Morphological and optical properties of carbonaceous aerosol particles from ship emissions and biomass burning during a summer cruise measurement in the South China Sea

Abstract

Abstract. Carbonaceous aerosols constitute a crucial component of atmospheric marine aerosols, among which black carbon (BC) and brown carbon (BrC) are important contributors to light absorption and hence the positive climatic radiative forcing in the marine atmosphere. We conducted a month-long (May 05–June 09, 2021) onboard sample collections and online measurements of carbonaceous aerosols to characterize their morphological and optical properties during a ship cruise in the South China Sea (SCS), covering a marine region of 11.9–24.5° N and 111.1–118.2° E. Single particles were collected by a single particle sampler and offline analyses were performed to investigate the mixing state and morphology using a transmission electron microscope (TEM). Online measurements of BC in PM2.5 were made by a seven-wavelength aethalometer and organic carbon (OC)/elemental carbon (EC) mass concentrations were measured by a semi-online OC/EC analyzer. Single particle samples were classified into two modes: “stop” when the ship was anchored and “navigation” when the ship sailed at high speed. Feret diameters of the single particles during navigation and stop showed size distributions with the lognormal fitting peaks at 307 and 325 nm, respectively. The fresh (without coating) and aged BC particles (after removal of coating by the electron beams in TEM) showed comparable median fractal dimensions (1.65 vs 1.66), in contrast to their different median lacunarities (0.53 vs 0.59). The aged BC particles showed narrower Feret diameters (298–1980 nm) during navigation than those (304–2982 nm) of freshly-emitted BC from the own ship during stop. Moreover, tar balls, as one important component of single particles from ship emissions and as the tracer of biomass burning, were identified with geometrical diameters of 160–420 nm in the TEM images. The energy dispersive X-ray spectroscopy (EDS) analyses showed those tar balls are mainly mixed with sea salt, organics, BC, and sulfate. We also found a significant fraction of aged BC in various mixing states (core-shell, embedded) with other components of the aerosol particles after long-range transport. The campaign was further divided into several periods (before monsoon period, BMP; transition monsoon period, TMP; after monsoon period, AMP; and ship pollution period, SPP) according to the wind direction during monsoon and the own ship pollution. The median OC/EC ratios were 8.14, 5.20, 6.35, and 2.63 during BMP, TMP, AMP, and SPP, respectively, showing higher OC/EC ratios for biomass burning emissions than for fossil fuel emissions. Additionally, the median absorption Angström exponent (AAE) values derived from all wavelengths were 1.14, 1.02, 1.08, and 1.06 for BMP, TMP, AMP and SPP, respectively. Particularly, a median AAE value of 1.93 was obtained during two significant biomass burning events. These results showed that biomass burning (BB) and fossil fuel (FF) combustion contributed to 18–22 % and 78–82 % of all the BC light absorption without the two intense biomass burning events, during which BB and FF accounted for 42 % and 58 %, respectively. The two BB events originated from the Philippines and Southeast Asia before and after the summer monsoon. Our results demonstrated that BC can serve as the core of aged particles but the fractal dimensions of BC aggerates were subject to little variation; moreover, such BC particles become much more aggerated after aging in the marine atmosphere, which further affects the light absorption of the BC particles in the SCS. This study provides information about the morphology and the optical properties of carbonaceous aerosols which can be used to evaluate their effects on light absorption and hence the climatic radiative forcing in the SCS region.

Atmospheric Brown Carbon: Sources, Optical Properties, and Chromophore Composition

“…The DRE of BrC absorption is more pronounced in areas with intense biomass-burning activity, such as South America, Africa, and Southeast Asia, where it exceeds +1 W m -2 (Wang et al, 2018;Zeng et al, 2020;Zhu et al, 2021;Sun et al, 2023, markedly surpassing the globalaverage DRE. This emphasises that BrC from BB is a crucial component of the global radiative balance.…”