engleski

Chemical nature of size segregated water soluble organic aerosols and their nitro-aromatic constituents

The contribution of light absorbing organic compounds to aerosol scattering and absorption is among the largest uncertainties in assessing the direct effect of aerosols on climate. To a large extent, understanding the processes involving organics in the atmosphere depends on how well these compounds are identified. It has been recently found that atmospheric water soluble organic matter (WSOM), apart from altering the hygroscopic properties, surface tension behavior and effective density of atmospheric particles, exhibits also light-absorbing (“brown carbon”-BrC) properties (Zhang et al., 2011). Water soluble BrC, thus potentially contributes to both a direct climate forcing and a semi-direct climate effect through heating and evaporation of clouds. Dominant subgroup of WSOM that can alter many aerosol properties including optical characteristics is referred as HUmic-LIke Substances (HULIS) (Hoffer et. al., 2006). Nitro-aromatic compounds (NAC) are known to absorb light in the UV/VIS range, hence, their fraction in atmosphere aerosols might contribute to the effects of aerosols on the solar radiation balance. Just recently, several NAC have been observed in PM10 urban aerosols (Kitanovski et al., 2012) and the first study on the presence of yellow-colored nitrocatechols in HULIS isolated from PM2.5 has been reported (Claeys et al., 2012). Although knowledge of the size distributions of aerosol components provides important information about their sources, formation and evolution mechanisms in the atmosphere, the size distribution of NAC and their quantitative contribution to WSOM and HULIS fraction is completely unknown. Our comprehensive study is focused on aerosol size-segregated WSOM, with a special emphasis on its NAC (4-nitrocatechol, 4NC ; methyl nitrocatechols, MNC ; di- and nitrophenols, (D)NP ; methyl nitrophenols, MNP ; nitrosalicylic acids, NSA ; nitroguaiacols, NG) through investigation of their solid phase extraction (SPE) analytical performances, molecular level speciation and quantification. Size segregated aerosol samples (0.038-15.6 μm aerodynamic diameters) were collected in the urban background location of Ljubljana, Slovenia. Study comprises seasonal data offering the opportunity to provide an insight into the ambient characteristics of NAC and to critically evaluate their potential main sources and transformation processes. As an important prerequisite for NAC successful chemical characterization, different SPE protocols for HULIS isolation were comprehensively analyzed with respect to selectivity, recovery and reproducibility of the specific NAC. Afterwards, aqueous aerosol extracts and HULIS fractions were obtained from the combined aerosol samples for each impactor stage. Chemical characterization of size segregated WSOM and HULIS included measurements of their light absorption properties by UV-VIS spectrometry and molecular level determination of target NAC by applying LC-MS approach. The obtained data were correlated with those for levoglucosan as a tracer for biomass burning emissions. To complement these studies, the size distributions of aerosol mass as well as total carbon (TC), water soluble organic (WSOC) and HULIS carbon (HULIS-C) content have been determined and related to WSOM production and formation processes.

atmospheric aerosols; HULIS; nitoaromatic compounds; brown carbon; chromatography

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)