Uniting In-situ Measurement And Integrating Sphere System To Study Absorption Enhancement Effect Of Black Carbon Aerosols

The impacts of atmospheric aerosols on climate effects,environmental change and human health have received widespread attention.Black carbon aerosol,which can absorb solar radiation and hence heat the atmosphere,have even been considered a type of warming factor second only to CO₂.During the aging process from source to sink,some of black carbon aerosols would form a core-shell structure,triggering the lensing effect in light absorption and thus enhancing the absorption ability of black carbon.Due to the existence of absorption enhancement effect,there are considerable uncertainties in assessing black carbon’s radiative forcing and climate effect.The existing methods for measuring the absorption enhancement coefficient of black carbon are mainly based on measuring the difference in light absorption values with and without coating layer.However,various limitations or deficiencies in existing methods used to accurately remove the coating layer usually bring biases in the measurements of absorption enhancement coefficient,which would add difficulty to the academic community in reaching a consensus on whether the absorption enhancement counts in climate and environment.Here a new measurement scheme is proposed,i.e.,"integrating sphere system+in-situ absorption measurement"united technology,to measure the absorption enhancement coefficient caused by lensing.In this scheme,the integrating sphere device combined with appropriate organic solvents is used to effectively remove the influence of lensing effect caused by core-shell structure,while the in-situ absorption measurement（Photoacoustic method,PAX）can measure the in-situ total absorbance when the coating layer exists（with lensing effect）.The absorption enhancement coefficient can thus be calculated from the relationship between the measuerments without and with lensing effects.This scheme was then applied to local ambient aerosols,and four typical months were selected to represent the four seasons in Beijing in 2019.While the in-situ light absorption values were continuously monitored by a PAX,PM_2.5 samples were collected in quartz filters.The light absorption values of PM_2.5-loaded filters were determined in the integrating sphere system.The results show that:（1）The absorption enhancement coefficient E_abs varies with the season.According to the PM_2.5 weighed in filters,the average concentration of PM_2.5 in Beijing in 2019 was 60.84μg·m^-3,and PM_2.5,organic carbon（OC）,and elemental carbon（EC）all showed obvious seasonal variations.The annual average absorption enhancement coefficient E_abs in Beijing in 2019 was 1.90±0.41,with a seasonal sequence of spring（1.75±0.54）<autumn（1.90±0.43）<summer（1.91±0.28）<winter（2.02±0.39）.Seasonal differences in pollution degree(represented by PM_2.5)may account for the sequence.（2）There is a statistically significant correlation between the absorption enhancement coefficient E_abs and PM_2.5 concentration,which can be expressed as an empirical equation E_abs=0.6×ln(PM_2.5-3.59)-0.43（R²=0.99）.This study arrived at above empirical equation by statistically analyzing the values of PM_2.5 and E_abs in 2019,which validated the hypothesis by a few of previous studies,through the analysis of ion composition,chemical composition,and particle size distribution in clean and heavily polluted periods:on the one hand,with the aggravation of pollution,the aging process causes the enhancement of light absorption by black carbon aerosols due to the core-shell structure;on the other hand,the heavy pollution process leads to sustained growth of black carbon coating layer,incurring a"shielding"effect,which weakens the the lensing effect.Under above two mechanisms,the absorption enhancement effect（lensing effect）of black carbon aerosols tends to be flattened in heavy pollution periods,which enriches the understanding of the influence of coating layer on the absorption enhancement of black carbon aerosols.（3）The impact of the changes in the absorption enhancement coefficent E_abson the 2025 radiative forcing change of ambient aerosols（relative to 2019）is close to 10%.The lensing effect of black carbon affects the prediction of the direct radiative forcing of ambient aerosols.The direct radiative forcing values for the four seasons of2025 local Beijing were calculated based on the AERONET website data and the results of E_abs in this study.The projected radiative forcing values of 2025 are spring(-10.25W·m^-2),summer(-13.52 W·m^-2),autumn(-9.14 W·m^-2),and winter(-9.91 W·m^-2).By comparison,the direct radiative forcing values for the four seasons of 2019 local Beijing were spring(-11.74 W·m^-2),summer(-14.90 W·m^-2),autumn(-10.85 W·m^-2),and winter(-10.90 W·m^-2).It is found that the cooling strength of future aerosols in2025 is weaker than that in 2019,with the contribution of the change in E_abs to the change in direct radiative forcing being about 9.33%（close to 10%）.