Analysis of gaseous I 2 in the marine boundary layer

Transcription

1 Analysis of gaseous I 2 in the marine boundary layer Thorsten Hoffmann, Ru-Jin Huang, Michael Kundel, Christopher Kampf TORERO workshop, NCAR, October 31 th,

2 Outline I. Introduction II. Denuder-GC/MS Method III. Comparison with DOAS Method IV. Practical Aspects for TORERO Cruise V. Outlook: GTRAP-TOF-AMS Method

3 I. Introduction Schematic diagram of the major halogen-related reactions in the gas and aqueous phase analytical methods available resonance-fluorescence (RF) cavity ring down spectroscopy GC-MS long path differential optical absorption (LP-DOAS) analytical methods not available Datum: Folie Nr. 3 [von Glasow and Crutzen, 2007]

4 II. Denuder-GC/MS Method Sampling of molecular iodine using an cyclodextrin coated diffusion denuder a-d-glucose molecule a-cyclodextrin hydrohobic cavity (diameter: 4,7-5,3Å) host-guest complex hydrophilic envelope

5 II. Denuder-GC/MS Method Sampling mechanism a-cyclodextrin [Yoshida 1999] Diffusion denuder Particles Gaseous non-target compound Gaseous target compound

6 II. Denuder-GC/MS Method Sample preparation and measurement Sampling addition of Iodide-129 standard solution (SRM 4949C) 127 I/ 129 I ratio of 0.17 Elution, derivatization and extraction Concentrated to 100 µl GC-MS [Huang and Hoffmann, 2009] N N I H + + I - I

7 II. Denuder-GC/MS Method Potential interference from highly reactive iodine compounds MeO OMe TMB OMe OMe + Br-I + (Cl-I) + (HO-I) MeO OMe OMe I OMe I I d + d - molecular iodine + Cl-Br Br Iodine MeO shows a much OMehigher + (HO-Br) reactivity + (Br towards electrophilic attack 2 ) to organic molecules Cl-I I 2 I 2 MeO OMe I Cl d + d - positively polarized iodine (interhalogens, cyclodextrin HOI, IONO 2 ) pump Datum: Folie Nr. 7 analysis by GC-MS

8 II. Denuder-GC/MS Method Sampling setup needle valve/ membrane pump a-cd/ 129 I - -coated denuder (brown glass) 1,3,5-TMB-coated denuder (brown glass) rain protection cap

9 III. Comparison with DOAS Method Analytical parameters Limit of Detection Time Resolution Spatial Resolution Denuder-GC/MS off-line 0.17 ppt 30 min single-point LP-DOAS on-line 20 ppt 30 min several km Mace Head Atmospheric Research Station (53 o 20 N, 9 o 54 W)

10 III. Comparison with DOAS Method Data correlation

11 IV. Practical Aspects for TORERO Cruise Sampling issues - Sensitivity: Time resolution of 30 min: Expected mixing ratios of I 2 between 1-5 ppt Time resolution of 60 min: If expected mixing ratios are lower - Temperature at sampling site: Sampling so far at temperatures 25 C Shadowing of sampling setup recommended - Contamination: Diesel exhaust problematic Suggestions for counter measures? (especially when ship is not moving)

12 IV. Practical Aspects for TORERO Cruise Storage and shipment - Shipment: GER US: To Boulder or to Hawaii CR GER: Cooling needed - Storage: Loaded denuders have to be stored in a refrigerator or freezer

13 V. Outlook: GTRAP-TOF-AMS Method Gaseous compound trapping in artificially generated particles followed by AMS analysis (GTRAP-AMS) I 2 (synth. Air) charcoal denuder 0.5 L flow tube ToF-AMS N L/min a-cd / NH 4 Br in MeOH/H 2 O (99:1 v/v) I 2 trapped in a-cd / NH 4 Br particles overflow Br - improves uptake efficiency uptake efficiency: 60-70%

14 V. Outlook: GTRAP-TOF-AMS Method TOF-AMS measurement m/z a-cd-fragment m/z I + m/z a-cd-fragment I m/z I 2 + m/z I 2 + m/z= a-cd-fragment x a-cd-fragment Datum: Folie Nr. 14

15 V. Outlook: GTRAP-TOF-AMS Method 30 min 10 min 5 min 3 min 2 min 1 min Analytical parameters ToF-AMS detection limit mainly depends on ion counting statistics increasing the averaging time of the data acquisition improves the detection limit (LOD ~ 1 / sqrt (t)) Succesful application in laboratory experiments Improvement of LOD necessary for field measurements Datum: Folie Nr. 15

16 Acknowledgement Prof. Dr. Thorsten Hoffmann Work group Hoffmann supported by the German Research Foundation (DFG) within the Research Training Group 826:Trace Analysis of Elemental Species: Development of Methods and Applications Thank you for your attention! Datum: Folie Nr. 16