Comparison of first Saint Mary’s University Open-Path Fourier Transform Infrared (OP-FTIR) spectrometer measurement results with National Air Pollution Surveillance (NAPS) air quality measurements in Halifax

Abstract

The atmosphere is very complex and it involves many chemical and physical processes that affect the air people breathe. This is why it is important to characterize the air in the atmosphere in order to determine what people are exposed to every day. Carbon monoxide (CO), a toxic air pollutant emitted primarily as a result of incomplete combustion and oxidation of hydrocarbons, was measured in Halifax, Nova Scotia using an Open-Path Fourier Transform Infrared (OP-FTIR) spectrometer and compared to National Air Pollution Surveillance (NAPS) CO measurements as a verification step in the characterization of this new instrument. Measured NAPS data was compared to OP-FTIR spectrometer results for three different measurement campaigns: Robie Street (at Inglis for 2 hours), Rice Building (at SMU for ~1 week), and Lake Major (in Dartmouth for ~1 week). For each campaign, spectra were recorded and a concentration of CO was retrieved for each spectrum (one per minute) by the program MALT. The retrieved CO concentrations were plotted in a time series for each campaign and compared to NAPS CO concentration measurements obtained on Barrington Street at the same time. Data quality of the OP-FTIR spectrometer was assessed in detail, with the majority of spectral fit residuals and their RMS values below 0.01 (1%), indicating a reasonable fit between the measured spectra and the fitted spectra simulated by MALT. The technique’s accuracy was previously conservatively estimated to be no worse than 10%; however, for all three campaigns, there was a clear systematic bias of up to 0.35 ppm (a factor of ~3) between the OP-FTIR spectrometer and NAPS measurements, along with unexplained enhancements in CO concentration at times and locations with minimal vehicle activity. Further studies are suggested in order to fully explain the reason for the systematic bias and unusual enhancements in CO concentration observed.