Identifying the sources driving observed PM_2.5 temporal variability over Halifax, Nova Scotia, during BORTAS-B

Abstract. The source attribution of observed variability of total PM_2.5 concentrations over Halifax, Nova Scotia, was investigated between 11 July and 26 August 2011 using measurements of PM_2.5 mass and PM_2.5 chemical composition (black carbon, organic matter, anions, cations and 33 elements). This was part of the BORTAS-B (quantifying the impact of BOReal forest fires on Tropospheric oxidants using Aircraft and Satellites) experiment, which investigated the atmospheric chemistry and transport of seasonal boreal wildfire emissions over eastern Canada in 2011. The US EPA Positive Matrix Factorization (PMF) receptor model was used to determine the average mass (percentage) source contribution over the 45 days, which was estimated to be as follows: long-range transport (LRT) pollution: 1.75 μg m⁻³ (47%); LRT pollution marine mixture: 1.0 μg m⁻³ (27.9%); vehicles: 0.49 μg m⁻³ (13.2%); fugitive dust: 0.23 μg m⁻³ (6.3%); ship emissions: 0.13 μg m⁻³ (3.4%); and refinery: 0.081 μg m⁻³ (2.2%). The PMF model describes 87% of the observed variability in total PM_2.5 mass (bias = 0.17 and RSME = 1.5 μg m⁻³). The factor identifications are based on chemical markers, and they are supported by air mass back trajectory analysis and local wind direction. Biomass burning plumes, found by other surface and aircraft measurements, were not significant enough to be identified in this analysis. This paper presents the results of the PMF receptor modelling, providing valuable insight into the local and upwind sources impacting surface PM_2.5 in Halifax and a vital comparative data set for the other collocated ground-based observations of atmospheric composition made during BORTAS-B.