Comparison of water-soluble inorganic ions and trace metals in PM2.5 between online and offline measurements in Beijing during winter

As the widespread application of online instruments in atmospheric monitoring, an increasing number of studies such as source apportionment researches are based on the high-resolution online monitoring data. Accurate online monitoring data are very critical in the model evaluation as well. However, there is a very limited number of comparison studies investigating the quality of online monitoring data compared to the traditional offline filter-based data. In this study, two high-time resolved instruments, including In-situ Gas and Aerosol Composition and Xact, were applied to measure the water-soluble inorganic ions (WSIIs) and trace elements in PM2.5, respectively, in Beijing during winter 2015. These online results were then compared with corresponding offline filter-based measurement for 8 ions and 13 trace elements. Good agreement (R-2 > 0.8, 0.8 < slope < 1.5) for most WSIIs (e.g. NO3-, NH4+, Cl-, Na+, and SO42-) and trace elements (e.g. Pb, K, Cu, Zn, Mn, Ni, Fe, Cr, and Se) can be found between the two methods. The comparison results for K+, Ba and Ca were marginally acceptable, whereas the relatively poor performance of Mg2+, As, and Cd need further evaluation due to their low concentrations, a large proportion of which was below the method detection limit. The overall performance of online monitoring has been shown comparable to offline measurement and the potential influences on source apportionment results due to online and offline measurements are discussed.