High-precision surface-level CO2 and CH4 using off-axis integrated cavity output spectroscopy OA-ICOS over Shadnagar, India

An enhanced performance model of the greenhouse gas analyser GGA instrument from Los Gatos Research LGR, which uses off-axis integrated cavity output spectroscopy OA-ICOS technology to obtain highly precise and accurate measurements, has been installed at the National Remote Sensing Center NRSC, Shadnagar with the objective of generating a long-term record of measurements conforming to standards set by the World Meteorological Organisation WMO. A calibration procedure to ensure the precision and accuracy of measurements was formulated using National Oceanic and Atmospheric Administration-Earth System Research Laboratory NOAA-ESRL calibration gases bearing WMO certification. The 10 s 1 σ average precision of carbon dioxide CO2 and methane CH4 measurements by the GGA was 101 parts per billion volume 0.30 ppbv and accuracy 78 ppbv 0.24 ppbv, respectively. The 5 s average of 1 Hz measurements was used to compute precision, which varied from 92.52 to 116.36 ppbv 1σ, 1 Hz = 0.66 parts per million volume ppmv, WMO-2009 for CO2 and CH4 0.45 ppbv to 0.55 ppbv 1 σ, 1 Hz = 2.48 ppbv, WMO-2009 respectively. Diurnal variation of greenhouse gas concentrations, temperature, and pressure, showed high fluctuations when cavity pressure and temperature varied from the standard values. The residuals of dry mixing ratios with respect to w.r.t. the NOAA span gases varied from +30 to −60 ppbv for CO2 and ±0.35 ppbv for CH4. A case study using the corrected observations over 1 year revealed the role of wind velocity and anthropogenic emissions, as well as the seasonal variations in the ambient concentrations of CO2 and CH4 gases near the surface.