DOI: 10.1594/pangaea.884313
title: OP-FTIR measurements of trace gases relevant to air quality in Western Sydney, Australia from 28 October 2016 to 13 September 2017

description: Location Spectrometer: Roof of 2 storey building at 2 Percy St, Auburn NSW 2144 Australia, -33.85472, 151.0373, 20.6 m rooftop above sea level, height rooftop above street level = 6.72 m; height measurement path above rooftop= 1.2 mLocation Reflectors: Roof of 3 story building, Cumberland City Council, Auburn Office, at 1 Susan St Auburn 2144, Australia, -33.85311, 151.0335, 40.8 m rooftop above sea level, height roof top above street = 12.8 m, height mid-mirrors above rooftop = 2.4 m,Distance between Spectrometer and reflectors = 395.8 m one-way (instrument to reflector) + 1 m Return Measurement path = 792.6 m (including 1 m internal reflectance) Measurement Path Slope: = 5.3 Degrees; Difference in altitude = 20.9 mMeasurement Path Bearing: 296.0278 degreesGas Species Reported (units): Carbon Monoxide (CO, ppbv) Carbon Dioxide (CO2, ppmv) Nitrous Oxide (N2O, ppbv), Ammonia (NH3, ppbv), Methane (CH4, ppbv).Instrument description: The open path FTIR system is based on an FTIR spectrometer (Matrix-M IR cube, Bruker Optik, Ettlingen, Germany) which provides modulated infrared radiation with 1 cm-1 resolution from a globar source in a nominally parallel 25 mm diameter output beam. The beam passes through a beamsplitter (ZnSe window, 50 x 3 mm) to the secondary mirror of an on-axis beam expander which expands the beam diameter to 250 mm and reduces the divergence by a factor of 10. The beam expander was constructed from a modified 10 inch Schmidt-Cassegrain telescope (Model LX200R, Meade Instrument Corp., California, USA) by removing the standard Schmidt correction plate and secondary mirror and replacing it with a convex mirror of effective focal length 50mm. The beam expander is focussed by shifting the secondary mirror along the optic axis so that its focus is coincident with that of the primary mirror. An optically black mask ~ 5 mm in diameter at the centre of the secondary mirror blocks the back reflection of radiation to the detector from the centre of the mirror. The expanded beam follows an open path to 3 x 300 mm diameter retroreflector arrays (PLX Industries, New York, USA) located ~500 m distant from the spectrometer. The retroreflector returns the beam back on itself through the beam expander to the beamsplitter. The reflected beam from the beamsplitter is focussed by a 29 mm focal length 90 degrees off-axis paraboloidal mirror to a cooled MCT detector (Infrared Associates Inc., Florida, USA). The detector is cooled to liquid nitrogen temperature by a Stirling cycle mechanical refrigerator (RicorK508), removing the need for a liquid nitrogen supply in field applications. The FTIR spectrometer, beamsplitter, beam expander and detector are mounted on a single 100 mm optical rail to allow simple and robust alignment. The optical rail is mounted to a heavy duty tripod (Gibralter model 4-60450-OA, Quickset International Inc., Illinois, USA ) with a computer controlled Automated Instrument Mount (AIM Colterlec, Unanderra, Australia) to the allow accurate and stable alignment of the beam between spectrometer and retroreflector. The reported precision of the instrument is: NH3 1 ppb, N2O 0.6 ppb, CO2 0.5 ppm, CH4 2 ppb, CO 1 ppb. Data Collection Rate: average 5 min, timestamped at start of data collection period.Spectral Analysis: MALT (Griffith, D.W.T., Synthetic calibration and quantitative analysis of gas phase infrared spectra, Applied Spectroscopy, 50 (1), 59-70, 1996) with spectral parameters from HITRAN08 database (www.hitram.com). Spectral Micro Windows: CO2, N2O, CO and H2O 2150-2280 cm-1 CH4, H2O 3001-3140 cm-1 NH3, H2O 900-945 and 955-995 cm-1 Data QA: Data were removed when the maximum spectral intensity in the 2300 cm-1 spectral region was reduced to &lt; 40% of the typical maximum value. Reduction in spectral intensity was typically due to rain, dew or dust on the telescope or retro-reflector surfaces.Calibration and Validation: Data (CO, CO2, CH4 and N2O) were compared with concurrent data from the Spectronus in-situ FTIR spectrometer. The regression results used to correct the OP-FTIR data (data collected at wind speed &lt; 1 ms-1 were removed from the regression).Interruptions and Issues:Instrument Failure: UoW OP-FTIR OP3 failed with laser failure on 28 May 2017 14:15 and was replaced by OP5 on 31 May 2017 14:14; Optics on OP-FTIR updated on 16 June 2017 10:35. Instrument shut down when air temperature exceeded 42 C to protect instrument against over-heating.