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. The Po Valley (Italy) is a well-known air quality hotspot characterized byparticulate matter (PM) levels well above the limit set by the European AirQuality Directive and by the World Health Organization, especially duringthe colder season. In the framework of Emilia-Romagna regional project“Supersito”, the southern Po Valley submicron aerosol chemical compositionwas characterized by means of high-resolution aerosol mass spectroscopy(HR-AMS) with the specific aim of organic aerosol (OA) characterization andsource apportionment. Eight intensive observation periods (IOPs) werecarried out over 4 years (from 2011 to 2014) at two different sites(Bologna, BO, urban background, and San Pietro Capofiume, SPC, ruralbackground), to characterize the spatial variability and seasonality of theOA sources, with a special focus on the cold season. On the multi-year basis of the study, the AMS observations show that OAaccounts for averages of 45±8 % (ranging from 33 % to 58 %) and 46±7 % (ranging from 36 % to 50 %) of the total non-refractory submicron particlemass (PM1-NR) at the urban and rural sites, respectively. Primaryorganic aerosol (POA) comprises biomass burning (23±13 % of OA) and fossil fuel (12±7 %) contributions with a marked seasonality in concentration. As expected, the biomass burning contribution to POA is moresignificant at the rural site (urban / rural concentration ratio of 0.67),but it is also an important source of POA at the urban site during the coldseason, with contributions ranging from 14 % to 38 % of the total OA mass. Secondary organic aerosol (SOA) contributes to OA mass to a much largerextent than POA at both sites throughout the year (69±16 % and83±16 % at the urban and rural sites, respectively), with importantimplications for public health. Within the secondary fraction of OA, themeasurements highlight the importance of biomass burning aging productsduring the cold season, even at the urban background site. This biomassburning SOA fraction represents 14 %–44 % of the total OA mass in the coldseason, indicating that in this region a major contribution of combustionsources to PM mass is mediated by environmental conditions and atmosphericreactivity. Among the environmental factors controlling the formation of SOA in the PoValley, the availability of liquid water in the aerosol was shown to play akey role in the cold season. We estimate that the organic fraction originatingfrom aqueous reactions of biomass burning products (“bb-aqSOA”) represents21 % (14 %–28 %) and 25 % (14 %–35 %) of the total OA mass and 44 %(32 %–56 %) and 61 % (21 %–100 %) of the SOA mass at the urban and ruralsites, respectively.

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