Guidelines for comparison of ACSM measurements with co-located external data

Airborne particles (or aerosols) are ubiquitous pollutants in ambient air, with significant impacts on the Earth’s climate, ecosystems and human health. A better understanding of these effects requires high quality measurements of the aerosol physical and chemical properties, especially within the fine fraction (e.g. particles with aerodynamic diameters less than 1 or 2.5 µm (PM1 and PM2.5, respectively)). In the last decade, the Aerosol Chemical Speciation Monitor (ACSM, Aerodyne Res. Inc.) has been deployed at a growing number of monitoring stations and research facilities. This instrument provides near real-time measurements of the major chemical components of non-refractory submicron particles (NR-PM1). It was built on the same operating principles than the Aerodyne Aerosol Mass Spectrometer (AMS), but with the advantage of being cheaper, simpler and more robust. It is therefore particularly well suited for continuous networking in situ observation activities. One of the main goals of the COLOSSAL COST Action CA16109 (https://www.costcolossal.eu/) is to establish common ACSM standard operation procedures, to guarantee comparability of measurements over Europe and to facilitate the implementation of research infrastructure such as ACTRIS (https://www.actris.eu/). 

In this context, the present document provides guidelines for the comparison of quadrupole or time-of-flight ACSM data with external data obtained from relevant colocated measurements. Such comparison exercises are essential to assess the validity of tuning and calibration parameters applied for ACSM operation. They include so-called “chemical mass closure”, where the sum of major chemical species is compared to total PM mass concentration measurements or estimates. In this case, NR-PM1 ACSM measurements should be combined with refractory species mass concentrations, starting with Elemental or equivalent Black Carbon (EC and eBC, respectively) and possibly including sea salt and/or mineral dust measurements. Comparisons of individual chemical species concentrations with external data are also highly recommended. More generally, the use of any available co-located measurements is worth considering.