Significance: Organic aerosol (OA) is a dominant component of atmospheric aerosol worldwide, and it is recognized as a key factor affecting air quality and possibly climate. Observations indicate that more than one-half of the global OA is of secondary origin. Traditional models typically underpredict secondary organic aerosol (SOA) mass, suggesting that a complete knowledge of SOA formation mechanisms is lacking. We show that aqueous-phase processing of biomass-burning emissions contributes to SOA formation. Such aqueous SOA absorbs UV and visible light more efficiently that other OA components. Aqueous chemistry processing of biomass-burning emissions should be taken into account in air quality and climate models for a correct description of the global OA budget and its climate-relevant optical properties.

Significance: To quantify the full implications of biomass burning emissions on the atmosphere, it is essential to accurately represent the emission plume after it has undergone chemical aging in the atmosphere. Atmospheric models typically consider the predominant aging pathway of biomass burning emissions to take place in the presence of sunlight (via the OH radical); however, this mechanism leads to consistent underpredictions of oxidized organic aerosol in wintertime urban areas. Here, we show, through a combination of laboratory experiments, ambient field measurements, and chemical transport modeling, that biomass burning emission plumes exposed to NO ₂ and O ₃ age rapidly without requiring any sunlight, thus providing an overlooked source of oxidized organic aerosol previously not accounted for in models.

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 Dva (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 Dva and close to 1 above 700 nm Dva. OA was able to affect scavenging efficiency of nitrate in particles smaller than 300 nm Dva. 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 Dva) 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.

To study the influence of regional biomass burning emissions and secondary processes, ambient samples of fog and aerosol were collected in the Po Valley (Italy) during the 2013 Supersito field campaign. After the extent of fresh vs. aged biomass burning influence was estimated from proton nuclear magnetic resonance (¹H NMR) and high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS), two samples of fog water and two samples of PM₁ aerosol were selected for ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis. Molecular compositions indicated that the water-soluble organic matter was largely non-polymeric without clearly repeating units. The selected samples had an atypically large frequency of molecular formulas containing nitrogen and sulfur (not evident in the NMR composition) attributed to multifunctional organonitrates and organosulfates. Higher numbers of organonitrates were observed in aerosol, and higher numbers of organosulfates were observed in fog water. Consistent with the observation of an enhanced aromatic proton signature in the ¹H-NMR analysis, the average molecular formula double-bond equivalents and carbon numbers were higher in the fresh biomass-burning-influenced samples. The average O : C and H : C values from FT-ICR MS were higher in the samples with an aged influence (O : C = 0.50–0.58, and H : C = 1.31–1.37) compared to those with fresh influence (O : C = 0.43–0.48, and H : C = 1.13–1.30). The aged fog had a large set of unique highly oxygenated CHO fragments in the HR-ToF-AMS, which reflects an enrichment of carboxylic acids and other compounds carrying acyl groups, highlighted by the NMR analysis. Fog compositions were more oxidized and SOA (secondary organic aerosol)-like than aerosols as indicated by their NMR measured acyl-to-alkoxyl ratios and the observed molecular formula similarity between the aged aerosol and fresh fog, implying that fog nuclei must be somewhat aged. Overall, functionalization with nitrate and sulfate moieties, in addition to aqueous oxidation, triggers an increase in the molecular complexity in this environment, which is apparent in the FT-ICR MS results. This study demonstrates the significance of the aqueous phase in transforming the molecular chemistry of atmospheric organic matter and contributing to secondary organic aerosol.

Climate warming affects the development and distribution of sea ice, but at present the evidence of polar ecosystem feedbacks on climate through changes in the atmosphere is sparse. By means of synergistic atmospheric and oceanic measurements in the Southern Ocean near Antarctica, we present evidence that the microbiota of sea ice and sea ice-influenced ocean are a previously unknown significant source of atmospheric organic nitrogen, including low molecular weight alkyl-amines. Given the keystone role of nitrogen compounds in aerosol formation, growth and neutralization, our findings call for greater chemical and source diversity in the modelling efforts linking the marine ecosystem to aerosol-mediated climate effects in the Southern Ocean.

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.