Abstract
Simultaneous measurements of black carbon (BC) and non-refractory PM1 aerosol chemical compositions were performed in autumn 2016 at a suburban site in Hong Kong. A thermodenuder (TD) was employed at different heating temperatures to remove semi-volatile aerosol fractions to varying degrees. The light absorption enhancement (Eabs) of BC due to semi-volatile coatings at seven wavelengths was evaluated, and the coating fractions were further analyzed. Results showed that the overall Eabs ranged from 1.58 ± 0.13 (470 nm) to 1.64 ± 0.16 (660 nm) at 280°C and increased very little from 50°C (1.02–1.04) to 200°C (1.13–1.20). The lensing-effect-related Eabs was probably attributable to the presence of ammonium and sulfate. Furthermore, the ratio of the coating thickness to the BC core radius was around 1.0–2.0 based on Mie calculations. This work evaluated the Eabs from coatings in Hong Kong and implied the BC mixing state, thus providing a critical reference for climate models on the role of aerosol in global warming.
Highlights
Black carbon (BC) is one of the major components of PM2.5 formed in flames during combustion of carboncontaining fuels, which often exists as an aggregate of small graphite spheres (Bond et al, 2013)
Organics (Org) and sulfate (SO42–) were two dominant species, with relative contribution of 51.3% and 29.0% to non-refractory PM1 (NR-PM1), followed by ammonium (NH4+, 11.2%), nitrate (NO3, 8.0%) and chloride (Chl, 0.4%), which is similar to other studies in Hong Kong (Sun et al, 2016) and in Beijing (Sun et al, 2013)
Characterization of Heated Aerosol We evaluated the filter-based light absorption enhancement of black carbon (BC) ( EaTb,s – fb ) as well as the remaining aerosol compositions indicated by remaining percentage (RP) for aerosol passing through TD at certain temperature
Summary
Black carbon (BC) is one of the major components of PM2.5 formed in flames during combustion of carboncontaining fuels, which often exists as an aggregate of small graphite spheres (Bond et al, 2013). In climatology, many academics regard BC as the second most important contributor to global warming after carbon dioxide (Jacobson 2001; IPCC 2007; Ramanathan and Carmichael, 2008; Srivastava et al, 2012). One key factor that defines the BC contribution to radiative forcing is its mixing state with other aerosol components, e.g., external mixture, volume mixture and encapsulated mixture, of which the last one ( known as coating-core mixing) has been recognized as the most realistic scenario, since BC is a solid and cannot physically be well mixed in a particle (Jacobson, 2001; Bond and Bergstrom, 2006) In this case, so-called coating materials surrounding BC core could act as a lens and enhance light absorption through the “lensing effect” (Fuller et al, 1999).
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