Redox-activity and in vitro effects of regional atmospheric aerosol pollution: Seasonal differences and correlation between oxidative potential and in vitro toxicity of PM1.

Particulate Matter (PM) is a complex and heterogeneous mixture of atmospheric particles recognized as a threat to human health. Oxidative Potential (OP) measurement is a promising and integrative method for estimating PM-induced health impacts since it is recognized as more closely associated with adverse health effects than ordinarily used PM mass concentrations. OP measurements could be introduced in the air quality monitoring, along with the parameters currently evaluated. PM deposition in the lungs induces oxidative stress, inflammation, and DNA damage. The study aimed to compare the OP measurements with toxicological effects on BEAS-2B and THP-1 cells of winter and summer PM1 collected in the Po Valley (Italy) during 2021. PM1 was extracted in deionized water by mechanical agitation and tested for OP and, in parallel, used to treat cells. Cytotoxicity, genotoxicity, oxidative stress, and inflammatory responses were assessed by MTT test, DCFH-DA assay, micronucleus, γ-H2AX, comet assay modified with endonucleases, ELISA, and Real-Time PCR. The evaluation of OP was performed by applying three different assays: dithiothreitol (OPDTT), ascorbic acid (OPAA), and 2',7'-dichlorofluorescein (OPDCFH), in addition, the reducing potential was also analysed (RPDPPH). Seasonal differences were detected in all the parameters investigated. The amount of DNA damage detected with the Comet assay and ROS formation highlights the presence of oxidative damage both in winter and in summer samples, while DNA damage (micronucleus) and genes regulation were mainly detected in winter samples. A positive correlation with OPDCFH (Spearman's analysis, p < 0.05) was detected for IL-8 secretion and γ-H2AX. These results provide a biological support to the implementation in air quality monitoring of OP measurements as a useful proxy to estimate PM-induced cellular toxicological responses. In addition, these results provide new insights for the assessment of the ability of secondary aerosol in the background atmosphere to induce oxidative stress and health effects.

Characterization and dark oxidation of the emissions of a pellet stove.

Pellet combustion in residential heating stoves has increased globally during the last decade. Despite their high combustion efficiency, the widespread use of pellet stoves is expected to adversely impact air quality. The atmospheric aging of pellet emissions has received even less attention, focusing mainly on daytime conditions, while the degree to which pellet emissions undergo night-time aging as well as the role of relative humidity remain poorly understood. In this study, environmental simulation chamber experiments were performed to characterize the fresh and aged organic aerosol (OA) emitted by a pellet stove. The fresh pellet stove PM1 (particulate matter with an aerodynamic diameter less than 1 µm) emissions consisted mainly of OA (93 ± 4%, mean ± standard deviation) and black carbon (5 ± 3%). The primary OA (POA) oxygen-to-carbon ratio (O : C) was 0.58 ± 0.04, higher than that of fresh logwood emissions. The fresh OA at a concentration of 70 µg m-3 (after dilution and equilibration in the chamber) consisted of semi-volatile (68%), low and extremely low volatility (16%) and intermediate-volatility (16%) compounds. The oxidation of pellet emissions under dark conditions was investigated by injecting nitrogen dioxide (NO2) and ozone (O3) into the chamber, at different (10-80%) relative humidity (RH) levels. In all dark aging experiments secondary organic aerosol (SOA) formation was observed, increasing the OA levels after a few hours of exposure to NO3 radicals. The change in the aerosol composition and the extent of oxidation depended on RH. For low RH, the SOA mass formed was up to 30% of the initial OA, accompanied by a moderate change in both O : C levels (7-8% increase) and the OA spectrum. Aging under higher RH conditions (60-80%) led to a more oxygenated aerosol (increase in O : C of 11-18%), but only a minor (1-10%) increase in OA mass. The increase in O : C at high RH indicates the importance of heterogeneous aqueous reactions in this system, that oxidize the original OA with a relatively small net change in the OA mass. These results show that the OA in pellet emissions can chemically evolve under low photochemical activity (e.g. the wintertime period) with important enhancement in SOA mass under certain conditions.

Exposure to urban nanoparticles at low PM[Formula: see text] concentrations as a source of oxidative stress and inflammation.

Exposures to fine particulate matter (PM[Formula: see text]) have been associated with health impacts, but the understanding of the PM[Formula: see text] concentration-response (PM[Formula: see text]-CR) relationships, especially at low PM[Formula: see text], remains incomplete. Here, we present novel data using a methodology to mimic lung exposure to ambient air (2[Formula: see text] 60 [Formula: see text]g m[Formula: see text]), with minimized sampling artifacts for nanoparticles. A reference model (Air Liquid Interface cultures of human bronchial epithelial cells, BEAS-2B) was used for aerosol exposure. Non-linearities observed in PM[Formula: see text]-CR curves are interpreted as a result of the interplay between the aerosol total oxidative potential (OP[Formula: see text]) and its distribution across particle size (d[Formula: see text]). A d[Formula: see text]-dependent condensation sink (CS) is assessed together with the distribution with d[Formula: see text] of reactive species . Urban ambient aerosol high in OP[Formula: see text], as indicated by the DTT assay, with (possibly copper-containing) nanoparticles, shows higher pro-inflammatory and oxidative responses, this occurring at lower PM[Formula: see text] concentrations (< 5 [Formula: see text]g m[Formula: see text]). Among the implications of this work, there are recommendations for global efforts to go toward the refinement of actual air quality standards with metrics considering the distribution of OP[Formula: see text] with d[Formula: see text] also at relatively low PM[Formula: see text].

Exposure to outdoor air pollution and risk of hospitalization for bronchiolitis in an urban environment: A 9-year observational study.

BACKGROUND: Outdoor air pollution is supposed to influence the course of bronchiolitis, but the evidence is limited. The present study aimed at evaluating the role of outdoor air pollutants on hospitalization for bronchiolitis. METHODS: Infants aged ≤12 months referred for bronchiolitis to our Pediatric Emergency Department in Bologna, Italy, from 1 October 2011 to 16 March 2020 (nine epidemic seasons) were retrospectively included. Daily concentrations of benzene (C6 H6 ), nitrogen dioxide (NO2 ), particulate matter ≤2.5 µm (PM2.5 ), and ≤10 µm (PM10 ), and the mean values of individual patient exposure in the week and the 4 weeks before hospital access were calculated. The association between air pollutants exposure and hospitalization was evaluated through logistic regression analysis. RESULTS: A total of 2902 patients were enrolled (59.9% males; 38.7% hospitalized). Exposure to PM2.5 in the 4 weeks preceding bronchiolitis was identified as the main parameter significantly driving the risk of hospitalization (odds ratio [95% confidence interval]: 1.055 [1.010-1.102]). After stratifying by season, higher values of other outdoor air pollutants were found to significantly affect hospitalization: 4-week exposure to C6 H6 (Season 2011-2012, 4.090 [1.184-14.130]) and PM2.5 (Season 2017-2018, 1.282 [1.032-1.593]), and 1-week exposure to C6 H6 (Season 2012-2013, 6.193 [1.552-24.710]), NO2 (Season 2013-2014, 1.064 [1.009-1.122]), PM2.5 (Season 2013-2014, 1.080 [1.023-1.141]), and PM10 (Season 2018-2019, 1.102 [0.991-1.225]). CONCLUSION: High levels of PM2.5 , C6 H6 , NO2 , and PM10 may increase the risk of hospitalization in children affected by bronchiolitis. Open-air exposure of infants during rush hours and in the most polluted areas should be avoided.

Secondary aerosol formation during the dark oxidation of residential biomass burning emissions.

Particulate matter from biomass burning emissions affects air quality, ecosystems and climate; however, quantifying these effects requires that the connection between primary emissions and secondary aerosol production is firmly established. We performed atmospheric simulation chamber experiments on the chemical oxidation of residential biomass burning emissions under dark conditions. Biomass burning organic aerosol was found to age under dark conditions, with its oxygen-to-carbon ratio increasing by 7-34% and producing 1-38 µg m-3 of secondary organic aerosol (5-80% increase over the fresh organic aerosol) after 30 min of exposure to NO3 radicals in the chamber (corresponding to 1-3 h of exposure to typical nighttime NO3 radical concentrations in an urban environment). The average mass concentration of SOA formed under dark-oxidation conditions was comparable to the mass concentration formed after 3 h (equivalent to 7-10 h of ambient exposure) under ultraviolet lights (6 µg m-3 or a 47% increase over the emitted organic aerosol concentration). The dark-aging experiments showed a substantial increase in secondary nitrate aerosol (0.12-3.8 µg m-3), 46-100% of which is in the form of organic nitrates. The biomass burning aerosol pH remained practically constant at 2.8 throughout the experiment. This value promotes inorganic nitrate partitioning to the particulate phase, potentially contributing to the buildup of nitrate aerosol in the boundary layer and enhancing long-range transport. These results suggest that oxidation through reactions with the NO3 radical is an additional secondary aerosol formation pathway in biomass burning emission plumes that should be accounted for in atmospheric chemical-transport models.

Phytoplankton Impact on Marine Cloud Microphysical Properties Over the Northeast Atlantic Ocean.

The current understanding of the impact of natural cloud condensation nuclei (CCN) variability on cloud properties in marine air is low, thus contributing to climate prediction uncertainty. By analyzing cloud remote sensing observations (2009-2015) at Mace Head (west coast of Ireland), we show the oceanic biota impact on the microphysical properties of stratiform clouds over the Northeast Atlantic Ocean. During spring to summer (seasons of enhanced oceanic biological activity), clouds typically host a higher number of smaller droplets resulting from increased aerosol number concentration in the CCN relevant-size range. The induced increase in cloud droplet number concentration (+100%) and decrease in their radius (-14%) are comparable in magnitude to that generated by the advection of anthropogenically influenced air masses over the background marine boundary layer. Cloud water content and albedo respond to marine CCN perturbations with positive adjustments, making clouds brighter as the number of droplets increases. Cloud susceptibility to marine aerosols overlaps with a large variability of cloud macrophysical and optical properties primarily affected by the meteorological conditions. The above findings suggest the existence of a potential feedback mechanism between marine biota and the marine cloud-climate system.

Leaching material from Antarctic seaweeds and penguin guano affects cloud-relevant aerosol production.

Within the Southern Ocean, the greatest warming is occurring on the Antarctic Peninsula (AP) where clear cryospheric and biological consequences are being observed. Antarctic coastal systems harbour a high diversity of marine and terrestrial ecosystems heavily influenced by Antarctic seaweeds (benthonic macroalgae) and bird colonies (mainly penguins). Primary sea spray aerosols (SSA) formed by the outburst of bubbles via the sea-surface microlayer depend on the organic composition of the sea water surface. In order to gain insight into the influence of ocean biology and biogeochemistry on atmospheric aerosol, we performed in situ laboratory aerosol bubble chamber experiments to study the effect of different leachates of biogenic material - obtained from common Antarctic seaweeds as well as penguin guano - on primary SSA. The addition of different leachate materials on a seawater sample showed a dichotomous effect depending on the leachate material added - either suppressing (up to 52%) or enhancing (22-88%) aerosol particle production. We found high ice nucleating particle number concentrations resulting from addition of guano leachate material. Given the evolution of upper marine polar coastal ecosystems in the AP, further studies on ocean-atmosphere coupling are needed in order to represent the currently poorly understood climate feedback processes.

Understanding the environmental factors related to the decrease in Pediatric Emergency Department referrals for acute asthma during the SARS-CoV-2 pandemic.

BACKGROUND: Lockdown measures during the SARS-CoV-2 pandemic determined radical changes to behavioral and social habits, that were reflected by a reduction in the transmission of respiratory pathogens and in anthropogenic atmospheric emissions. OBJECTIVE: This ecological study aims to provide a descriptive evaluation on how restrictive measures during the SARS-CoV-2 pandemic impacted Pediatric Emergency Department (PED) referrals for asthma exacerbations, and their potentially associated environmental triggers in Bologna, a densely populated urban area in Northern Italy. METHODS: Files of children evaluated for acute asthma during 2015 to 2020 at the PED of Sant'Orsola University Hospital of Bologna were retrospectively reviewed. Historical daily concentration records of particulate (PM2.5 , PM10 ) and gaseous (NO2 , C6 H6 ) air pollutants, and pollen were concurrently evaluated, including specific PM chemical tracers for traffic-related air pollution (TRAP). RESULTS: In 2020, asthma-related PED referrals decreased compared to referral rates of the previous 5 years (p < 0.01). This effect was particularly marked during the first lockdown period (March to May), when the drastic drop in PED referrals was associated with a reduction of high-priority cases up to 85% and by 54%, on average. A concomitant reduction in the concentrations of traffic-related air pollutants was observed in the range of 40%-60% (p < 0.01). CONCLUSIONS: The lower rate of asthma exacerbations in childhood was in this study paralleled with reduced TRAP levels during the pandemic. Synergic interactions of the multiple consequences of lockdowns likely contributed to the reduced exacerbations, including decreased exposure to ambient pollutants and fewer respiratory infections, identified as the most important factor in the literature.

Chemical composition and radiative forcing of atmospheric aerosols over the high-altitude Western Himalayas of India.

Aerosol behavior over the Himalayas plays an important role in the regional climate of South Asia. Previous studies at high-altitude observatories have provided evidence of the impact of long-range transport of pollutants from the Indo-Gangetic Plain (IGP). However, little information exists for the valley areas in the high Himalayas where significant local anthropogenic emissions can act as additional sources of short-living climate forcers and pollutants. The valley areas host most economic activities based on agriculture, forestry, and pilgrimage during every summer season. We report here first measurements at a valley site at ~2600 m a.s.l. on the trek to the Gangotri glacier (Gaumukh), in the Western Himalayas, where local infrastructures for atmospheric measurements are absent. The study comprised short-term measurement of aerosols, chemical characterization, and estimation of aerosol radiative forcing (ARF) during the winter and summer periods (2015-2016). The particulate matter mass concentrations were observed to be higher than the permissible limit during the summer campaigns. We obtained clear evidence of the impact of local anthropogenic sources: particulate nitrate is associated with coarse aerosol particles, the black carbon (BC) mass fraction appears undiluted with respect to measurements performed in the lower Himalayas, and in winter, both BC and sulfate concentrations in the valley site are well above the background levels reported from literature studies for mountain peaks. Finally, high concentrations of trace metals such as copper point to anthropogenic activities, including combustion and agriculture. While most studies in the Himalayas have addressed pollution in the high Himalayas in terms of transport from IGP, our study provides clear evidence that local sources cannot be overlooked over the high-altitude Himalayas. The estimated direct clear-sky ARF was estimated to be in the range of -0.1 to +1.6 W m-2, with significant heating in the atmosphere over the high-altitude Himalayan study site. These results indicate the need to establish systematic aerosol monitoring activities in the high Himalayan valleys.

Historical Changes in Seasonal Aerosol Acidity in the Po Valley (Italy) as Inferred from Fog Water and Aerosol Measurements.

Acidity profoundly affects almost every aspect that shapes the composition of ambient particles and their environmental impact. Thermodynamic analysis of gas-particle composition datasets offers robust estimates of acidity, but they are not available for long periods of time. Fog composition datasets, however, are available for many decades; we develop a thermodynamic analysis to estimate the ammonia in equilibrium with fog water and to infer the pre-fog aerosol pH starting from fog chemical composition and pH. The acidity values from the new method agree with the results of thermodynamic analysis of the available gas-particle composition data. Applying the new method to historical (25 years) fog water composition at the rural station of San Pietro Capofiume (SPC) in the Po Valley (Italy) suggests that the aerosol has been mildly acidic, with its pH decreasing by 0.5-1.5 pH units over the last decades. The observed pH of the fog water also increased 1 unit over the same period. Analysis of the simulated aerosol pH reveals that the aerosol acidity trend is driven by a decrease in aerosol precursor concentrations, and changes in temperature and relative humidity. Currently, NOx controls would be most effective for PM2.5 reduction in the Po valley both during summer and winter. In the future, however, seasonal transitions to the NH3-sensitive region may occur, meaning that the NH3 reduction policy may become increasingly necessary.

Aerosol Toxins Emitted by Harmful Algal Blooms Susceptible to Complex Air-Sea Interactions.

Critical research is needed regarding harmful algal blooms threatening ecosystem and human health, especially through respiratory routes. Additional complexity comes from the poorly understood factors involved in the physical production of marine aerosols coupled with complex biogeochemical processes at ocean surfaces. Here-by using a marine aerosol generation tank-five bubble-bursting experiments (with contrasting incubation times and, likely, physiological microalgal states) were run to investigate simultaneously the concentrations of the toxins, synthesized by a natural Ostreopsis cf. ovata bloom, in suspension in the water and in the atmosphere. The first two experiments (EXP1-2) were run with moderate levels of O. cf. ovata cell numbers (ca. 105 cells·L-1) and total toxin in suspension (4 × 106 pg·Lwater-1) obtained at an early phase of the bloom. After 0.75-4 h incubation, toxin concentration in the aerosols accounted for 49-69 pg·Lair-1. By striking contrast, three experiments (EXP3-5)-conducted with samples collected two weeks later with higher cell abundances and higher toxin concentration in the seston (respectively, about 1 × 106 cells·L-1 and 2 × 108 pg·Lwater-1) and incubated for 21 h-showed about 15-fold lower atmospheric concentrations (3-4 pg·Lair-1), while important foam accumulation was observed in the water surface in the tank. Offline spectroscopic analysis performed by proton-nuclear magnetic resonance spectroscopy showed that the particulate organic carbon in the water was drastically different from that of bubble-bursting aerosols from the tank experiments-suggesting a selective transfer of organic compounds from seawater into the atmosphere. Overall, the results suggest that aerosol production and diffusion of marine toxins in the atmosphere are regulated by complex interactions between biological processes and air-sea aerosol production dynamics.

Rapid dark aging of biomass burning as an overlooked source of oxidized organic aerosol.

Oxidized organic aerosol (OOA) is a major component of ambient particulate matter, substantially impacting climate, human health, and ecosystems. OOA is readily produced in the presence of sunlight, and requires days of photooxidation to reach the levels observed in the atmosphere. High concentrations of OOA are thus expected in the summer; however, our current mechanistic understanding fails to explain elevated OOA during wintertime periods of low photochemical activity that coincide with periods of intense biomass burning. As a result, atmospheric models underpredict OOA concentrations by a factor of 3 to 5. Here we show that fresh emissions from biomass burning exposed to NO2 and O3 (precursors to the NO3 radical) rapidly form OOA in the laboratory over a few hours and without any sunlight. The extent of oxidation is sensitive to relative humidity. The resulting OOA chemical composition is consistent with the observed OOA in field studies in major urban areas. Additionally, this dark chemical processing leads to significant enhancements in secondary nitrate aerosol, of which 50 to 60% is estimated to be organic. Simulations that include this understanding of dark chemical processing show that over 70% of organic aerosol from biomass burning is substantially influenced by dark oxidation. This rapid and extensive dark oxidation elevates the importance of nocturnal chemistry and biomass burning as a global source of OOA.

Global Importance of Hydroxymethanesulfonate in Ambient Particulate Matter: Implications for Air Quality.

Sulfur compounds are an important constituent of particulate matter, with impacts on climate and public health. While most sulfur observed in particulate matter has been assumed to be sulfate, laboratory experiments reveal that hydroxymethanesulfonate (HMS), an adduct formed by aqueous phase chemical reaction of dissolved HCHO and SO2, may be easily misinterpreted in measurements as sulfate. Here we present observational and modeling evidence for a ubiquitous global presence of HMS. We find that filter samples collected in Shijiazhuang, China, and examined with ion chromatography within 9 days show as much as 7.6 µg m-3 of HMS, while samples from Singapore examined 9-18 months after collection reveal ~0.6 µg m-3 of HMS. The Shijiazhuang samples show only minor traces of HMS 4 months later, suggesting that HMS had decomposed over time during sample storage. In contrast, the Singapore samples do not clearly show a decline in HMS concentration over 2 months of monitoring. Measurements from over 150 sites, primarily derived from the IMPROVE network across the United States, suggest the ubiquitous presence of HMS in at least trace amounts as much as 60 days after collection. The degree of possible HMS decomposition in the IMPROVE observations is unknown. Using the GEOS-Chem chemical transport model, we estimate that HMS may account for 10% of global particulate sulfur in continental surface air and over 25% in many polluted regions. Our results suggest that reducing emissions of HCHO and other volatile organic compounds may have a co-benefit of decreasing particulate sulfur.

Contribution of Water-Soluble Organic Matter from Multiple Marine Geographic Eco-Regions to Aerosols around Antarctica.

We present shipborne measurements of size-resolved concentrations of aerosol components across ocean waters next to the Antarctic Peninsula, South Orkney Islands, and South Georgia Island, evidencing aerosol features associated with distinct eco-regions. Nonmethanesulfonic acid Water-Soluble Organic Matter (WSOM) represented 6-8% and 11-22% of the aerosol PM1 mass originated in open ocean (OO) and sea ice (SI) regions, respectively. Other major components included sea salt (86-88% OO, 24-27% SI), non sea salt sulfate (3-4% OO, 35-40% SI), and MSA (1-2% OO, 11-12% SI). The chemical composition of WSOM encompasses secondary organic components with diverse behaviors: while alkylamine concentrations were higher in SI air masses, oxalic acid showed higher concentrations in the open ocean air. Our online single-particle mass spectrometry data exclude a widespread source from sea bird colonies, while the secondary production of oxalic acid and sulfur-containing organic species via cloud processing is suggested. We claim that the potential impact of the sympagic planktonic ecosystem on aerosol composition has been overlooked in past studies, and multiple eco-regions act as distinct aerosol sources around Antarctica.

Antarctic sea ice region as a source of biogenic organic nitrogen in aerosols.

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.

Results of an interlaboratory comparison of analytical methods for quantification of anhydrosugars and biosugars in atmospheric aerosol.

An interlaboratory comparison was performed to evaluate the analytical methods for quantification of anhydrosugars - levoglucosan, mannosan, galactosan - and biosugars - arabitol, glucose and mannitol - in atmospheric aerosol. The performance of 10 laboratories in Italy currently involved in such analyses was investigated on twenty-six PM (particulate matter) ambient filters, three synthetic PM filters and three aqueous standard solutions. An acceptable interlaboratory variability was found, determined as the mean relative standard deviation (RSD%) of the results from the participating laboratories, with the mean RSD% values ranging from 25% to 46% and decreasing with increasing sugar concentration. The investigated methods show good accuracy, evaluated as the percentage error (ε%) related to mean values, since method biases ranged within ±20% for most of the analytes measured in the different laboratories. The detailed investigation (ANOVA analysis at p < 0.05) of the contribution of each laboratory to the total variability and the measurement accuracy shows that comparable results are generated by the different methods, despite the great diversity in terms of extraction conditions, chromatographic separation - more recent LC (liquid chromatography) and EC (exchange chromatography) methods compared to more widespread GC (gas chromatography) - and detection systems, namely PAD (pulsed amperometric detection) or mass spectrometry.

Direct observation of aqueous secondary organic aerosol from biomass-burning emissions.

The mechanisms leading to the formation of secondary organic aerosol (SOA) are an important subject of ongoing research for both air quality and climate. Recent laboratory experiments suggest that reactions taking place in the atmospheric liquid phase represent a potentially significant source of SOA mass. Here, we report direct ambient observations of SOA mass formation from processing of biomass-burning emissions in the aqueous phase. Aqueous SOA (aqSOA) formation is observed both in fog water and in wet aerosol. The aqSOA from biomass burning contributes to the "brown" carbon (BrC) budget and exhibits light absorption wavelength dependence close to the upper bound of the values observed in laboratory experiments for fresh and processed biomass-burning emissions. We estimate that the aqSOA from residential wood combustion can account for up to 0.1-0.5 Tg of organic aerosol (OA) per y in Europe, equivalent to 4-20% of the total OA emissions. Our findings highlight the importance of aqSOA from anthropogenic emissions on air quality and climate.

On the water-soluble organic nitrogen concentration and mass size distribution during the fog season in the Po Valley, Italy.

The study of organic nitrogen gained importance in recent decades due to its links with acid rain, pollution, and eutrophication. In this study, aerosol and fog water samples collected from two sites in Italy during November 2011 were analyzed to characterize their organic nitrogen content. Organic nitrogen contributed 19-25% of the total soluble nitrogen in the aerosol and around 13% in fog water. The largest water soluble organic nitrogen concentrations in the PM1.2 fraction occurred during the diurnal period with mean values of 2.03 and 2.16 µg-N m(-3) (154 and 145 nmol-N m(-3)) at Bologna and San Pietro Capofiume (SPC), respectively. The mean PM10 WSON concentration during diurnal periods at SPC was 2.30 µg-N m(-3) (164 nmol-N m(-3)) while it was 1.34 and 0.82 µg-N m(-3) (95.7 and 58.5 nmol-N m(-3)) in the night and fog water samples, respectively. Aerosol mass distribution profiles obtained during fog changed significantly with respect to those estimated in periods without fog periods due to fog scavenging, which proved to be over 80% efficient. Linear correlations suggested secondary processes related to combustion and, to a lesser extent, biomass burning, as plausible sources of WSON. Regarding the inorganic nitrogen fraction, the results showed that ammonium was the largest soluble inorganic nitrogen component in the samples.