Impact of Chemical Speciation Network method changes on time series ion and carbon species concentrations

Numerous studies rely on long-term PM_2.5 speciation data from the EPA's Chemical Speciation Network (CSN), for example, to estimate health impacts or investigate the sources and transport of PM_2.5 pollution. These studies rely on consistent, long-term time series measurements of PM_2.5 species to draw conclusions about PM_2.5 emissions sources and their health impacts. However, changes in contractors and associated methodological changes in 2015 and 2018 led to disruptions in the consistency of the CSN data, specifically, concentration discontinuities in the CSN time series for ions and elemental carbon (EC) from November 2015 to September 2018 and from October 2018 onward, respectively. To address the impact of these changes on downstream air quality and health analyses, this study developed correction factors by comparing collocated CSN measurements to measurements from the Interagency Monitoring of Protected Visual Environment (IMPROVE) network, which used consistent instrumentation and contractors throughout the study period. These correction factors reduced the discontinuities in the ions and EC concentration time series data, which could be critical for time series source apportionment receptor modeling, air pollution policy and accountability investigations, and health effect studies.