These data are retrievals of column-average dry-air mole fractions of CO2, CH4, CO, and H2O  (referred to as xco2, xch4, xco, xh2o) collected using a Bruker Optics EM27/SUN Fourier Transform Spectrometer (FTS) at four of the five long-term monitoring sites in the Greater Toronto Area (see map). Retrievals from the two EM27/SUN FTS owned by the University of Toronto are published separately: ta (UTSG; doi: 10.5683/SP3/WMQFDC) and tb (UTSC; doi: 10.5683/SP3/80TERS).  
These observations are ongoing as of the date these are published. The data were processed using GGG2020, which is also used by the Total Carbon Column Observing Network (TCCON). The retrieval algorithm uses a scaling profile retrieval in which a prior estimate of the atmospheric profile of trace gas concentrations is derived from a model. The relative concentrations between atmospheric layers (i.e., the profile shape) in the prior remains fixed and least squares methods are used to calculate a scaling factor for this profile that optimizes the fit to the measured spectra. The resulting scaled profile is then integrated to yield a total column-average dry-air mole fraction (more details in doi: 10.14291/tccon.ggg2014.documentation.R0/1221662).   Quality filtering and post-processing methods are described by Mostafavi Pak et al., 2023 (doi: 10.5194/amt-16-1239-2023). In post-processing, an airmass dependent bias correction is applied, followed by an additive bias correction to bring all the EM27 instruments into agreement with a standard reference instrument, and finally a scaling factor is applied to correct to the WMO scale based on comparisons to AirCcore measurements. 
Because of their geographic proximity, the retrievals for all Toronto sites, excluding the remote northern site at CARE , used common prior profiles  based on model data interpolated to 43.63N, 79.38W (shown on map). This was done to minimize the impacts of using different priors when analyzing gradients across the city of Toronto, but the CARE site is too far from the other sites for this to be reasonably applied. For the CARE site, prior profiles are based on model data interpolated to the exact site location. Measurements provided here end on the dates before the instruments for each site were modified to use a new fibre optic solar tracking system.
Note that data files for the tg instrument at UTM during 2023-06-02 to 2023-11-30 and 2024-07-08 to 2024-09-02 had a malfunction in the detector that measures xco, but other gases were still retrieved. All the data files are roughly organized by year, but in some cases when data collection only occurred in a small fraction of the year, those data are combined with the previous or following year. 
