Monitoring Urban Greenhouse Gases in Downtown Toronto 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 useful data for estimating urban emissions. Open-path Fourier transform infrared (OP-FTIR) spectroscopy is a non-intrusive technique that can be used to simultaneously measure multiple atmospheric trace gases. Here, we present two years of continuous measurements by an OP-FTIR system located in downtown Toronto, Canada. This system is used to retrieve CO₂, CO, CH₄, N₂O, H₂O, and HDO volume mixing ratios (VMRs) over a two-way total atmospheric open path of 320 m using non-linear least squares fitting. Our results show that retrieved VMRs of CO₂, CO, CH₄ and N₂O are sensitive to urban emissions from Toronto. The VMRs of these gases are compared and their diurnal variations are investigated to assess potential urban sources. By performing comparisons with measurements from an in situ analyzer 5.4 km south of the OP-FTIR system, it is demonstrated that the diurnal horizontal gradients in CO₂ and CO VMRs between these sites are enhanced for weekdays relative to weekends consistent with bottom-up emission inventories. Emissions of CO₂ and CO are calculated from the horizontal gradients between the sites, and are compared with bottom-up estimates. This presentation describes the measurement results and the limitations of the methodology of calculated emissions.