Organic aerosol evolution and transport observed at Mt. Cimone (2165 m a.s.l.), Italy, during the PEGASOS campaign

M Rinaldi,V Poluzzi,S Sandrini,P Massoli,P Bonasoni,S Fuzzi,M Paglione,A Marinoni,S Decesari,S Gilardoni,P Cristofanelli

doi:10.5194/acp-15-11327-2015

Abstract

Abstract. High-resolution aerosol mass spectrometer measurements were performed, for the first time, at the Mt. Cimone Global Atmosphere Watch (GAW) station between June and July 2012, within the EU project PEGASOS and the ARPA–Emilia-Romagna project SUPERSITO. Submicron aerosol was dominated by organics (63 %), with sulfate, ammonium and nitrate contributing the remaining 20, 9 and 7 %, respectively. Organic aerosol (OA) was in general highly oxygenated, consistent with the remote character of the site; our observations suggest that oxidation and secondary organic aerosol (SOA) formation processes occurred during aerosol transport to high altitudes. All of the aerosol component concentrations as well as the OA elemental ratios showed a clear daily trend, driven by the evolution of the planetary boundary layer (PBL) and by the mountain wind regime. Higher loadings and lower OA oxidation levels were observed during the day, when the site was within the PBL, and therefore affected by relatively fresh aerosol transported from lower altitudes. Conversely, lower loadings and higher OA oxidation levels were observed at night, when the top of Mt. Cimone resided in the free troposphere although affected by the transport of residual layers on several days of the campaign. Analysis of the elemental ratios in a Van Krevelen space shows that OA oxidation follows a slope comprised between −0.5 and −1, consistent with addition of carboxylic groups, with or without fragmentation of the parent molecules. The increase of carboxylic groups during OA ageing is confirmed by the increased contribution of organic fragments containing more than one oxygen atom in the free troposphere night-time mass spectra. Finally, positive matrix factorization was able to deconvolve the contributions of relatively fresh OA (OOAa) originating from the PBL, more aged OA (OOAb) present at high altitudes during periods of atmospheric stagnation, and very aged aerosols (OOAc) transported over long distances in the free troposphere.

Highlights

Atmospheric aerosols have been intensively studied in the last decades because of their effects on climate, air quality and ecosystems
NR-PM1 was clearly dominated by organic aerosol (OA) through the whole campaign: OA average atmospheric concentration and standard deviation were 2.8 ± 2.4 μg m−3, for an average mass contribution of 63 %
Elemental analysis of the high-resolution aerosol mass spectrometer (AMS) data showed highly oxygenated OA, suggesting that strong oxidation and secondary organic aerosol (SOA) formation processes occur during aerosol transport to high altitudes

Summary

Introduction

Atmospheric aerosols have been intensively studied in the last decades because of their effects on climate, air quality and ecosystems. OA is made of thousands of individual compounds that can either be emitted directly into the atmosphere (i.e. primary OA or POA) or formed through chemical and physical processes (i.e. secondary OA or SOA). OA is a dynamic component, experiencing both atmospheric oxidation and reversible partitioning. This processing (usually referred to as ageing) is generally not completely understood and not well represented in models (Heald et al, 2010). Online instruments like the Aerodyne Research Inc. aerosol mass spectrometer (AMS) (Jayne et al, 2000; Canagaratna et al, 2007) have provided new insights into OA chemical composition and simplified ways of characterizing atmospheric OA ageing. Online instruments like the Aerodyne Research Inc. aerosol mass spectrometer (AMS) (Jayne et al, 2000; Canagaratna et al, 2007) have provided new insights into OA chemical composition and simplified ways of characterizing atmospheric OA ageing. Ng et al (2010)

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