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. The aerosol chemical speciation monitor (ACSM) isnowadays widely used to identify and quantify the main components of fineparticles in ambient air. As such, its deployment at observatory platformsis fully incorporated within the European Aerosol, Clouds and Trace GasesResearch Infrastructure (ACTRIS). Regular intercomparisons are organized atthe Aerosol Chemical Monitoring Calibration Center (ACMCC; part of theEuropean Center for Aerosol Calibration, Paris, France) to ensure theconsistency of the dataset, as well as instrumental performance andvariability. However, in situ quality assurance remains a fundamental aspectof the instrument's stability. Here, we present and discuss the main outputsof long-term quality assurance efforts achieved for ACSM measurements at theresearch station Melpitz (Germany) since 2012 onwards. In order to validatethe ACSM measurements over the years and to characterize seasonalvariations, nitrate, sulfate, ammonium, organic, and particle massconcentrations were systematically compared against the collocatedmeasurements of daily offline high-volume PM1 and PM2.5 filtersamples and particle number size distribution (PNSD) measurements. Massclosure analysis was made by comparing the total particle mass (PM)concentration obtained by adding the mass concentration of equivalent blackcarbon (eBC) from the multi-angle absorption photometer (MAAP) to the ACSMchemical composition, to that of PM1 and PM2.5 during filterweighing, as well as to the derived mass concentration of PNSD. Acombination of PM1 and PM2.5 filter samples helped identifying thecritical importance of the upper size cutoff of the ACSM during suchexercises. The ACSM–MAAP-derived mass concentrations systematically deviatedfrom the PM1 mass when the mass concentration of the latter representedless than 60 % of PM2.5, which was linked to the transmissionefficiency of the aerodynamic lenses of the ACSM. The best correlations areobtained for sulfate (slope =0.96; R2=0.77) and total PM(slope =1.02; R2=0.90). Although, sulfate did not exhibita seasonal dependency, total PM mass concentration revealed a small seasonalvariability linked to the increase in non-water-soluble fractions. Thenitrate suffers from a loss of ammonium nitrate during filter collection,and the contribution of organo-nitrate compounds to the ACSM nitrate signalmake it difficult to directly compare the two methods. The contribution ofm∕z 44 (f44) to the total organic mass concentration was used toconvert the ACSM organic mass (OM) to organic carbon (OC) by using a similar approach as for theaerosol mass spectrometer (AMS). The resulting estimated OCACSM was compared with the measuredOCPM1 (slope =0.74; R2=0.77), indicating that thef44 signal was relatively free of interferences during this period. ThePM2.5 filter samples use for the ACSM data quality might suffer from asystematic bias due to a size truncation effect as well as to the presenceof chemical species that cannot be detected by the ACSM in coarse mode (e.g.,sodium nitrate and sodium sulfate). This may lead to a systematicunderestimation of the ACSM particle mass concentration and/or a positiveartifact that artificially decreases the discrepancies between the twomethods. Consequently, ACSM data validation using PM2.5 filters has tobe interpreted with extreme care. The particle mass closure with the PNSDwas satisfying (slope =0.77; R2=0.90 over the entireperiod), with a slight overestimation of the mobility particle sizespectrometer (MPSS)-derived mass concentration in winter. This seasonalvariability was related to a change on the PNSD and a larger contribution ofthe supermicrometer particles in winter. This long-term analysis between the ACSM and other collocated instrumentsconfirms the robustness of the ACSM and its suitability for long-termmeasurements. Particle mass closure with the PNSD is strongly recommended toensure the stability of the ACSM. A near-real-time mass closure procedurewithin the entire ACTRIS–ACSM network certainly represents an optimalquality control and assurance of both warranting the quality assurance ofthe ACSM measurements as well as identifying cross-instrumental biases.

atmospheric measurement techniques

Multi-year ACSM measurements at the central European research station Melpitz (Germany) – Part 1: Instrument robustness, quality assurance, and impact of upper size cutoff diameter