Humic-like substances in Austrian PM10: : occurrence and possible origin

Abstract

Humic-Like Substances (HULIS), water and alkaline extractable organic compounds with molecular weights of several hundred Dalton and chemical characteristics very similar to naturally occurring terrestrial and aquatic humic acids, therefore termed HUmic-Like Substances (HULIS), are major contributors to organic aerosol. Although these substances got in the scope of interest decades ago there are still open questions regarding their sources and composition. The present work aims at elucidating the atmospheric abundance, seasonality and sources of HULIST - with HULIST denoting the sum of alkaline and water-extractable fractions of HULIS in different environments. A three city study, comprising 10 sites with (sub-) urban and rural characteristics in Vienna, Graz and Salzburg revealed annual averaged HULIST levels in a range of 0.46 to 1.24 micro g C/m3; and contributions to organic carbon (OC) up to 28.9%. All ten sites showed winter enrichment factors between 2.0 and 2.8. The establishment of soot stemming from diesel engined vehicles (ECD) as dilution tracer at two urban traffic impacted sites revealed, that additional sources and/ or higher formation rates are almost exclusively responsible for the observed winter enrichment. HULIST carbon in source emission samples from wood, lignite and anthracite combustion, diesel engined cars and cooking activities revealed contributions to PM10 in the range of 1-7%, 4-5%, 0.9-1.3% and up to 0.7%, respectively. These results indicate that beside wood smoke, combustion of fossil fuels can not be ignored in considerations on the potential sources of HULIST. Processing ratios of HULIST carbon and unique or main tracers in source emissions on ambient tracer levels revealed that, on annual average, up to 25% of HULIST could be of primary origin, emitted by diesel vehicles and wood stoves. Seasonal resolved correlation analysis indicated a strong connection of wintry HULIST levels with secondary species, the wood smoke tracer levoglucosan and with tracers connected to fossil fuel combustion. Correlation patterns in summer indicate as well secondary formation and the combustion of fossil fuels and partly biomass combustion to contribute to atmospheric HULIST. Abundance and seasonality of HULIST was investigated and compared at 23 Austrian sites and no trend for HULIST was observed to occur preferred at sites with urban or rural characteristics. The majority of the 23 sampling sites showed maximum HULIST concentrations in fall or winter with a winter enrichment factor in the range of 2-4. The impact of wood smoke and its relation to HULIST was assessed by comparison of HULIST and levoglucosan levels. The winter average of HULIST carbon and levoglucosan ratios occurred in a rather narrow range of 1.1- 5.9 at the 23 sites. The average ratio in samples from wood stove emissions was much lower (0.33). Seasonal averaged HULIS carbon and levoglucosan ratios in Vienna, Graz and Salzburg maximise in summer with values 4-7 times higher than in winter. Significant correlation of HULIS and levoglucosan was observed for most, but not all, sites in winter. We observed significant and good correlation for the sites in Vienna (r~0.74-0.93) and Salzburg (r~0.70-0.88). Lacking correlation of HULIST and levoglucosan levels at the background and urban fringe sites in Upper Austria and Carinthia indicate other sources than wood combustion being important for the atmospheric abundance of HULIST. HULIST levels at the 23 sites furthermore did not show a uniform trend to maximise in regions with highest wood smoke impact. The investigations of water and alkaline extracted atmospheric HULIS fractions with a thermal method shed light on seasonal and spatial variations of HULIS at two sampling sites in Upper Austria. All thermograms showed peak maxima from slightly below 300 up to 350 deg C and very refractory behaviour. The differences regarding thermal characteristics between HULISAS and HULISWS are rather small. Wintry urban HULIS showed a considerable shift to higher refractive material with peak maxima shifted from ~300 to 350 deg C, evidencing a change of composition compared to summerly and rural wintry isolates. These regional differences have not been observed for summerly HULIS isolates. Both HULIS - fractions showed almost congruent thermogram curves, which also suggests that they are structurally similar. The similar thermal stability of both HULIS factions at the rural and urban sampling site could indicate that the type of sources which contribute to the atmospheric abundance of HULIST in summer plays a minor role with regard to their composition.