Abstract
Abstract. The source attribution of observed variability of total PM2.5 concentrations over Halifax, Nova Scotia, was investigated between 11 July and 26 August 2011 using measurements of PM2.5 mass and PM2.5 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 PM2.5 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 PM2.5 in Halifax and a vital comparative data set for the other collocated ground-based observations of atmospheric composition made during BORTAS-B.
Highlights
The HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) 2-day ensemble air mass back trajectories are provided in Fig. 2, with the mean location of the trajectory over the 2-day travel time used to group the trajectories into four clusters: (1) N (315◦ to 45◦), (2) marine (45◦ to 235◦), (3) SW (235◦ to 265◦), and (4) W–NW (265◦ to 315◦)
Our analysis showed that over 80 % of the back trajectories were below 1.5 km for the entire 48 h
The profiles from the N show the highest probability of air subsiding from the free troposphere; it was anticipated that these profiles would be associated with clean air regardless of the altitude of the back trajectories
Summary
The ACSM 30 min data points were averaged to match the 24 h PM2.5 filter samples. Environment Canada used the meteorological data from Halifax International Airport (26.8 km distant at a heading of 012◦) to provide an overview of meteorological conditions within the Halifax Regional Municipality during the 45 days of filter sampling at the BORTAS-B DGS. The period of the filter-based measurements at the DGS in summer 2011 was influenced by numerous weak low-pressure systems during the first half of the sampling period (to 4 August). These systems, along with onshore moist southerly airflows provided extended periods with low-level clouds and occasional periods of rain, drizzle and fog. These data were accessed via http://www.climate.weather.gc.ca/index e.html
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