10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Long-term High Temporal Resolution Measurements of Carbonaceous Aerosol at a Suburban Station, in Athens, Greece – Assessment of Secondary Organic Aerosol Formation
Abstract Number: 1239 Working Group: Carbonaceous Aerosol
Abstract Carbonaceous aerosol is mainly comprised of organic carbon (OC), in the form of a wide range of non-volatile and semi-volatile organic compounds, and elemental carbon (EC). These components have been found to contribute significantly to the total PM mass, at both rural and urban locations, especially in the fine particle fraction. OC may originate from a number of sources, through primary emission or secondary formation. EC is of primary origin only and is formed from the incomplete combustion of fossil fuels or biomass. Recent epidemiological studies have provided evidence that carbon-containing components may be more strongly associated with adverse health outcomes in comparison to other particulate matter (PM) components. In addition, carbonaceous aerosol has been shown to have significant impacts on the global climate change through direct and indirect radiation forcing. The present study employs high resolution data of EC/OC concentrations collected over a long-term period (5 years) in Athens (Greece), in order to provide insight into the sources and processes affecting carbonaceous aerosol in urban environments.
The measurements were conducted at the NCSR Demokritos (DEM) suburban station in Athens, Greece, during 2013-2017. Near-real time EC and OC concentrations were monitored on a 3 h basis, by thermo-optical transmittance (TOT) method, by the use of an OCEC semi-continuous field analyzer (Sunset Laboratory, Inc). The instrument was sampling at a flow rate of 8 lpm, from a PM2.5 cut-off inlet and was equipped with an in-line parallel carbon denuder for the removal of organic gases. The EUSAAR2 protocol was applied for sample analysis. In addition, real-time data of aerosol absorption at seven wavelengths (370, 470, 520, 590, 660, 880 and 950 nm) and of equivalent black carbon (eBC) at 880 nm were derived from a 7-wavelength aethalometer (Rack Mount Aethalometer Model AE31, Magee Scientific Corp.). The aethalometer was recording every 5 min. The raw aethalometer data were corrected for the multiple scattering and shadowing effects, with the use of parallel measurements of aerosol scattering, obtained on a 1 min basis by a 3-wavelength nephelometer (Aurora 3000, Ecotech Pty Ltd). Real-time standard meteorological data were also collected. In addition, NOx and O3 concentration data were available from a National Monitoring Station located at around 300 m from the DEM station. Diurnal, weekly and seasonal variability of EC and OC levels, OC-EC relationships and OC/EC concentration ratios were analysed in order to assess carbonaceous aerosol levels and identify the main sources and/or atmospheric conditions responsible for the observed concentration levels. In addition, different calculation algorithms for the apportionment of OC to primary and secondary sources were evaluated. The contribution of secondary organic aerosol has been assessed and the factors governing secondary formation mechanisms have been studied. Relationships of EC-OC with other aerosol parameters (such as BC) and gaseous pollutants were studied in order to further understand the origin and characteristics of the Athens suburban carbonaceous aerosol.
Mean 24 h elemental and organic carbon concentrations over the 5-year measurement period were found equal to 0.7± 0.2 μg m-3 and 3.4 ± 1.5 μg m-3, respectively. The corresponding OC-to-EC concentration ratios were calculated equal to 6 ± 17. High ratios during the warm period pointed towards increased photochemical secondary formation of organic carbon. The diurnal trends of OC and EC concentrations followed dissimilar patterns. EC displayed a clear morning peak (between 6 and 9 am), which may be attributed to the morning rash hours. During the cold period, both OC and EC exhibited high concentrations at night (9pm to 12 am), probably due to the shallowing of the mixing layer. Nevertheless, an enhanced increase was observed for organic carbon (as documented by the diurnal patter of the OC/EC ratio), suggesting an additional source for OC, such as condensation of semi-volatile organic compounds due to the lower night-time temperatures. The OC/EC ratio displayed also a clear day-time peak (between 12 and 15 pm) which points towards secondary aerosol formation promoted by the enhanced solar radiation. The overall results indicate low levels for the primary carbonaceous components, due to the suburban character of the site. Nevertheless, the impact by local combustion sources (traffic and domestic heating, including the use of biomass burning) is clearly shown through the seasonal, weekly and diurnal variability in the concentrations. A significant contribution from secondary organic aerosol is also documented. The results highlight the value of high temporal resolution data in the continuous efforts of the scientific community to understand the sources and processes affecting the evolution of carbonaceous aerosol components.