Abstract: Open-Path Fourier Transform Infrared (OP-FTIR) spectroscopy involves the usage of an internal infrared source whose radiation is transmitted to a retroreflector, reflected back, captured by a telescope, and directed into a FTIR spectrometer. Spectral information is integrated along the path, providing a number of benefits: a reduced sensitivity to local emissions and wind, compared to in situ measurements, and 24-hour coverage, as opposed to sunlight-limited solar spectroscopy. This makes OP-FTIR useful for the monitoring of urban greenhouse gas concentrations – which, in turn, are very useful to understand in the process of monitoring anthropogenic climate change. This talk will provide an overview of the OP-FTIR technique and the Bruker IFS 125M OP-FTIR system installed in our very own McLennan Physical Laboratories and present several example studies of OP-FTIR usage, including insights into transport emissions and trends during the COVID-19 pandemic here in Toronto.