Quantitative interpretation of divergence between PM10 and PM2.5 mass measurement by TEOM and gravimetric (Partisol) instruments

An intercomparison of PM10 and PM2.5 mass measured simultaneously by tapered element oscillating microbalance (TEOM) instruments and Partisol samplers, with laboratory gravimetric determination of particle mass from the latter, has been carried out at a rural site in Oxfordshire, UK. The TEOM gives lower readings of both PM10 and PM2.5 mass and the underestimation is greater at high concentrations. The strong day-to-day variation between instruments is the result of the variability of the particle composition and the influence of temperature and relative humidity. Results show that most of the particulate material not measured by the TEOMs belongs to the PM2.5 fraction and suggest that the loss of semi-volatile compounds constitutes an important part of the difference between instruments; even though it is not the only cause of divergence. Ammonium nitrate is one of the major particulate compounds lost in the inlet of the TEOMs located at Harwell. Its volatilisation can explain a significant part of the difference between TEOMs and filter-based Partisol methods. At Harwell, events of high concentrations are often events of high concentrations of semi-volatile compounds including particulate nitrate and as a consequence, the TEOM instrument may miss events above the daily air quality standard of 50 μg m−3. The influences of temperature and relative humidity are demonstrated. The difference between measurements decreases with increasing temperature and with decreasing relative humidity. This result is consistent with the mass deficit being associated with loss of semi-volatile substances such as NH4NO3 whose formation is temperature and humidity-dependent. It may also be related to positive artefacts associated with the filter-based method such as particle-bound water and adsorbed semi-volatile components.