Two-year observations of fine carbonaceous particles in variable sampling intervals

Daily to weekly integrated PM2.5 samples were collected consecutively from August 2005 through November 2007 at Egbert, a rural site in southern Ontario with two collocated samplers to characterize temporal variations of organic and elemental carbon (OC and EC) concentrations and to review the effects of different sampling intervals on their respective concentrations. OC and EC concentrations from both samplers had reasonable agreements for the samples with identical sampling intervals, whereas for the samples with different sampling intervals, they did not coincide. Considering sampling artifacts, analytical challenges and highly varied daily concentrations, it is suggested and would be advantageous to adopt the weekly integrated sampling strategy other than 24-h sampling on selected days for long-term trend studies on ambient carbonaceous PM. Both OC and EC concentrations varied seasonally by factors of 2.4 and their concentrations in warm seasons (May–October) were higher than those in cold seasons (November–April). Greater fluctuations were also found in warm seasons during the study period. Weekly concentrations of carbonaceous species at Egbert and urban Toronto show similar temporal variations and small urban excess during the one-year parallel sampling period, indicating a relatively large regional contribution from transport instead of local emissions to the paired urban–rural sites. The continental background level of total carbon mass (TCM) in PM2.5 in southern Ontario, constrained by surface air mass directions and back trajectories, was estimated to be 0.5 μg m−3. It is likely that the elevated OC level during the warm season in 2007 compared with those in 2006 was due to more smog days in 2007.

► Weekly sampling yields more representative mean OC–EC levels on a long-term basis.
► The elevated warm-season carbonaceous levels were accompanied with more smog days.
► There is a relatively larger regional influence on OC–EC levels in south Ontario.
► The continental background level of total carbon in southern Ontario is 0.5 μg m−3.