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Carbonaceous aerosols have great influence on the air quality, human health and climate change. Except for organic carbon (OC) and elemental carbon (EC), brown carbon (BrC), mainly originates from biomass burning, as a group of OC with strong absorption from the visible to near-ultraviolet wavelengths, makes a considerable contribution to global warming. Large amounts of studies have reported long-term observation of OC and EC concentrations throughout the word, but studies of BrC based on long-term observations are rather limited. In this study, we established a two-wavelength method (658 nm and 405 nm) applied in the Sunset thermo/optical carbon analyzer. Based on one-year observation, we firstly investigated the characteristics, meteorological impact and transport process of OC and EC. Due to BrC absorbs light at 405 nm more effectively than 658 nm, we defined the enhanced concentrations (dEC = EC405 nm−EC658 nm) and gave the possibility to provide an indicator of BrC. The receptor model and MODIS fire information were used to identify the presence of BrC aerosols. Our results showed that the carbonaceous aerosols concentrations were highest in winter and lowest in summer. Traffic emission was an important source of carbonaceous aerosols in Nanjing. Receptor model results showed that strong local emissions were found in OC and EC aerosols, however dEC aerosols were significantly affected by regional or long-range transport. The dEC / OC and OC / EC ratios showed similar diurnal patterns and the dEC / OC increased when the OC / EC ratios increased, indicating strong secondary sources or biomass burning contributions to dEC. Two biomass burning events both in summer and winter were analyzed and the results showed that the dEC concentrations were obvious higher in biomass burning days, however, no similar levels of the OC and EC concentrations were found both in biomass burning days and normal days in summer, suggesting that biomass burning emission made a great contribution to dEC and the sources of OC and EC were more complicated. Large number of open fire counts from the northwest and southwest areas of the study site were monitored in winter, significantly contributed to OC, EC and dEC. In addition, the near-by YRD area was one of the main potential source areas of dEC, suggesting that anthropogenic emissions could also be important sources of dEC. The results proved that dEC can be an indicator of BrC in biomass burning days. Our modified two-wavelength instrument provided more information than traditional single-wavelength thermo/optical carbon analyzer and gave a new idea about the measurement of BrC, the application of dEC data need to be further investigated. |
