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)

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.

The impact of biomass burning and aqueous-phase processing on air quality: a multi-year source apportionment study in the Po Valley, Italy

“…On the multi-year basis of the project, OA represents on average 45 ± 8 % (33 %-58 %) and 46 ± 7 % (36 %-50 %) of the total non-refractory submicron particles (PM 1 -NR) at the urban and rural sites, respectively, within the range reported in the literature for other European sites and the Asian regions (Hu et al, 2017;Li et al, 2015;Wu et al, 2018, for China and East Asia;Chakraborty et al, 2018, for India), and slightly less than the values reported for the southeastern US (50 %-75 %, Xu et al, 2015;Budisulistiorini et al, 2016). Among this fraction, primary sources (POA) are dominated by biomass burning (23 ± 13 %), especially at the rural site (SPC), whereas the fossil fuel combustion (12 ± 7 %) is higher at the urban background site (Bologna), where it also presents a marked seasonality.…”

“…However, the biomass burning contribution to POA remains the most important source of POA also at the urban site during the cold fall/winter seasons. The BBOA contribution ranging from 17 % to 38 % at Bologna during the fall/winter seasons is not far from the values reported for other European cities (10 %-40 % in Paris, Crippa et al, 2013b; 5 %-27 % from the EUCAARI multi-site study, Crippa et al, 2014) and United States areas (e.g., 15 %-33 % for the southeastern US, Budisulistiorini et al, 2016), and slightly higher than that of other highly populated and polluted cities/regions of Asia (11 %-14 % at Beijing, China, Sun et al, 2018;10 %-20 % at Kunpur, India, Chakraborty et al, 2018), where, however, other combustion sources (i.e., coal) contribute to the POA fraction.…”

“…The HR-AMS data indicate that the OA mass contributions of this SOA factor influenced by wood combustion were on the order of 14 %-44 %, which translates into biomass burning emissions representing the 31 %-82 % of the OM mass in the Po Valley during cold months (fall and winter). Significant contribution of aged BB emissions on the OA mass loadings has already been suggested by previous studies regarding the Po Valley (Saarikoski et al, 2012) and different European (Paris, France, Crippa et al, 2013b;eastern Mediterranean, Bougiatioti et al, 2014;Athens, Greece, Stavroulas et al, 2019), Asian (Beijing, China, Hu et al, 2017;Sun et al, 2018;Kunpur, India, Chakraborty et al, 2018) and American sites (southeastern US, Xu et al, 2015;Budisulistiorini et al, 2016). However, studies reporting the identification and quantification in ambient air of specific BB-influenced OOA factors are still very limited Xu et al, 2017).…”

Sources and Secondary Production of Organic Aerosols in the Northeastern United States during WINTER

“…In general, wintertime studies have shown lower total concentrations of submicron aerosols than summer. Similar to summer studies, OA is a major (and often the largest) fraction of submicron aerosol in the winter, typically with increases in primary OA (POA), mostly driven by increases in biomass burning OA (BBOA) from residential wood burning (Bressi et al, 2016;Budisulistiorini et al, 2016;Crippa et al, 2013;Lanz et al, 2008;Xu et al, 2015Xu et al, , 2016. The amount of oxidized OA (OOA) observed in warmer months of the year has been documented extensively in the literature, throughout much of the world (Jimenez et al, 2009, and references within).…”

Transport of Po Valley aerosol pollution to the northwestern Alps – Part 2: Long-term impact on air quality

“…Although heavy pollution episodes can occur also on the French side of the Alps (e.g. Chazette et al, 2005;Brulfert et al, 2006;Bonvalot et al, 2016;Chemel et al, 2016;Sabatier et al, 2018), it is very rare to clearly spot an aerosol layer transported from that region to Aosta, since in those cases air masses coming from the west must have crossed the Alps and are almost always mixed with clean air from the uppermost layers; therefore, only a dim aerosol backscatter signal is usually noticed from the ALC.…”