Residential wood combustion, particulate matter 2.5 sampling project, Whitehorse: chapter 4


Conclusions

The use of levogucosan, 14C and black carbon measurements as markers during the two-month sampling campaign was successful in determining the presence and contribution of wood smoke to PM2.5 in Whitehorse, thereby meeting the primary objective of this study. Average concentrations of PM2.5 were 7.7 µg/m3 and 10.2 µg/m3 at the Downtown and Riverdale sites, respectively. Overall, concentrations ranged from 1.6 µg/m3 to a maximum of 31.3 µg/m3 on February 11 at Riverdale, which exceeded 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%. Assuming a consistent linear relationship between black carbon measured by the aethalometer at Riverdale and PM2.5 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. Winter-time particulate matter concentrations measured during this study are 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 found at the Whitehorse NAPS site for the January to March period.

Calculations based on the analysis of 14C data indicate that the average wood smoke contribution to PM2.5 ranged from 70-84% at Riverdale. These results are supported by levoglucosan data, which also indicate a strong wood smoke influence. Ratios (% w/w) of levoglucosan to PM2.5 were 4.7 ± 1.6 and 6.0 ± 2.4 (avg. ± s.d.) at Downtown and Riverdale, respectively, indicating a slightly higher wood smoke contribution at Riverdale. Black carbon results from the aethalometer also support the strong wood smoke presence at Riverdale, with the wood smoke signal being 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., lending credibility to the results.

The highest concentrations of PM2.5 in Whitehorse were associated with calm winds and poor ventilation, 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 south and southwesterly air masses.

Results from this study confirm that efforts to reduce winter-time PM2.5 in Whitehorse, particularly in the residential subdivision 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.

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