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Multiscale observations of NH3 around Toronto, Canada

Ammonia (NH3) is a major source of fine particulate matter and nitrates in the atmosphere. As such, there have been increasing efforts to monitor NH3. This study examines long-term measurements of NH3 around Toronto, Canada, derived from three multiscale datasets: 16-years of total column measurements using ground-based Fourier Transform InfraRed (FTIR) spectroscopy, 3-years of surface in-situ measurements, and 10-years of total columns from the Infrared Atmospheric Sounding Interferometer (IASI) sensor onboard the Metop satellites. These datasets were examined to quantify NH3 temporal variabilities (trends, inter-annual and seasonal) over Toronto, as well as to assess the observational footprint of the FTIR measurements. In addition, two case studies of pollution events due to transport of biomass burning plumes occurring in August 2014 and May 2016 have been studied. All three timeseries (FTIR, in-situ, and IASI) showed increasing trends in NH3 over Toronto: 3.34 ± 0.44 %/year from 2002 to 2018 in the FTIR columns, 8.88 ± 2.49 %/year from 2013 to 2017 in the surface in-situ data, and 8.98 ± 0.73 %/year from 2008 to 2018 in the IASI columns. To assess the observational footprint of the FTIR NH3 columns, correlations between the datasets were examined. The best correlation between FTIR and IASI was found for a coincidence criterion of ≤ 20 km and ≤ 20 minutes, with r = 0.73 and a slope of 1.125 ± 0.060. The FTIR columns and in-situ measurements were standardized and correlated, with 24-day averages and monthly averages yielding correlation coefficients of r = 0.72 and r = 0.75, respectively. FTIR and IASI were also compared against the GEOS-Chem model, run at 2° x 2.5° resolution, to assess model performance and investigate correlation of the model output with local column measurements (FTIR) and measurements on a regional scale (IASI). Comparisons on a regional scale (domain spanning from 35°N to 53°N, and 93.75°W to 63.75°W) resulted in r = 0.57 (r2 = 0.33), but comparing a single model grid point against the FTIR resulted in a poorer correlation, with r2 = 0.26, indicating that a finer spatial resolution is needed to adequately model the variability of NH3.