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

Özet

As the widespread application of online instruments in atmospheric monitoring, an increasing number of studiessuch as source apportionment researches are based on the high-resolution online monitoring data. Accurateonline monitoring data are very critical in the model evaluation as well. However, there is a very limited numberof comparison studies investigating the quality of online monitoring data compared to the traditional offlinefilter-based data. In this study, two high-time resolved instruments, including In-situ Gas and AerosolComposition and Xact, were applied to measure the water-soluble inorganic ions (WSIIs) and trace elements inPM2.5, respectively, in Beijing during winter 2015. These online results were then compared with correspondingoffline filter-based measurement for 8 ions and 13 trace elements. Good agreement (R2 > 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 marginallyacceptable, whereas the relatively poor performance of Mg2+, As, and Cd need further evaluation due to theirlow concentrations, a large proportion of which was below the method detection limit. The overall performanceof online monitoring has been shown comparable to offline measurement and the potential influences on sourceapportionment results due to online and offline measurements are discussed.