Monitoring Urban Greenhouse Gases Using Open-Path Fourier Transform Spectroscopy
Urban areas are large sources of greenhouse gases (GHGs) to the atmosphere. Measurements of atmospheric GHGs in urban areas provide information on these emissions, which can complement bottom-up estimates. Here, we present an Open-Path Fourier Transform Infrared (OP-FTIR) spectroscopy system for GHG monitoring in Toronto, Canada. We describe the installation of the OP-FTIR and retrieval of CO2, CO, CH4, N2O, and H2O dry-air mole fractions and δD over a two-way atmospheric open path of approximately 320 m using non-linear least squares fitting. The OP-FTIR measurements of CO2, CO, and CH4 are then calibrated using measurements from two Picarro GHG Cavity Ringdown Spectrometers deployed at both ends of the system. Our results show that retrieved dry-air mole fractions of CO2, CO, CH4, and N2O are sensitive to urban emissions from Toronto. In addition, CH4 measurements are influenced by a localized source southwest of the observing system, presumably a natural gas leak, and N2O measurements are influenced by an undetermined source to the northeast of the OP-FTIR. By performing comparisons with measurements from an in situ detector 5.4 km south of the OP-FTIR system, it is demonstrated that the diurnal gradients in CO2 and CO between these sites are enhanced for weekdays relative to weekends, consistent with bottom-up emission inventories. Emissions of CO2 and CO are then calculated from the gradients between the sites. The emissions are found to be consistent with bottom-up estimates but are too imprecise to further refine the bottom-up inventories.