Multiscale observations of NH3 around Toronto, Canada

Ammonia (NH3) is a major source of nitrates in the atmosphere and a major source of fine particulate matter. As such, there have been increasing efforts to measure the atmospheric abundance of NH3 and its spatial and temporal variability. In this study, long-term measurements of NH3 derived from multiscale datasets are examined. These NH3 datasets include 16 years of total column measurements using Fourier transform infrared (FTIR) spectroscopy, 3 years of surface in situ measurements, and 10 years of total column measurements from the Infrared Atmospheric Sounding Interferometer (IASI). The datasets were used to quantify NH3 temporal variability over Toronto, Canada. The multiscale datasets were also compared to assess the representativeness of the FTIR measurements. All three time series showed positive trends in NH3 over Toronto: 3.34 +/- 0.89 %/yr from 2002 to 2018 in the FTIR columns, 8.88 +/- 5.08 %/yr from 2013 to 2017 in the surface in situ data, and 8.38 +/- 1.54 %/yr from 2008 to 2018 in the IASI columns. To assess the representative scale of the FTIR NH3 columns, correlations between the datasets were examined. The best correlation between FTIR and IASI was obtained with coincidence criteria of <= 25 km and <= 20 min, with r = 0.73 and a slope of 1.14 +/- 0.06. Additionally, FTIR column and in situ measurements were standardized and correlated. Comparison of 24 d averages and monthly averages resulted in correlation coefficients of r = 0.72 and r = 0.75, respectively, although correlation without averaging to reduce high-frequency variability led to a poorer correlation, with r = 0.39. The GEOS-Chem model, run at 2 degrees x 2.5 degrees resolution, was compared to FTIR and IASI to assess model performance and investigate the correlation of observational data and model output, both with local column measurements (FTIR) and measurements on a regional scale (IASI). Comparisons on a regional scale (a domain spanning 35 to 53 degrees N and 93.75 to 63.75 degrees W) resulted in r = 0.57 and thus a coefficient of determination, which is indicative of the predictive capacity of the model, of r(2) = 0.33, but comparing a single model grid point against the FTIR resulted in a poorer correlation, with r(2) = 0.13, indicating that a finer spatial resolution is needed for modeling NH3.