The impact of biomass burning and aqueous-phase processing on air quality: a multi-year source apportionment study in the Po Valley, Italy
Authors: Paglione Marco , Gilardoni Stefania , Rinaldi Matteo , Decesari Stefano , Zanca Nicola , Sandrini Silvia , Giulianelli Lara , Bacco Dimitri , Ferrari Silvia , Poluzzi Vanes , Scotto Fabiana , Trentini Arianna , Poulain Laurent , Herrmann Hartmut , Wiedensohler Alfred , Canonaco Francesco , Prévôt André S. H. , Massoli Paola , Carbone Claudio , Facchini Maria Cristina , Fuzzi Sandro 
Published: February 2020

Abstract. The Po Valley (Italy) is a well-known air quality hotspot characterized by
particulate matter (PM) levels well above the limit set by the European Air
Quality Directive and by the World Health Organization, especially during
the colder season. In the framework of Emilia-Romagna regional project
“Supersito”, the southern Po Valley submicron aerosol chemical composition
was characterized by means of high-resolution aerosol mass spectroscopy
(HR-AMS) with the specific aim of organic aerosol (OA) characterization and
source apportionment. Eight intensive observation periods (IOPs) were
carried out over 4 years (from 2011 to 2014) at two different sites
(Bologna, BO, urban background, and San Pietro Capofiume, SPC, rural
background), to characterize the spatial variability and seasonality of the
OA sources, with a special focus on the cold season. On the multi-year basis of the study, the AMS observations show that OA
accounts for averages of 45±8 % (ranging from 33 % to 58 %) and 46±7 % (ranging from 36 % to 50 %) of the total non-refractory submicron particle
mass (PM1-NR) at the urban and rural sites, respectively. Primary
organic 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 more
significant 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 cold
season, with contributions ranging from 14 % to 38 % of the total OA mass. Secondary organic aerosol (SOA) contributes to OA mass to a much larger
extent than POA at both sites throughout the year (69±16 % and
83±16 % at the urban and rural sites, respectively), with important
implications for public health. Within the secondary fraction of OA, the
measurements highlight the importance of biomass burning aging products
during the cold season, even at the urban background site. This biomass
burning SOA fraction represents 14 %–44 % of the total OA mass in the cold
season, indicating that in this region a major contribution of combustion
sources to PM mass is mediated by environmental conditions and atmospheric
reactivity. Among the environmental factors controlling the formation of SOA in the Po
Valley, the availability of liquid water in the aerosol was shown to play a
key role in the cold season. We estimate that the organic fraction originating
from aqueous reactions of biomass burning products (“bb-aqSOA”) represents
21 % (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 rural
sites, respectively.