Experimental Methods for the Detection of Atmospheric Trace Gases Andreas Hofzumahaus Forschungszentrum Jülich, IEK-8 Literature: D.E. Heard, Analytical Techniques for Atmospheric Measurement, Blackwell Publ., 2006 B.J. Finlayson-Pitts and J.N. Pitts, Upper and Lower Atmosphere, Academic Press, 2000 B. Kolb, Gaschromatographie in Bildern, Wiley-VCH, 1999 H.K. Roscoe and K.C. Clemitshaw, Science 276, 1065-1072, 1997 M.W. Sigrist, Air Monitoring by Spectroscopic Techniques, Wiley Interscience, 1994 Model + Theory -Development Numerical Simulations Field Observations and Measurements Atmospheric Chemistry Knowledge Laboratory Studies Reaction Kinetics Photochemistry Spectroscopy Thermodynamics Simulation Chamber Experiments 1
Field Observations Antarctic Ozone Hole NASA ER-2 Research Aircraft O 3 detected by Farman, Gardiner and Shanklin, 1985 ClO Anderson et al., 1989 Measurements in Atmospheric Research Trace Gases Photolysis Frequencies Aerosols (chem. composition, hygroscopicity, optical properties, size distributions) Meteorological Parameters (p, T, wind, humidity, precipitation, radiation) Emission- and Deposition Rates (gases, aerosols) 2
Applications Atmospheric State Description Process Studies Monitoring - TÜV (Technische Überwachung) - LUA (Landesumweltämter) - UBA (Umweltbundesamt) - DWD (Deutscher Wetterdienst) - GAW (Global Atmospheric Watch) - NDSC (Network for the Detection of Stratospheric Change) Local < 100 m Regional mesoscale, < 2000 km Synoptic Global > 1000 km Experimental Methods [Trace gases] Which substances? General requirements for detection and measurements? Measurement methods? 3
Relevant Atmospheric Gases Examples O-, H- Compounds X- Compounds (X = F, Cl, Br, J) O 3 H 2 O, HO, HO 2, H 2 O 2 N- Compounds NH 3 NO, NO 2, NO 3 HONO, HONO 2, HO 2 NO 2 S- Compounds H 2 S, COS (CH 3 ) 2 S SO 2, H 2 SO 4 HX, XO, X n O m FCKW, Halones C- Compounds CO, CO 2 CH 4, Alkanes, Alkenes, Alkines, Aromatics HCHO, CH 3 CHO, Aldehydes (CH 3 ) 2 CO, Ketones Biogenic Compounds (Isoprene, Terpenes, Alcoholes...) RO 2 ROOH RO 2 NO 2 (e.g. Peroxyacetylnitrate, PAN) RONO RONO 2 General Requirements Wishes? uff! Sensitivity? As high as possible! Selectivity? Yes Multicomponent analysis! Specificity? No interferences wanted! Sampling? Wall less + contamination free! Temporal resolution? Spatial resolution? As high as necessary! (depends on the scientific question) Logistics? Simple and inexpensive! 4
Temporal and Spatial Scales Most difficult to measure Platforms and Sensors "in-situ" local (point) measurements "remote" optical measurements along a light path through the atmosphere Roscoe and Clemitshaw (1997) 5
Measurement Methods Universal methods - measure several components - must be selective - must be specific Specialized methods- measure only one component - must be specific Surrogate methods - measure pseudo components representing a class of components or a chemical family e.g. NO x = NO + NO 2 RO x = OH + HO 2 + RO + RO 2 Universal Methods Workhorses Type Technique Substances (Examples) in-situ GC CO, H 2, CH 4, HCs PTR-MS VOCs MS Noble gases, HCs CIMS OH, HNO 3, H 2 SO 4 in-situ opt. Spectrospopy "everything" and - UV-VIS DOAS remote sensing - FTIR - Microwave GC PTR-MS MS CIMS DOAS FTIR Gaschromatography (combined with different detectors) Proton Transfer Reaction Mass Spectrometry Mass spectrometry Chemical Ionisation Mass Spectrometry Differential Optical Absorption Spectroscopy Fourier-Transform Infrared Spectroscopy 6
Specialized Optical Methods Type Technique Substances (Examples.) in-situ Chemiluminescence NO, O 3 Fluorometry HCHO, H 2 O 2, ROOH Photometry O 3, CO 2 Laserspectroscopy "everything" - DOAS OH - LIF OH, HO 2, RO 2, NO - TDLAS HCHO, H 2 O 2, C 2 H 2 - PF HNO 2, HNO 3, NH 3 - PAS CH 4, CO 2, CO, SF 6 - CRDS NO 3 LIF TDLAS PF PAS CRDS Laser-induced Fluorescence Tunable diode laser absorption spectroscopy Photofragmentation Photoacoustic spectroscopy Cavity ring-down absorption spectroscopy Specialized Optical Methods Type Technique Substances (Examples) remote LIDAR sensing - DIAL O 3, SO 2, NO 2, C 6 H 6 - Fluorescence OH, Na (h ~100 km) - Raman N 2, O 2, H 2 O - Mie Aerosols LIDAR DIAL Light detection and ranging Differential absorption LIDAR 7
Specialized Techniques (others) Type Technique Substances (Examples) in-situ MIESR HO 2, CH 3 O 2, NO 2, NO 3 CA (OH + HO 2 + RO + RO 2 ) Derivatization + HPLC Electrochemical HCHO, CH 3 CHO O 3 (sonde) Wet chem. analysis - + gas stripping HNO 3, Nitrate (Aerosols) - + Denuder HNO 3 - + Filter HNO 3, Nitrate (Aerosols) MIESR CA HPLC Matrix-Isolation Electron Spin Resonance Chemical Amplifier High performance liquid chromatography Experimental Methods (Examples) Differential Optical Absorption Spectroscopy (DOAS) numerous trace gases + radicals (OH, NO 3 ) Laser Induced Fluorescence (LIF) radicals (OH, HO 2, RO 2 ) Gas Chromatography (GC) numerous trace gases (VOCs, CO, H 2 etc.) 8
Differential Optical Absorption Spectroscopy Differential Optical Absorption Spectroscopy 9
Differential Optical Absorption Spectroscopy Intensity Aerosol Extinction I 0 ( ) Rayleigh Extinction Lambert-Beer's law ln(i/i' 0 ) = ' c L I 0 ( ) I( ) ' differential absorption cross-section (cm 2 ) c number density (cm -3 ) L absorption length (cm) 1 0 2 Wavelength adapted from S. Trick (2004) Differential Optical Absorption Spectroscopy Detection Limit pptv (5 km pathlength) 10
Differential Optical Absorption Spectroscopy OH SO 2 CS 2 CH 2 O C 10 H 8 Differential Optical Absorption Spectroscopy DOAS Frequency doubled dye laser Spectrograph 20 m Optical Multireflection Cell Detector Atmosphere Field Container 11
[OH] = ~ 10 +6 cm -3 OH = 1.6 x 10-16 cm 2 L = ~ 1 km I I 0 0.001% Laser Spectroscopy Energy of OH Molecule E k h (laser) h (fluorescence) Absorption Emission E i E k E i = h ik = hc / ik h Planck's constant c light velocity frequency wavelength 12
Laser Spectroscopy I o ( ) incident laser beam Sample OH I( ) = I o ( ) e - [OH]L Attenuated laser beam follows Lamber-Beer's law - requires no calibration - needs long light path (~ 1km) Fluorescence I ~ [OH] - requires calibration - small detection volume (~ 1cm) Laser Induced Fluorescence (LIF) Detection OH-detection Ambient Air HO 2 -detection Ambient Air Inlet Nozzles NO-Injector laser (308 nm) Gas Expansions PMT Field of View Vacuum Pump OH fluorescence detection using a photomultiplier (PMT) for photon counting Vacuum Pump HO 2 + NO OH + NO 2 13
Atmospheric OH Measurements Atmospheric OH Measurements HO x -Radical Measurements at SAPHIR 14
Airborne Measurements HO x, MaxDOAS, j-values Top platform with LIF T, p, wind O 3, NO x, CO, H 2 O HCHO, HONO, VOC, Aerosol MaxDOAS, j-values Zeppelin NT, Friedrichshafen am Bodensee Gondola 15