Automated alternating sampling of PM₁₀ and PM_2.5 with an online XRF spectrometer

Measuring the chemical composition of ambient particulate matter in different size classes is a relevant gain in information, which can be used for improved source apportionment. The design and implementation of a switching system between a PM₁₀ and a PM_2.5 inlet for an Xact 625i ambient metals monitor is presented. First applications in the field reveal its practicability and applicability to determine the PM_2.5/PM₁₀ ratio. For example, in Beijing crustal elements such as Si, Ca, or Ti occur mainly in the coarse fraction (ratio < 0.2), whereas combustion related elements such as Se, Pb or As are found mainly in the fine fraction (ratio > 0.8). Simulations of the uncertainty caused by alternating sampling of the two size classes show that the increase of uncertainty is within acceptable limits, depending on the characteristics of the time series. The gain in information outweighs the increase of uncertainty in most cases.