Long Term Aerosol Composition Measurements at the CESAR Tower at Cabauw, NL

Abstract

In this work, intensive mass spectrometric measurements of PM1 aerosol size distribution and chemical composition were performed at Cabauw, the Netherlands, using a High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS), an Aerosol Chemical Speciation Monitor (ACSM), a Thermal-Desorption Proton-Transfer-Reaction Time-of- Flight Mass Spectrometer (TD-PTR-ToF-MS), and supplementary instruments. The field campaigns took place in November 2011, during May to July 2012 (both periods with the AMS and the collocated TD-PTR-MS) and between July 2012 and June 2013 (ACSM). Average total aerosol mass loadings of 9.26 μg m−3, 6.40 μg m−3, and 9.50 μg m−3 were observed during the campaign periods, respectively. Within the ACSM campaign, 12 exceedances of the PM2.5 daily mean limit, established by the World Health Organization (WHO), were observed. In almost all campaigns, the highest contribution to total mass were seen by nitrate (21% - 39 %, mainly as ammonium nitrate) and organic compounds (23% - 33 %) on average, especially in periods with relatively high total mass loadings (> 25 μg m−3). The presence of organic nitrate and excess ammonium indicate the high impact of nitrogen containing compounds on the formation and composition of ambient aerosols in Cabauw. Factor analysis was applied to organic aerosols (OA) for all data sets. AMS and ACSM data showed that secondary organic aerosols (SOA, 53% - 84% average contribution to OA) dominated the organic fraction throughout all campaigns. A factor which is attributed to humic-like substances (HULIS) was identified as a highly oxidised background aerosol in Cabauw. Primary organic aerosols (POA) were mainly emitted by traffic (8% - 35% average contribution to OA) and biomass burning (8% - 23 %). A first approach of the application of factor analysis to TD-PTR-MS data was performed in this work, showing good agreement with factors obtained from the collocated AMS. The dominance of secondary aerosol in PM1 shows the high importance of atmospheric ageing processes of aerosol concentration at this rural site. Due to the large secondary fraction of aerosol reduction of particulate mass is challenging on a local scale