice. J. Phys. Chem. A 107, ,doi: /jp021956u (2003). 42 Hu, J. H. et al. Reactive uptake of Cl 2 (g) and Br 2 (g) by aqueous surfaces as a function of Br - and I - ion concentration- The effect of chemical-reaction at the interface. Journal of Physical Chemistry 99, , doi: /j100021a050 (1995). 43 Atkinson, R. et al. Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I - gas phase reactions of O(x), HO(x), NO(x) and SO(x) species. Atmos. Chem. Phys. 4, (2004). 44 Erickson, D. J., Seuzaret, C., Keene, W. C. & Gong, S. L. A general circulation model based calculation of HCl and ClNO2 production from sea salt dechlorination: Reactive Chlorine Emissions Inventory. J. Geophys. Res.-Atmos. 104, , doi: /98jd01384 (1999). NATURE GEOSCIENCE 13

14 45 Sjostedt, S. J. et al. Observations of hydroxyl and the sum of peroxy radicals at Summit, Greenland during summer Atmos. Environ. 41, , doi: /j.atmosenv (2007). 46 Foster, K. L. et al. The role of Br-2 and BrCl in surface ozone destruction at polar sunrise. Science 291, , doi: /science (2001). 47 Finley, B. D. & Saltzman, E. S. Observations of Cl(2), Br(2), and I(2) in coastal marine air. J. Geophys. Res.-Atmos. 113, doi:d /2008jd (2008). 48 Fickert, S., Adams, J. W. & Crowley, J. N. Activation of Br-2 and BrCl via uptake of HOBr onto aqueous salt solutions. J. Geophys. Res.-Atmos. 104, , doi: /1999jd (1999). 49 Adams, J. W., Holmes, N. S. & Crowley, J. N. Uptake and reaction of HOBr on frozen and dry NaCl/NaBr surfaces between 253 and 233 K. Atmos. Chem. Phys. 2, (2002). 50 Huff, A. K. & Abbatt, J. P. D. Gas-phase Br-2 production in heterogeneous reactions of Cl-2, HOCl, and BrCl with halide-ice surfaces. J. Phys. Chem. A 104, , doi: /jp001155w (2000). 14 NATURE GEOSCIENCE

15 SUPPLEMENTARY INFORMATION Table 1S. Reactions included in the time dependent model. Gas phase reactions Reaction rates (molec cm -3 s -1 ) Reaction rates (250K) 1. Cl + O 3 ClO + O exp(-200/t) Cl + CH 4 HCl + CH exp(-1280/t) Cl + CH 2 O HCl + CHO exp(-30/t) Cl + C 2 H 6 HCl + C 2 H exp(-70/t) Cl + C 2 H 4 ClC 2 H 4 k 0 = (T/300) k = (T/300) Cl + C 2 H 2 ClC 2 H 2 k 0 = (T/300) k = (T/300) Cl + C 3 H 8 HCl + CH 3 CHCH 3 HCl + CH 2 CH 2 CH exp(-80/t) Cl + C 3 H Cl + n-c 4 H 10 HCl + 1-C 4 H 9 HCl + 2-C 4 H exp(-120/T) exp(55/T) ) Cl + i-c 4 H 10 HCl + C 4 H Cl +CH 3 CHO HCl + CH 3 CO HCl + CH 2 OH Cl + CH 3 OH HCl + CH 2 OH Cl + CH 3 C(O)CH 3 HCl exp(-1000/t) CH 3 C(O)CH Cl + CH 3 C(O)CH 2 CH 3 HCl + CH 3 C(O)CHCH 3 HCl + CH 2 C(O)CH 2 CH exp(80/t) HCl + CH 3 C(O)CH 2 CH Cl + n-c 5 H 12 HCl + C 5 H NATURE GEOSCIENCE 15

16 16. Cl + i-c 5 H 12 HCl + C 5 H Cl + benzene Cl + HO 2 HCl + O 2 ClO + OH exp(170/t) exp(-450/t) ClO + HO 2 HOCl + O exp(220/t) ClO + ClO Cl 2 + O 2 ClOO + Cl OClO + Cl exp(-1590/t) exp(-2450/t) exp(-1370/t) Cl 2 O 2 k 0 = (T/300) k = (T/300) ClO + NO Cl + NO exp(220/t) k = (T/300) ClO + BrO Br + OClO exp(550/t) Br + ClOO exp(260/t) BrCl + O exp(290/t) ClO + OH Cl + HO exp(270/t) HCl + O exp(230/t) Cl 2 + OH HOCl + Cl exp(-900/t) HOCl + OH ClO + H 2 O exp(-500/t) HCl + OH Cl + H 2 O exp(-350/t) Photolysis reactions (s -1 ) 28. Cl + hv 2Cl 29. ClO + hv Cl + O 30. HOCl + hv Cl + OH Heterogeneous reactions * * * Lifetime due to heterogeneous loss 31. HOCl + Cl - + H + Cl 2 + H 2 O 15.3 min 16 NATURE GEOSCIENCE

17 SUPPLEMENTARY INFORMATION 32. HCl Cl H 0.4 hr * Midday 11: 00 16: 00 average Table 2S. The typical input levels in the box model. Input species Typical levels Sources O 3 18 ppbv Chemiluminescence observations CH 2 O 275 pptv Tunable Diode Laser Spectroscopy observations NO NO 2 HO 2 /OH 5 pptv 7 pptv molec.cm -3 / molec.cm -3 (daytime) Chemiluminescence observations (exclude polluted periods when NO > 20 pptv) Chemiluminescence observations (exclude polluted periods when NO 2 > 20 pptv) CIMS observations CH ppm Tropospheric average C 2 H ppbv canister observations C 2 H 4 55 pptv canister observations C 2 H pptv canister observations C 3 H pptv canister observations C 3 H 6 20 pptv canister observations n-c 4 H pptv canister observations i-c 4 H pptv canister observations n-pentane 60 pptv canister observations i-pentane 90 pptv canister observations CH 3 CHO 70 pptv TOGA VOC analyzer NATURE GEOSCIENCE 17

18 CH 3 OH 667 pptv TOGA VOC analyzer CH 3 C(O)CH ppbv TOGA VOC analyzer MEK 190 pptv TOGA VOC analyzer benzene 130 pptv TOGA VOC analyzer Aerosol surfaces 56 um 2 cm -3 SMPS observations Daytime = 06 : 00-20:00 Table 3S. Bromine reactions included in the HO x model. Chlorine reactions are the same as listed in Table 1S. Bromine reactions Reaction rates (molec cm -3 s -1 or s -1 ) Reaction rates (250K) 1. BrO + HO 2 HOBr exp(460/t ) BrO + BrO 2Br + O exp(40/t ) BrO + BrO Br 2 + O 2 3. BrO + NO Br + NO 4. Br + O 3 BrO + O 2 5. Br + CH 2 O HBr + CHO 6. Br + HO 2 HBr + O 2 7. HOBr + hv Br + OH exp(860/t ) exp(260/t ) exp( 800/T ) exp( 800/T ) exp(-310/t) BrO + hv Br + O NATURE GEOSCIENCE

19 SUPPLEMENTARY INFORMATION Figure captions: Figure 1S. The raw CIMS signal at mass 199 amu (I 35 Cl 37 Cl - ) is plotted versus 197 amu (I 35 Cl 35 Cl - ) for the ambient measurements of molecular chlorine. The black line is the ideal ratio of chlorine isotopes in nature. The red line is an equally weighted bivariate linear regression of the two isotopes. Figure 2S. (a) Map of QuikSCAT sea ice backscatter coefficient. The sea ice with a higher fraction of multi-year has a higher sea ice backscatter coefficient (blue). (b). Mean footprint residence time in the boundary layer (0-400 m) of 2-day retro-plumes released on daytime hours (6:00-18:00 local time) during 2 days with the highest (>51.24 pptv) daytime mean Cl 2 concentrations ([Cl 2 ] DM ). Note that for the two high chlorine days the plumes from both the northeast and south but that the residence time in the boundary layer near Barrow is large in both cases. (c). Mean footprint residence time in the boundary layer (0-400 m) of 2-day retro-plumes released on daytime hours (6:00-18:00 local time) during the 2 days with the lowest (<1.23 pptv) daytime mean Cl 2 concentrations ([Cl 2 ] DM ). For the two low chlorine days the wind is primarily from the north and does not spend much time in the boundary layer near Barrow. Figure 3S. Predicted HCl plotted versus observed HCl. The red line is the equally weighted bivariate regression and the grey dashed line is the 1:1 line. The correlation coefficient (R 2 ) is Figure 4S. Correlation plot of observed HOBr and predicted HOBr with a model that considers only bromine chemistry (blue) or bromine and chlorine chemistry (red). NATURE GEOSCIENCE 19

20 Figure 5S. Observed BrCl plotted versus observed Cl 2 multiplied by the Cl 2 heterogeneous deposition rate to snow and divided by J BrCl when J BrCl > s -1. The red line is the linear regression. Figure 6S. Scatter plot of Cl 2 versus NO measurements for the campaign. Figures: Figure 1S 20 NATURE GEOSCIENCE

21 SUPPLEMENTARY INFORMATION Figure 2S Figure 3S. NATURE GEOSCIENCE 21

22 Figure 4S Figure 5S 22 NATURE GEOSCIENCE

23 SUPPLEMENTARY INFORMATION Figure 6S NATURE GEOSCIENCE 23