Amplification of black carbon light absorption induced by atmospheric aging: temporal variation at seasonal and diel scales in urban Guangzhou

Abstract

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.

Evolution of organic carbon during COVID-19 lockdown period: Possible contribution of nocturnal chemistry

“…In most of these previous studies, the lowest value (5–20%) of OC/EC was used empirically ( Yao et al, 2020 ; Zhang et al, 2019a ). In this study, the appropriate value of (OC/EC) pri was determined using the MRS method ( Bian et al, 2018 ; Ji et al, 2018b ; Millet et al, 2005 ; Sun et al, 2020a ; Wu and Yu, 2016 ), which determining the (OC/EC) pri when the smallest correlation coefficient (R 2 ) between SOC and EC achieved by assuming a series of continuous OC/EC ratios. More details can be found elsewhere ( Wu et al, 2019 ; Wu and Yu, 2016 ; Yao et al, 2020 ).…”

Mixing state of refractory black carbon in fog and haze at rural sites in winter on the North China Plain

“…10d). The evaporation of volatile components due to increased temperature could be the explanation (Sun et al, 2020). Higher temperatures facilitate the transition of volatile components such as nitrates and volatile organic compounds from the solid 525 phase to the gas phase.…”

Chemical Composition and Source Apportionment of Total Suspended Particulate in the Central Himalayan Region

“…In addition, data corresponding to the lowest 25% of OC/EC ratios were also considered for the primary OC/EC ratio and, via linear regressions of OC vs. EC, an (OC/EC) pri ratio of 5.13 was estimated. On the other hand, the MRS method [51,82] estimated a slightly higher (OC/EC) pri ratio of 6.0. These varying (OC/EC) pri ratios indicate the uncertainty in the quantification of POC and SOC components with the EC tracer method, which increases significantly in environments highly affected by BB aerosols [53].…”