Variations in the chemical composition of the submicron aerosol and in the sources of the organic fraction at a regional background site of the Po Valley (Italy)

Michael Bressi,Jean-Philippe Putaud,Sebastiao Martins Dos Santos,Antonella Malaguti,Massimo Berico,Roman Fröhlich,Fabrizia Cavalli,Ettore Petralia,Claudio A Belis,Francesco Canonaco,André S H Prévôt

doi:10.5194/acp-16-12875-2016

Abstract

Abstract. Fine particulate matter (PM) levels and resulting impacts on human health are in the Po Valley (Italy) among the highest in Europe. To build effective PM abatement strategies, it is necessary to characterize fine PM chemical composition, sources and atmospheric processes on long timescales (> months), with short time resolution (< day), and with particular emphasis on the predominant organic fraction. Although previous studies have been conducted in this region, none of them addressed all these aspects together. For the first time in the Po Valley, we investigate the chemical composition of nonrefractory submicron PM (NR-PM1) with a time resolution of 30 min at the regional background site of Ispra during 1 full year, using the Aerodyne Aerosol Chemical Speciation Monitor (ACSM) under the most up-to-date and stringent quality assurance protocol. The identification of the main components of the organic fraction is made using the Multilinear-Engine 2 algorithm implemented within the latest version of the SoFi toolkit. In addition, with the aim of a potential implementation of ACSM measurements in European air quality networks as a replacement of traditional filter-based techniques, parallel multiple offline analyses were carried out to assess the performance of the ACSM in the determination of PM chemical species regulated by air quality directives. The annual NR-PM1 level monitored at the study site (14.2 µg m−3) is among the highest in Europe and is even comparable to levels reported in urban areas like New York City and Tokyo. On the annual basis, submicron particles are primarily composed of organic aerosol (OA, 58 % of NR-PM1). This fraction was apportioned into oxygenated OA (OOA, 66 %), hydrocarbon-like OA (HOA, 11 % of OA) and biomass burning OA (BBOA, 23 %). Among the primary sources of OA, biomass burning (23 %) is thus bigger than fossil fuel combustion (11 %). Significant contributions of aged secondary organic aerosol (OOA) are observed throughout the year. The unexpectedly high degree of oxygenation estimated during wintertime is probably due to the contribution of secondary BBOA and the enhancement of aqueous-phase production of OOA during cold months. BBOA and nitrate are the only components of which contributions increase with the NR-PM1 levels. Therefore, biomass burning and NOx emission reductions would be particularly efficient in limiting submicron aerosol pollution events. Abatement strategies conducted during cold seasons appear to be more efficient than annual-based policies. In a broader context, further studies using high-time-resolution analytical techniques on a long-term basis for the characterization of fine aerosol should help better shape our future air quality policies, which constantly need refinement.

Highlights

The Po Valley region – located in northern in the Po Valley (Italy) – is amongst the most polluted areas in Europe
High levels of fine aerosol are mostly due to the conjunction of (i) high pollutant emissions related to industrial, transport, biomass burning and agricultural activities – the Po river basin hosting 37 % of the Italian industries, 55 % of the livestock and contributing 35 % of the Italian agricultural production (WMO et al, 2012) – and (ii) the specific geography and topography of this area – a flat basin surrounded by the Alps and Apennine Mountains dominated by weak winds that favour the accumulation of pollutants (Decesari et al, 2014; Kukkonen et al, 2005; Pernigotti et al, 2012)
It is comparable to NR-PM1 levels reported during specific campaigns in the urban areas of New York City (USA, 12 μg m−3; Weimer et al, 2006), Tokyo (Japan, 12–15 μg m−3; Takegawa et al, 2006) or Manchester (UK, 14 μg m−3; Allan et al, 2003a, b)

Summary

Introduction

The Po Valley region – located in northern Italy – is amongst the most polluted areas in Europe (van Donkelaar et al, 2010; EEA, 2013). Effective PM abatement strategies are needed in the Po Valley and require an in-depth knowledge of the chemical composition of fine PM to quantify its sources and the atmospheric processes leading to its secondary formation In this region, high levels of fine aerosol are mostly due to the conjunction of (i) high pollutant emissions related to industrial, transport, biomass burning and agricultural activities – the Po river basin hosting 37 % of the Italian industries, 55 % of the livestock and contributing 35 % of the Italian agricultural production (WMO et al, 2012) – and (ii) the specific geography and topography of this area – a flat basin surrounded by the Alps and Apennine Mountains dominated by weak winds that favour the accumulation of pollutants (Decesari et al, 2014; Kukkonen et al, 2005; Pernigotti et al, 2012). Measurements of fine PM mass and chemical composition at rural background sites are in addition required in the current European directive on air quality (EU, 2008)

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Conclusion