||Attempts were made to measure the fraction of elemental carbon (EC) in ultrafine aerosol
by modifying an Ambient Carbonaceous Particulate Monitor (ACPM, R&P 5400). The main
modification consisted in placing a quartz filter in one of the sampling lines of
this dual-channel instrument. With the filter all aerosol and EC contained in it is
collected, while in the other line of the instrument the standard impactor samples
only particles larger than 0.14 μm. The fraction of EC in particles smaller than 0.14
μm is derived from the difference in concentration as measured via the two sampling
lines. Measurements with the modified instrument were made at a suburban site in Amsterdam,
The Netherlands. An apparent adsorption artefact, which could not be eliminated by
the use of denuders, precluded meaningful evaluation of the data for total carbon.
Blanks in the measurements of EC were negligible and the EC data were hence further
evaluated. We found that the concentration of EC obtained via the channel with the
impactor was systematically lower than that in the filter-line. The average ratio
of the concentrations was close to 0.6, which indicates that approximately 40% of
the EC was in particles smaller than 0.14 μm. Alternative explanations for the difference
in the concentration in the two sampling lines could be excluded, such as a difference
in the extent of oxidation. This should be a function of loading, which is not the
case. Another reason for the difference could be that less material is collected by
the impactor due to rebound, but such bounce of aerosol is very unlikely in The Netherlands
due to co-deposition of abundant deliquesced and thus viscous ammonium compounds.
The conclusion is that a further modification to assess the true fraction of ultrafine
EC, by installing an impactor with cut-off diameter at 0.1 μm, would be worth pursuing.
© 2005 Elsevier Ltd. All rights reserved.