1. Ally Lewis, University of York, U.K Data from Katie Read, Jim Hopkins, Jacqui Hamilton. NMHCs, & DMS measurements by GC-FID Remote automated NMHC, - simple robust instrument. Addition of OVOC measurements – 2 columns / detectors

2. CASLab, British Antarctic Survey Halley Base, Antarctica -Year round NMHC -2005 Jan-Mar O 3 / HO x experiment Long term NMHC measurements: Halley Antarctic

3. Seasonal distribution of alkanes at Halley, Antarctica

4. From previous work the seasonal trend should follow a sinusoidal oscillation 1,2,3, this work showed a deviation in early Spring, particularly for ethane. OH chemistry is not thought to be significant at this time. Could potentially be an influence of Cl chemistry over this period? 1 Sharma, U. K., Y. Kajii, et al. (2000). "Seasonal variation of C 2 -C 6 NMHCs at Happo, a remote site in Japan." Atmos. Env. 34: 447-4458. 2 Swanson, A. L., N. Blake, et al. (2003). "Seasonal variations of C 1 -C 4 nonmethane hydrocarbons and C1-C4 alkyl nitrates at the Summit research station in Greenland." J. Geophys. Res 108(D2): 4065, doi:10.1029/2001JD001445. 3 Clarkson, T. S., R. J. Martin, et al. (1997). "Ethane and propane in the southern marine troposphere." Atmos. Environ. 31(22): 3763-3771. Deviation OH dependence of ethane and propane concentrations

5. Used the behaviour of NMHC and Cl kinetics to determine the [Cl] (and [Br] during periods of ozone depletion 1 [RH] LO = [RH] BKG exp(-k Cl ∫[Cl]dt) A plot of ln([RH] LO /[RH] BKG should give a linear relationship with a gradient equal to -∫[Cl]dt. So by integrating over set time periods we can obtain [Cl]. [Br] can also be calculated from the additional ethene and acetylene removal. [-k Br ∫[Br]dt =ln([C 2 H 2 ] LO /[C 2 H 2 ] BKG ) observed -ln([C 2 H 2 ] LO /[C 2 H 2 ] BKG ) Clatom For this depletion period this analysis gives a [Cl] of between 1.7 x 10 3 and 3.4 x 10 4 atom cm -3 and a [Br] of between 1.4 x 10 6 to 2.9 x 10 7 atom cm -3. 1 Jobson, B. T., H. Niki, et al. (1994). "Measurements of C 2 -C 6 hydrocarbons during the Polar Sunrise 1992 Experiment: Evidence for Cl atom and Br atom chemistry." J. Geophys. Res 99(D12): 25355-25368. Halogen atom presence during NMHC and O 3 depletion episodes

6. CO remained high during September whilst the O 3 and ethane decreased. We can apply the same analysis as with the ozone depletion episodes for this austral spring period. The gradient of:- ln ([HC] t =2 weeks /[HC] t=0 ) vs k HC+Cl implied NMHC reactions with a persistent [Cl] of 2.3 x 10 3 atom cm -3 over a two week period. 3 x 10 3 atom cm -3 Cl atom input into GEOS-CHEM model for just 15 days 1 1 Mat Evans and Sarah Walker- School of Earth and Environment (SEE), University of Leeds ethane-black line GEOS-CHEM model-blue line Cl atom presence during early spring

7. NMHC + OVOC in background N Atlantic air Mace Head, West coast Ireland methacrolien acetone acetaldehyde Methanol ethanol MVK benzene C2-C7 NMHCs + DMS Measurements by GC-2xFID

8. Sum NMHC=All NMHC except ethane. Sum O-VOC=Acetone + Acetaldehyde + Methanol + Methacrolein concentrations. Only in periods of anticyclonic + local easterly pollution does the mass sum of the NMHC compare to the sum of the O-VOC. 18 8 35 Time / mins 10 30 Time / mins

9. Combining NMHCs, methanol and DMS

10. Simulating wind speed dependence of measurements Calculated ocean uptake rate varied from 0.03 to 0.19 cm s -1 depending upon the wind speed. [Global vD of 0.1 cm s-1 estimated by Singh et al. (2002)]

11. Isoprene and degradation products during summer 2003