Sampling system influence on gaseous air pollution measurements

Abstract

Efficient sampling systems are key components of ambient air pollution measurement systems used for both regulatory and research purposes. Gaseous sampling systems were tested at 83 monitoring sites incorporating three different sampling system designs. The mean sample collection efficiency was 98% for CO, 99.1% for NO2, 88.7% for O3 and 96.7% for SO2. In most cases, the test uncertainties were greater than the 2% threshold for sample losses allowed by the relevant European Standards. These uncertainties were largely due to repeatability and signal noise within the equipment being tested and the low test gas concentrations required to avoid conditioning the sampling system. There is therefore little scope for improving the test uncertainty. For O3, test uncertainties and repeatability issues meant that firm conclusions could not be reached. For other pollutants, manifold systems offered no systematic advantages over simple PTFE tubing. PTFE tubes should therefore be favoured in site design, providing sample residence times can be met. The high average sampling efficiencies of PTFE sample tubes combined with the uncertainties inherent in the test procedure suggest that regular testing is not worthwhile for this type of sampling system providing sample lines are regularly changed or cleaned. Where a manifold is used, periodic maintenance should include cleaning, as well as flow and leak tests.