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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, NO x controls would be most effective for PM 2.5 reduction in the Po valley both during summer and winter. In the future, however, seasonal transitions to the NH 3 -sensitive region may occur, meaning that the NH 3 reduction policy may become increasingly necessary.

Relevant concentrations of dimethyl- and diethylammonium salts (DMA+ and DEA+) were measured in submicrometer marine aerosol collected over the North Atlantic during periods of high biological activity (HBA) in clean air masses (median concentration (minimum-maximum)=26(6-56) ng m(-3)). Much lower concentrations were measured during periods of low biological activity (LBA): 1 (<0.4-20) ng m(-3) and when polluted air masses were advected to the sampling site: 2 (<0.2-24) ng m(-3). DMA+ and DEA+ are the most abundantorganic species, second only to MSA, detected in fine marine particles representing on average 11% of the secondary organic aerosol (SOA) fraction and a dominant part (35% on average) of the water-soluble organic nitrogen (WSON). Several observations support the hypothesis that DMA+ and DEA+ have a biogenic oceanic source and are produced through the reaction of gaseous amines with sulfuric acid or acidic sulfates. Moreover, the water-soluble fraction of nascent marine aerosol particles produced by bubble-bursting experiments carried out in parallel to ambient aerosol sampling over the open ocean showed WSON, DMA+, and DEA+ concentrations always below the detection limit, thus excluding an important primary sea spray source.

The functional group compositions of atmospheric aerosol water-soluble organic compoundswere obtained employing proton nuclear magnetic resonance (1H NMR) spectroscopy in a series of recent experiments in several areas of the world characterized by different aerosol sources and pollution levels. Here, we discuss the possibility of using 1H NMR functional group distributions to identifythe sources of aerosol in the different areas. Despite the limited variability of functional group compositions of atmospheric aerosol samples, characteristic 1H NMR fingerprints were derived for three major aerosol sources: biomass burning, secondary formation from anthropogenic and biogenic VOCs, and emission from the ocean. The functional group patterns obtained in areas characterized by one of the above dominant source processes were then compared to identify the dominant sources for samples coming from mixed sources. This analysis shows that H NMR spectroscopy can profitably be used as a valuable tool for aerosol source identification. In addition, compared to other existing methodologies, it is able to relate the source fingerprints to integral chemical properties of the organic mixtures, which determine their reactivity and their physicochemical properties and ultimately the fate of the organic particles in the atmosphere.