TROPOspheric Monitoring Instrument (TROPOMI), on‐board the Sentinel‐5 Precurser satellite, is a nadir‐viewing spectrometer measuring reflected sunlight in the ultraviolet, visible, near‐infrared, and shortwave infrared. From these spectra several important air quality and climate‐related atmospheric constituents are retrieved, including nitrogen dioxide (NO2) at unprecedented spatial resolution from a satellite platform. We present the first retrievals of TROPOMI NO2 over the Canadian Oil Sands, contrasting them with observations from the Ozone Monitoring Instrument satellite instrument, and demonstrate TROPOMI's ability to resolve individual plumes and highlight its potential for deriving emissions from individual mining facilities. Further, the first TROPOMI NO2 validation is presented, consisting of aircraft and surface in situ NO2 observations, and ground‐based remote‐sensing measurements between March and May 2018. Our comparisons show that the TROPOMI NO2 vertical column densities are highly correlated with the aircraft and surface in situ NO2 observations, and the ground‐based remote‐sensing measurements with a low bias (15–30 %); this bias can be reduced by improved air mass factors.

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Results are reported from an ongoing passive air monitoring study for polycyclic aromatic compounds (PACs) in the Athabasca oil sands region in Alberta, Canada. Polyurethane foam (PUF) disk passive air samplers were deployed for consecutive 2-month periods from November 2010 to June 2012 at 17 sites. Samples were analyzed for polycyclic aromatic hydrocarbons (PAHs), alkylated PAHs, dibenzothiophene and its alkylated derivatives (DBTs). Relative to parent PAHs, alkylated PAHs and DBTs are enriched in bitumen and therefore considered to be petrogenic markers. Concentrations in air were in the range 0.03-210 ng/m(3), 0.15-230 ng/m(3) and 0.01-61 ng/m(3) for ∑PAHs, ∑alkylated PAHs and ΣDBTs, respectively. An exponential decline of the PAC concentrations in air with distance from mining areas and related petrogenic sources was observed. The most significant exponential declines were for the alkylated PAHs and DBTs and attributed to their association with mining-related emissions and near-source deposition, due to their lower volatility and greater association with depositing particles. Seasonal trends in concentrations in air for PACs were not observed for any of the compound classes. However, a forest fire episode during April to July 2011 resulted in greatly elevated PAH levels at all passive sampling locations. Alkylated PAHs and DBTs were not elevated during the forest fire period, supporting their association with petrogenic sources. Based on the results of this study, an "Athabasca PAC profile" is proposed as a potential source marker for the oil sands region. The profile is characterized by ∑PAHs/∑Alkylated PAHs = ∼0.2 and ∑PAHs/∑DBTs = ∼5.

[1] Three nitrogen species in air, HNO 3 , particle nitrate (pNO 3 À ), and particle ammonium (pNH 4 + ), have been routinely measured for estimating nitrogen dry deposition in the Canadian Air and Precipitation Monitoring Network (CAPMoN) and the U.S. Clean Air Status and Trends Network (CASNET) for several decades. To investigate the relative contributions of other nitrogen species to total nitrogen dry and dry + wet deposition, 14 short-term field campaigns were conducted between 2001 and 2005 at eight selected rural sites across eastern Canada. Air concentrations were measured for the total oxidized nitrogen (NO y ) and major species comprising NO y (NO, NO 2 , PAN, PPN, HNO 3 , pNO 3 À ), and for the two reduced nitrogen species (NH 3 , pNH 4 + ). Dry deposition fluxes of NO y and NH x (= NH 3 + pNH 4 + ) were then estimated by combining measured concentrations of individual nitrogen species with their respective dry deposition velocities estimated from big-leaf models using on-site meteorological measurements. Nitrogen dry deposition were estimated to be 0.8-4.0 kg N ha À1 a À1 , depending on location, with 60-75% from NO y and 25-40% from NH x . HNO 3 and NO 2 dominated NO y dry deposition while NH 3 and pNH 4 + contributed equally to NH x dry deposition. The pNO 3 À , PAN, and unidentified NO y species also contributed appreciable amounts to NO y dry deposition. Nitrogen dry + wet deposition from NO y + NH x was estimated at 4.3-11 kg N ha À1 a À1 , with dry deposition accounting for 10-50%. The routinely monitored species accounted for less than 50% of total N dry deposition; thus, total dry + wet deposition from the Canadian monitoring network had a lower bias by 5-25% at most of the sites.

As part of the BAQS-Met 2007 field campaign, Aerodyne time-of-flight aerosol mass spectrometers (ToF-AMS) were deployed at two sites in southwestern Ontario from 17 June to 11 July 2007. One instrument was located at Harrow, ON, a rural, agriculture-dominated area approximately 40 km southeast of the Detroit/Windsor/Windsor urban area and 5 km north of Lake Erie. The second instrument was located at Bear Creek, ON, a rural site approximately 70 km northeast of the Harrow site and 50 km east of Detroit/Windsor. Positive matrix factorization analysis of the combined organic mass spectral dataset yields factors related to secondary organic aerosol (SOA), direct emissions, and a factor tentatively attributed to the reactive uptake of isoprene and/or condensation of its early generation reaction products. This is the first application of PMF to simultaneous AMS measurements at different sites, an approach which allows for self-consistent, direct comparison of the datasets. Case studies are utilized to investigate processing of SOA from (1) fresh emissions from Detroit/Windsor and (2) regional aerosol during periods of inter-site flow. A strong correlation is observed between SOA/excess CO and photochemical age as represented by the NO_x/NO_y ratio for Detroit/Windsor outflow. Although this correlation is not evident for more aged air, measurements at the two sites during inter-site transport nevertheless show evidence of continued atmospheric processing by SOA production. However, the rate of SOA production decreases with airmass age from an initial value of ~10.1 μg m⁻³ ppmv_CO^−1 h^−1 for the first ~10 h of plume processing to near-zero in an aged airmass (i.e. after several days). The initial SOA production rate is comparable to the observed rate in Mexico City over similar timescales