Mass Absorption Cross-section And Absorption Enhancement Of Black Carbon Aerosol At Typical Urban And Rural Areas In China

Carbonaceous aerosols play an important role in air quality and Earth’s climate systems. Black carbon aerosols (BC) are the highly condensed residue of incomplete combustion of biomass and fossil fuel. Black carbon has a positive radiative forcing that contributing to greenhouse effect, which is estimated to be second only to CO2. The reduction of BC emission could be an effective strategy to combat global warming in short term. However, scientific conclusions are low on emissions, concentrations, sources, and optical properties of BC.Non-absorbing aerosols such as sulfates and organic matters may surround a BC core (as coating or internal mixing) in the atmosphere. Such coatings may cause lensing effect for BC absorption, which potentially leads to the enhancement of mass absorption cross-section (MAC). The absorption enhancement of BC in ambient aerosols was poorly constrained, contributing to the one of the largest uncertainties of BC radiative forcing estimates for climate models.To determine the MAC and enhancement of MAC (EMAC) of BC in ambient aerosols, the high-intensity sampling and monitoring campaigns were achieved at Jinan and Yucheng, as an urban and regional rural site respectively, in the center of the densely populated North China Plain (NCP). An aerosol filter dissolution-filtration (AFD) system was then developed for a two-step removal of both inorganic and organic matter coatings in ambient aerosols. Aerosol samples in urban and regional rural areas in NCP were analyzed for MAC of BC before and after the AFD decoating treatment to establish EMAC values.The observed MAC for Jinan and Yucheng were 8.99±2.29 m2/g and 9.58±1.83 m2/g at 678 nm, respectively. The range of MAC was compatible with literature reports (3.4～16.8 m2/g). The variations of MAC showed a relationship with ratios of OC/EC, NO3-/EC, NH4+/EC and SO42-/EC, indicating that aerosol aging has effects on the MAC.The AFD treatment dissolved all water-soluble salts and 90% organic carbon in the aerosol samples, with the method having～99% recovery of decoated/pure BC. The MAC of decoated/pure BC during the campaigns at Jinan and Yucheng showed a narrow range of 4.78±1.72 m2/g and 4.38±0.79 m2/g, respectively, and both were lower than the levels of their original aerosol samples. The AFD solvent treatment thus offers a means to measure the MAC of ambient aerosol samples and their decoated BC isolates, with the ratio of the two measurements representing the absorption enhancement (EMAC) of BC due to coatings in ambient aerosols.The observed absorption enhancement (EMAC) was 2.07±0.72 and 2.25±0.55 at urban Jinan and rural area Yucheng, respectively. The EMAC at daytime (2.22±0.74 and 2.63±0.36 at Jinan and Yucheng, respectively) was higher than nighttime levels (1.76±0.60 and 1.75±0.31 at Jinan and Yucheng, repectively), which could be explained by stronger photochemical reaction in daytime. An increase trend of EMAC correlated well with the ratios SO42-/EC (R2=0.65) at the rural area. Coatings on aged aerosols could increase the EMAC to about a factor of three, which is twice that in fresh emissions from in complete fossil fuel combustion. The observation constrained absorption enhancements suggest that BC radiative forcing in severe polluted area may be stronger than previously thought.This thesis focused on the optical properties of the black carbon. The MAC and EMAC at typical severe polluted urban and rural areas in NCP were determined. The observational data constrained the uncertainties in estimation of BC radiative absorption and subsequent climatic effects. Such results add the understanding of BC driven climate forcing and increase scores for mitigating China air pollution.