Abstract
Abstract. 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, and makes a considerable contribution to global warming. Large numbers of studies have reported long-term observation of OC and EC concentrations throughout the world, but studies of BrC based on long-term observations are rather limited. In this study, we established a two-wavelength method (658 and 405 nm) applied in the Sunset thermal–optical carbon analyzer. Based on a 1-year observation, we firstly investigated the characteristics, meteorological impact and transport process of OC and EC. Since 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 of providing 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 aerosol concentrations were the 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 for OC and EC; however, dEC was 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. A total of two biomass burning events both in summer and winter were analyzed, and the results showed that the dEC concentrations were obviously higher on 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 emissions made a great contribution to dEC, and the sources of OC and EC were more complicated. Large number of open fire counts from the northwestern and southwestern areas of the study site were observed in winter and significantly contributed to OC, EC and dEC. In addition, the nearby Yangtze River Delta 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 on biomass burning days. Our modified two-wavelength instrument provided more information than the traditional single-wavelength thermal–optical carbon analyzer and gave a new idea about the measurement of BrC; the application of dEC data needs to be further investigated.
Highlights
Carbonaceous aerosols, including organic carbon (OC) and elemental carbon (EC), which have significant influence on the global radiative transfer, human health and atmospheric visibility, have been the focus of research in the atmospheric environment field for many years (Lelieveld et al, 2015; Wu and Yu, 2016; Wang et al, 2018; Zhang et al, 2017, 2019; Liu et al, 2019)
The results are comparable to those reported by Chen et al (2017) in the Xianlin Campus of Nanjing University (5.7 μg m−3 for OC and 3.2 μg m−3 for EC), for which the site was located in the southeastern suburb of Nanjing and close to the G25 highway and was affected by traffic sources
The average contributions of OC and EC to the total measured PM2.5 mass were 12.8 % and 4.3 %, respectively, suggesting that carbonaceous fraction made an important contribution to fine particulate matter
Summary
Carbonaceous aerosols, including organic carbon (OC) and elemental carbon (EC), which have significant influence on the global radiative transfer, human health and atmospheric visibility, have been the focus of research in the atmospheric environment field for many years (Lelieveld et al, 2015; Wu and Yu, 2016; Wang et al, 2018; Zhang et al, 2017, 2019; Liu et al, 2019). EC mainly originates from fossil fuel and biomass combustion and is estimated to be the second-largest warming factor behind CO2 contributing to climate change (Liu et al, 2015; Zhang and Kang, 2019; Cao and Zhang, 2015). Brown carbon (BrC), as a kind of lightabsorbing organic carbon which can absorb light especially from near-UV to visible wavelength, has caused global concern due to its positive climate effect (Andreae and Gelencsér, 2006; Zhang et al, 2020). In developing countries such as China and India, the contribution of fossil fuel combustion to BrC cannot be ignored (Satish et al, 2017; Yan et al, 2017; Kirillova et al, 2014). Due to the lack of understanding of BrC at the molecular level and in situ BrC data, there are still large uncertainties in the estimates of the distribution and the magnitude of the BrC climate effect in both remote sensing and modeling (Arola et al, 2011; Feng et al, 2013)
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