Positive and negative artifacts in particulate organic carbon measurements with denuded and undenuded sampler configurations
Abstract
Measurement of ambient particulate organic carbon (POC) with quartz filters is prone
to positive and negative sampling artifacts. One approach for estimating these artifacts
is to sample with a backup quartz filter placed behind either the main quartz filter
or a Teflon filter in a parallel line. Another approach is to use a denuder to reduce
the positive artifact in combination with a highly adsorbent backup filter to capture
any negative artifact. Results obtained using both of these approaches in parallel
for over one year in Pittsburgh, PA are presented in this article. A sampler using
an activated carbon monolith denuder has been developed and tested extensively. Transmission
losses were found to be negligible, and the denuder is on average 94% efficient at
removing gas-phase organics. Denuder breakthrough is corrected for each run using
a dynamic blank in parallel with the sample line. Comparisons with the dynamic blank
indicate that the denuder almost eliminates the positive artifact on the quartz filter.
Negative artifact from the denuded quartz filter is quantified using a carbon-impregnated
glass fiber (CIG) backup filter and was found to be small, typically less than 10%
of the ambient POC. Compared to the denuded sampler POC, 24 h bare quartz samples
showed an almost constant positive artifact of 0.5 μg-C/m3 for samples taken throughout
the year-long study period. Sampling for shorter durations (4-6 h) resulted in a larger
positive artifact. A quartz filter behind a Teflon filter (QBT) provides a consistent
estimate of the positive artifact on the bare quartz filter irrespective of sample
duration, though it overcorrects for the positive artifact by 16-20% (attributed to
particulate matter volatilizing off the upstream Teflon filter). The quartz behind
quartz (QBQ) approach provides a reasonable estimate of the positive artifact on the
bare quartz filter for the 24 h samples but not for the shorter samples. A slight
seasonal variation is observed in the absolute value of the positive artifact, with
higher values observed during the summer months.
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