The potential of high temporal resolution automatic measurements of PM_2.5 composition as an alternative to the filter-based manual method used in routine monitoring

Under the EU Air Quality Directive (AQD) 2008/50/EC member states are required to undertake routine monitoring of PM_2.5 composition at background stations. The AQD states for PM_2.5 speciation this should include at least: nitrate $\left({\text{NO}}_3^{-}\right)$ , sulfate $\left({\text{SO}}_4^{2-}\right)$ , chloride (Cl^-), ammonium (NH4⁺), sodium (Na⁺), potassium (K⁺), magnesium (Mg²⁺), calcium (Ca²⁺), elemental carbon (EC) and organic carbon (OC). Until 2017, it was the responsibility of each country to determine the methodology used to report the composition for the inorganic components of PM_2.5. In August 2017 a European standard method of measurement of PM_2.5 inorganic chemical components ( ${\text{NO}}_3^{-},{\text{SO}}_4^{2-}$ , Cl^-, ${\text{NH}}_4^{+}$ , Na⁺, K⁺, Mg²⁺, Ca²⁺) as deposited on filters (EN16913:2017) was published. From August 2019 this then became the European standard method. This filter method is labour-intensive and provides limited time resolution and is prone to losses of volatile compounds. There is therefore increasing interest in the use of alternative automated methods. For example, the UK reports hourly PM_2.5 chemical composition using the Monitor for AeRosols and Gases in Ambient air (MARGA, Metrohm, NL). This study is a pre-assessment review of available data to demonstrate if or to what extent equivalence is possible using either the MARGA or other available automatic methods, including the Aerosol Chemical Speciation Monitor (ACSM, Aerodyne Research Inc. US) and the Ambient Ion Monitor (AIM, URG, US). To demonstrate equivalence three objectives were to be met. The first two objectives focused on data capture and were met by all three instruments. The third objective was to have less than a 50% expanded uncertainty compared to the reference method for each species. Analysis of this objective was carried out using existing paired datasets available from different regions around the world. It was found that the MARGA (2006-2019 model) had the potential to demonstrate equivalence for all species in the standard, though it was only through a combination of case studies that it passed uncertainty criteria. The ACSM has the potential to demonstrate equivalence for ${\text{NH}}_4^{+}$ , ${\text{SO}}_4^{2-}$ and in some conditions ${\text{NO}}_3^{-}$ , but did not for Cl^- due to its inability to quantify refractory aerosol such as sea salt. The AIM has the potential for ${\text{NH}}_4^{+}$ , ${\text{NO}}_3^{-}$ , ${\text{SO}}_4^{2-}$ , Cl^- and Mg²⁺. Future investigations are required to determine if the AIM could be optimised to meet the expanded uncertainty criterion for Na⁺, K⁺ and Ca²⁺. The recommendation is that a second stage to demonstrate equivalence is required which would include both laboratory and field studies of the three candidate methods and any other technologies identified with the potential to report the required species.