The interaction of aerosol with atmospheric water affects the processing and wet removal of atmospheric particles. Understanding such interaction is mandatory to improve model description of aerosol lifetime and ageing. We analyzed the aerosol-water interaction at high relative humidity during fog events in the Po Valley within the framework of the Agenzia Regionale per la Prevenzione e l'Ambiente (ARPA) -Emilia Romagna supersite project. For the first time in this area, the changes in particle chemical composition caused by fog are discussed along with changes in particle microphysics.During the experiment, 14 fog events were observed. The average mass scavenging efficiency was 70 % for nitrate, 68 % for ammonium, 61 % for sulfate, 50 % for organics, and 39 % for black carbon. After fog formation, the interstitial aerosol was dominated by particles smaller than 200 nm D va (vacuum aerodynamic diameter) and enriched in carbonaceous aerosol, mainly black carbon and water-insoluble organic aerosol.For each fog event, the size-segregated scavenging efficiency of nitrate and organic aerosol (OA) was calculated by comparing chemical species size distribution before and after fog formation. For both nitrate and OA, the size-segregated scavenging efficiency followed a sigmoidal curve, with values close to zero below 100 nm D va and close to 1 above 700 nm D va . OA was able to affect scavenging efficiency of nitrate in particles smaller than 300 nm D va . A linear correlation between nitrate scavenging and particle hygroscopicity (κ) was observed, indicating that 44-51 % of the variability of nitrate scavenging in smaller particles (below 300 nm D va ) was explained by changes in particle chemical composition.The size-segregated scavenging curves of OA followed those of nitrate, suggesting that organic scavenging was controlled by mixing with water-soluble species. In particular, functional group composition and OA elemental analysis indicated that more oxidized OA was scavenged more efficiently than less oxidized OA. Nevertheless, the small variability of organic functional group composition during the experiment did not allow us to discriminate the effect of different organic functionalities on OA scavenging.

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Abstract. We investigate optical–microphysical–chemical properties of brown carbon (BrC) in the urban ambient atmosphere of the Po Valley. In situ ground measurements of aerosol spectral optical properties, PM1 chemical composition (HR-ToF-AMS), and particle size distributions were carried out in Bologna. BrC was identified through its wavelength dependence of light absorption at visible wavelengths, as indicated by the absorption Ångström exponent (AAE). We found that BrC occurs in particles with a narrow monomodal size distribution peaking in the droplet mode, enriched in ammonium nitrate and poor in black carbon (BC), with a strong dependance on OA-to-BC ratios, and SSA530 of 0.98 ± 0.01. We demonstrate that specific complex refractive index values (k530 = 0.017 ± 0.001) are necessary in addition to a proper particle size range to match the large AAEs measured for this BrC (AAE467 − 660 = 3.2 ± 0.9 with values up to 5.3). In terms of consistency of these findings with literature, this study i. provides experimental evidence of the size distribution of BrC associated with the formation of secondary aerosol;ii. shows that in the lower troposphere AAE increases with increasing OA-to-BC ratios rather than with increasing OA – contributing to sky radiometer retrieval techniques (e.g., AERONET);iii. extends the dependence of AAE on BC-to-OA ratios previously observed in chamber experiments to ambient aerosol dominated by wood-burning emissions. These findings are expected to bear important implications for atmospheric modeling studies and remote sensing observations as regards the parametrization and identification of BrC in the atmosphere.

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 (PM 1 -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.Published by Copernicus Publications on behalf of the European Geosciences Union. 1234M. Paglione et al.: Impact of biomass burning and aqueous-phase processing on air quality 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.

13Traditional aerosol mechanisms underestimate the observed organic aerosol concentration, especially due to 14 the lack of information on secondary organic aerosol (SOA) formation and processing. In contrast VBS slightly underestimates the contribution from fossil-fuel combustion (HOA), indicating that 31 POA emissions related to road transport are either underestimated or associated to higher volatility classes. 32The VBS scheme under-predictes the SOA too, but to a lesser extent than CAMx-SOAP. SOA 33 underestimation can be related to corresponding underestimation of either aging processes or precursor 34emissions. This indicates that improvements in the emission inventories for semi-and intermediate-volatility 35 organic compounds are needed for further progress in this area. Finally, the comparison between modeled 36 and observed SOA sources points out the urgency to include processing of OA in particle water phase into 37 SOA formation mechanisms, to reconcile model results and observations. 38 Highlights 39• CAMx performance for OA are worse than for other PM components 40• SOAP scheme shows a better performance than VBS, due to an error compensation 41• VBS allows a better repartition of primary and secondary OA than SOAP scheme 42• POA volatility distribution and SVOC and IVOC emissions need improvement 43• Aqueous phase mechanism is necessary to reconcile OA observations and modeling 44 45 CTMs implementing standard SOA chemistry often over-predict fresh POA and underpredict SOA, 74 especially in summer (Bergstrom et al., 2012). 75 KeywordsThe volatility basis set (VBS) approach Donahue et al. (2006Donahue et al. ( , 2011 allows to take into account POA 76 volatility and multiple generation SOA production. VBS can be used in two configurations. The 1-dimension 77 approach (1D-VBS) describes OA evolution based on OA volatility (Donahue et al., 2006). The 2-dimension 78 approach (2D-VBS) describes OA evolution in the 2-D space defined by effective saturation concentration 79 C* (μg m -3 ) and the oxidation degree (Donahue et al., 2012). We use a hybrid VBS approach 1.5D-VBS 80 (Koo et al., 2014), where OA evolution follows specific path in such space, reducing computational cost. The 81 implementation of VBS approach in CTMs introduced a valuable improvement both in model performance 82 as well as in the knowledge of the key processes influencing the modeled results. For example Zhang et al. 83 (2013) showed that in a simulation with non-volatile POA and a simplified SOA formation mechanism POA 84 are largely overestimated, while SOA are underestimated. The application of the VBS scheme indicated that 85 also the volatility distribution of the aerosols is extremely important (Lane et al., 2008; Fountoukis et al., 86 2011). Namely, the distribution of OA emissions into the low volatility bins appears to be important for the 87 predicted POA because it has great impact on the initial partitioning between the aerosol and the gas phase 88 (Tsimpidi et al., 2010). Bergstrom et al. (2012) showed...