Residential wood combustion, particulate matter 2.5 sampling project, Whitehorse: summary

Summary

Residential wood combustion (RWC) in wood stoves, fireplaces and fireplace inserts is a commonly used method for heating homes in Canada and is a significant source of fine particulate matter (PM_2.5) air emissions, which can negatively impact human health. The impact of RWC emissions in the City of Whitehorse has been a concern for several decades. Previous studies by Environment Canada in the 1980s documented high levels of total suspended particulate in the subdivision of Riverdale, particularly under stagnant conditions. Despite lower levels of particulate matter measured in more recent years, local government agencies often field complaints from residents regarding wood smoke, and there is anecdotal evidence that levels may be higher in certain parts of the city, such as Riverdale. An air quality project working group, including representatives from the Pacific and Yukon Region of Environment Canada, the City of Whitehorse, Yukon Environment, and Health and Social Services Yukon in Whitehorse undertook this study with the objective of assessing current PM_2.5 levels in Whitehorse during the winter months and to quantify the contribution of wood smoke. This would allow the establishment of a baseline for wood smoke contributions for this community, against which the effectiveness of any future intervention could be assessed, such as an appliance change-out program.

The PM_2.5 sampling campaign in Whitehorse took place from January to March 2009. Filter-based sampling was employed at two sites in downtown Whitehorse and the subdivision of Riverdale. From the filter samples, the commonly used wood smoke markers levoglucosan and a carbon isotope, ¹⁴C (carbon 14), were measured to assess the contribution of RWC to PM_2.5. An aethalometer was employed at the Riverdale site to measure black carbon.

Average/median concentrations were 7.7/6.4 µg/m³ (n=18) and 10.2/7.1 µg/m³ (n=12) at the Downtown and Riverdale sites, respectively. Overall, concentrations ranged from 1.6 µg/m³ to a maximum of 31.3 µg/m³ (Riverdale on February 11), exceeding the value of the Canada-wide Standard of 30 µg/m³. Note that this does not represent a formal exceedance, as it is not based on the annual 98th percentile value. On average, fine particulate at Riverdale was composed primarily of organic carbon (55%), while elemental carbon made up 7%. Winter-time particulate matter concentrations measured during this study were significantly lower than those found in previous studies in the early 1980s and comparable to current levels in several communities in the interior of B.C. However, results from this study were higher than typically measured at the Whitehorse National Air Pollution Surveillance site for the January to March period. Causative factors here were likely a combination of a different sampling location and, to a lesser extent, different measurement methodology.

To estimate the 24-hour average concentrations of PM_2.5 throughout the sampling period, a simple linear regression was used to relate 24-hour average black carbon data, measured by aethalometer at Riverdale, to daily average PM_2.5 concentrations at the two sites. Assuming a consistent linear relationship between black carbon at Riverdale and PM_2.5 at both sites throughout the study period and within the range of black carbon measured, a simple linear regression model predicted that the value of the Canada-wide Standard was likely exceeded on five days at Riverdale (all during a nine-day period in February) and none at the Downtown site.

Calculations based on the analysis of ¹⁴C data indicate that the average wood smoke contribution to PM_2.5 ranged from 70-84% (average minimum-maximum %) at the Riverdale site. These results were supported by levoglucosan data, which also indicated a strong wood smoke influence in Whitehorse. Ratios (% w/w) of levoglucosan to PM_2.5 were 4.7 ± 1.6 and 6.0 ± 2.4 (avg. ± s.d.) at Downtown and Riverdale, respectively, indicating a slightly lower wood smoke contribution at Downtown, which is expected to have more contribution from traffic and heating from commercial buildings. Results from the aethalometer were also indicative of a strong wood smoke presence at Riverdale. Hourly concentrations of black carbon ranged from 0 to 8.6 µg/m³, with an average of 0.9 µg/m³ and a median of 0.2 µg/m³. The wood smoke signal was observed to be highest in the early morning and late evening. Source apportionment results for wood smoke contribution from this study are in relatively close agreement to earlier studies in Riverdale, the most recent emission inventory for Whitehorse and a previous study in B.C.

The highest concentrations of PM_2.5 in Whitehorse were associated with calm winds, which limit dispersion, and low temperatures, which are associated with increased wood burning. These conditions were found to be present on days with a strong surface-level inversion caused by a stable ridge of cold arctic air or suspected subsidence of southwesterly or westerly air masses.

Results from this study confirm that any efforts to reduce winter-time PM_2.5 in Whitehorse, particularly in the residential subdivision of Riverdale, should consider RWC reduction strategies. If RWC emission reduction strategies are introduced, future work in Whitehorse could include follow-up monitoring to assess their effectiveness. Future studies should ideally include monitoring over a longer period to more fully capture the impact of RWC on cold season air quality in Whitehorse.