Abstract. Black carbon (BC) aerosols have been widely recognized as
a vital climate forcer in the atmosphere. Amplification of light absorption
can occur due to coatings on BC during atmospheric aging, an effect that
remains uncertain in accessing the radiative forcing of BC. Existing studies
on the absorption enhancement factor (Eabs) have poor coverage on both
seasonal and diurnal scales. In this study, we applied a recently developed
minimum R squared (MRS) method, which can cover both seasonal and diurnal
scales, for Eabs quantification. Using field measurement data in
Guangzhou, the aims of this study are to explore (1) the temporal dynamics of
BC optical properties at seasonal (wet season, 31 July–10 September; dry
season, 15 November 2017–15 January 2018) and diel scales (1 h time
resolution) in the typical urban environment and (2) the influencing factors on
Eabs temporal variability. Mass absorption efficiency at 520 nm by
primary aerosols (MAEp520) determined by the MRS method exhibited a strong
seasonality (8.6 m2 g−1 in the wet season and 16.8 m2 g−1
in the dry season). Eabs520 was higher in the wet season (1.51±0.50) and lower in the dry season (1.29±0.28). Absorption
Ångström exponent (AAE470–660) in the dry season (1.46±0.12) was higher than that in the wet season (1.37±0.10). Collective
evidence showed that the active biomass burning (BB) in the dry season
effectively altered the optical properties of BC, leading to elevated MAE,
MAEp and AAE in the dry season compared to those in the wet season.
Diurnal Eabs520 was positively correlated with AAE470–660 (R2=0.71) and negatively correlated with the AE33 aerosol loading
compensation parameter (k) (R2=0.74) in the wet season, but these
correlations were significantly weaker in the dry season, which may be
related to the impact of BB. This result suggests that during the wet
season, the lensing effect was more likely dominating the AAE diurnal
variability rather than the contribution from brown carbon (BrC). Secondary
processing can affect Eabs diurnal dynamics. The Eabs520 exhibited
a clear dependency on the ratio of secondary organic carbon to organic carbon (SOC∕OC), confirming the contribution of secondary organic aerosols to
Eabs; Eabs520 correlated well with nitrate and showed a clear
dependence on temperature. This new finding implies that gas–particle
partitioning of semivolatile compounds may potentially play an important
role in steering the diurnal fluctuation of Eabs520. In the dry season,
the diurnal variability in Eabs520 was associated with photochemical
aging as evidenced by the good correlation (R2=0.69) between oxidant
concentrations (Ox=O3+NO2) and Eabs520.
Because of the strong Coulomb interaction and quantum confinement effect, 2-dimensional transition metal dichalcogenides possess a stable excitonic population. To realize excitonic device applications, such as excitonic circuits, switches, and transistors, it is of paramount importance for understanding the optical properties of transition metal dichalcogenides. Furthermore, the strong quantum confinement in 2-dimensional space introduces exotic properties, such as enhanced phonon bottlenecking effect, many-body interaction of excitons, and ultrafast nonequilibrium exciton–exciton annihilation. Exciton diffusion is the primary energy dissipation process and a working horse in excitonic devices. In this work, we investigated time-resolved exciton propagation in monolayer semiconductors of WSe
2
, MoWSe
2
, and MoSe
2
, with a home-built femtosecond pump-probe microscope. We observed ultrafast exciton expansion behavior with an equivalent diffusivity of up to 502 cm
2
s
−1
at the initial delay time, followed by a slow linear diffusive regime (20.9 cm
2
s
−1
) in the monolayer WSe
2
. The fast expansion behavior is attributed to energetic carrier-dominated superdiffusive behavior. We found that in the monolayers MoWSe
2
and MoSe
2
, the energetic carrier-induced exciton expansion is much more effective, with diffusivity up to 668 and 2295 cm
2
s
−1
, respectively. However, the “cold” exciton transport is trap limited in MoWSe
2
and MoSe
2
, leading to negative diffusion behavior at later time. Our findings are helpful to better understand the ultrafast nonlinear diffusive behavior in strongly quantum-confined systems. It may be harnessed to break the limit of conventional slow diffusion of excitons for advancing more efficient and ultrafast optoelectronic devices.
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