Chromophoric water-soluble organic matter in atmospheric aerosols potentially plays an important role in aqueous reactions and light absorption by organics. The fluorescence and chemical-structural characteristics of the chromophoric water-soluble organic matter in submicron aerosols collected in urban, forest, and marine environments (Nagoya, Kii Peninsula, and the tropical Eastern Pacific) were investigated using excitation-emission matrices (EEMs) and a high-resolution aerosol mass spectrometer. A total of three types of water-soluble chromophores, two with fluorescence characteristics similar to those of humiclike substances (HULIS-1 and HULIS-2) and one with fluorescence characteristics similar to those of protein compounds (PLOM), were identified in atmospheric aerosols by parallel factor analysis (PARAFAC) for EEMs. We found that the chromophore components of HULIS-1 and -2 were associated with highly and less-oxygenated structures, respectively, which may provide a clue to understanding the chemical formation or loss of organic chromophores in atmospheric aerosols. Whereas HULIS-1 was ubiquitous in water-soluble chromophores over different environments, HULIS-2 was abundant only in terrestrial aerosols, and PLOM was abundant in marine aerosols. These findings are useful for further studies regarding the classification and source identification of chromophores in atmospheric aerosols.

Abstract. Traditional yield curve analysis shows that semi-volatile organic compounds are a major component of secondary organic aerosols (SOAs). We investigated the volatility distribution of SOAs from α-pinene ozonolysis using positive electrospray ionization mass analysis and dilution- and heat-induced evaporation measurements. Laboratory chamber experiments were conducted on α-pinene ozonolysis, in the presence and absence of OH scavengers. Among these, we identified not only semi-volatile products, but also less volatile highly oxygenated molecules (HOMs) and dimers. Ozonolysis products were further exposed to OH radicals to check the effects of photochemical aging. HOMs were also formed during OH-initiated photochemical aging. Most HOMs that formed from ozonolysis and photochemical aging had 10 or fewer carbons. SOA particle evaporation after instantaneous dilution was measured at  < 1 and  ∼ 40 % relative humidity. The volume fraction remaining of SOAs decreased with time and the equilibration timescale was determined to be 24–46 min for SOA evaporation. The experimental results of the equilibration timescale can be explained when the mass accommodation coefficient is assumed to be 0.1, suggesting that the existence of low-volatility materials in SOAs, kinetic inhibition, or some combined effect may affect the equilibration timescale measured in this study.

Water-soluble organic nitrogen (WSON) affects the formation, chemical transformations, hygroscopicity, and acidity of organic aerosols as well as biogeochemical cycles of nitrogen. However, large uncertainties exist in the origins and formation processes of WSON. Submicrometer aerosol particles were collected at a suburban forest site in Tokyo in summer 2015 to investigate the relative impacts of anthropogenic and biogenic sources on WSON formations and their linkages with aerosol liquid water (ALW). The concentrations of WSON (ave. 225 ± 100 ngN m–3) and ALW exhibited peaks during nighttime, which showed a significant positive correlation, suggesting that ALW significantly contributed to WSON formation. Further, the thermodynamic predictions by ISORROPIA-II suggest that ALW was primarily driven by anthropogenic sulfate. Our analysis, including positive matrix factorization, suggests that aqueous-phase reactions of ammonium and reactive nitrogen with biogenic volatile organic compounds (VOCs) play a key role in WSON formation in submicrometer particles, which is particularly significant in nighttime, at the suburban forest site. The formation of WSON associated with biogenic VOCs and ALW was partly supported by the molecular characterization of WSON. The overall result suggests that ALW is an important driver for the formation of aerosol WSON through a combination of anthropogenic and biogenic sources.

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