engleski

Surfactant properties of water soluble nitro- aromatic compounds and their contribution to HULIS fraction of size segregated organic aerosols

In spite of decades of investigations, the effects of atmospheric aerosols via cloud formation and properties still constitutes the largest uncertainties in assessing the climate budget today (IPCC, 2007). These difficulties result in part from the limited understanding of the microphysical processes controlling the transformations of atmospheric aerosol particles into cloud droplets. Diffusional growth of cloud droplets occurs via activation of aerosols serving as cloud condensation nuclei (CCN) followed by the subsequent condensation of water vapour. The ability of particles to act as CCN depends on their chemical composition, as well as size. Some atmospheric water soluble organics known as surface active substances (SAS) or surfactants can substantially influence the surface tension (ST) of the gas-liquid interface of solution droplets in the air which plays a critical role in governing their CCN abilities. One of the most important aerosol surfactants defined as a subgroup of water soluble organic matter (WSOM) are humic-like substances (HULIS). Despite numerous works tried to identify organics able to lower the ST of atmospheric aerosols, their CCN properties responsible for their activation into cloud droplets are still not completely understood, giving rise to large uncertainties in the climate budget. This is in part because some water-soluble SAS remain unidentified and the effects of interactions among these species under typical aerosol conditions are generally unknown. Thus, knowing the ST of atmospheric particles or, alternatively, identifying the SAS, determining their surface active properties as well as concentrations in aerosol WSOM appear to be highly beneficial for accurate prediction of CCN properties leading to a better understanding of the role WSOM plays in the atmospheric and climatic systems. Just recently, nitro-aromatic compounds (NAC) like nitrocatehols have been shown to be present in HULIS samples of PM2.5 aerosols, corresponding to their yellow-coloured substances (Claeys et al., 2012). Several other NAC have been observed in urban PM10 aerosols as well, including nitrophenols, nitrocatechols, nitroguaiacols and nitrosalicylic acids (Kitanovski et al., 2012). The NAC, as potentially toxic and carcinogenic compounds present in relatively high concentrations, especially in urban aerosols may have adverse effects on human health and the environment. Moreover, due to their polarity, NAC may enhance the capacity of ambient aerosol to act as CCN, thereby affecting the global climate. Recently, alternating current (ac.) voltammetry was applied as a new high sensitive approach for qualitative and quantitative characterization of surfactants in atmospheric WSOM avoiding time- consuming sample pretreatments (Frka et al., 2012). In this study ac. voltammetry has been applied to characterize surface active properties of size segregated aerosols (0.038-15.6 μm aerodynamic diameters) collected seasonally in the urban background environment of Ljubljana, Slovenia. Firstly, the method proved to be sensitive enough to enable determination of SAS in bulk WSOM and HULIS samples of each impactor stage. We determined seasonal size distributions of HULIS surfactant concentrations as well as the contribution of HULIS surfactant concentrations to the overall surfactant activity of bulk WSOM for each size fraction. Concentrations of SAS expressed as the equivalent amounts of real HULIS material isolated from PM10 aerosols collected in the same season from the same location were correlated with those expressed in equivalents of Suwannee River fulvic acid (SRFA) often used as a standard for atmospheric HULIS material. Results are supported by water soluble organic carbon (WSOC) and HULIS carbon (HULIS-C) content. To get more insight to the seasonal variability of predominant surfactant material in each size fraction, the aerosol SAS concentrations were normalized to the WSOC content and corelated with normalized surfactant activities obtained for model organic compounds, including model NAC (4- nitrocatechol, methyl nitrocatechols, di- and nitrophenols, methyl nitrophenols, nitrosalicylic acids and nitroguaiacols). Results were also correlated with determined target NAC concentrations in size segregated WSOM and HULIS fractions by applying LC-MS approach.

tmospheric aerosols; HULIS; surfactants; nitroaromatic compounds; electrochemistry; brown carbon;

Rad je financiran kroz: AERONAR project - Marie Curie FP7-PEOPLE-2011- COFUND NEWFELPRO (European Commission and the Croatian Ministry of Science, Education Sports) and Slovenian Research Agency project (Contract no. P1-0034-0140)