Picarro Cavity Ring Down Spectrometry (CRDS) for High-Precision Continuous Monitoring of Atmospheric CO 2 and CH 4 Dave Lowry, Rebecca Fisher, Mathias Lanoisellé, Sri Sriskantharajah and Euan Nisbet Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey TW20 0EX, United Kingdom Presented at: APRIL Meeting, London, 12/09 Funded by NERC since 2008

CRDS Theory When the laser is on, the cavity quickly fills with circulating laser light. A fast photo- detector senses the small mount of light leaking through one of the mirrors (not 100% reflective) to produce a signal that is directly proportional to the intensity in the cavity. When the photodetector signal reaches a threshold level (in a few tens of microseconds), the continuous wave (CW) laser is abruptly turned off. The light already within the cavity continues to bounce between the mirrors (about 100,000 times). The light intensity inside the cavity steadily leaks out and decays to zero in an exponential fashion. This decay, or "ring down", is measured in real-time by the photodetector.

The Model We Use Picarro Inc.WS-CRDS Analyzer for CO 2 /CH 4 /H 2 O in air - Model G1301 (Made in California) Wavelength-specific Simultaneous, precise measurement of CO 2, CH 4 and H 2 O How much does it cost? At current exchange rates about £40,000 once installed Automated inlet parts - £7,000 Inlet control software development - £5,000 Calibration gas suite - £3,000

Tasks Instrument Tests –Standardization with NOAA calibration gases –Precision –Water vapour correction –Long-term drift Development of Automated Inlet –Software –Automated valves and flow Deployment at Remote Sites –South Atlantic islands –RHUL

Picarro G1301

NOAA standardization Currently 6 NOAA standards in use in lab - 2 ambient air, 4 synthetic CRDS only measures 12 C CO2 and 12 C CH4 peaks Synthetic are depleted in 13 C CO2 and enriched in 13 C CH4 so extra correction necessary if not using ambient air standards

Precision - International Requirements for Background Stations Northern Hemisphere - CO 2 ± 0.05 ppm, CH 4 ± 2.0 ppb Southern Hemisphere - CO 2 ± ppm, CH 4 ± 1.0 ppb Residuals for 5 NOAA standards - CO 2 ± ppm, CH 4 ± 0.16 ppb Achieved on 10 min ave (120 points) -CO 2 ± ppm, CH 4 ± 0.15 ppb

Water Vapour Correction Factory applied for CO 2 but…. Currently no consensus for the CH 4 correction Calculated by having two instruments side by side on the same air line, one of which has a Nafion-dried airstream - reduces water from 1.5% to 0.1% = raw CH 4 (ppb) x (1 + %H 2 O x constant) Calculated constant = Nafion reduces correction from ≈27 to 2 ppb ppm (18.98 ppb) / 1% H 2 O ppm / 1% H 2 O

Long Term Drift CO 2 - no obvious drift - up to 0.1 ppm fluctuation CH 4 drifted by 3-4 ppb in 1 year (less when air is dried) - <0.2%

Automated Inlet Development 6 standard bottles: 3 calibration 1 target 1 zero 1 long term

Automated Inlet Software Controls cylinder outlet pressure at 0.5 bar (7 psi) Controls flow at cc/min Gives real-time data display Creates calibration file and makes drift correction between calibrations Remote data transfer capabilities

Deployment in the South Atlantic Falkland Islands and Ascension Island Close to airports for communications

Deployment at RHUL Very local source now affecting all air from W-NW during working hours Source ? P2 P3 LiCor