Estimation of radiative forcing by the dust and non-dust content in mixed East Asian pollution plumes on the basis of depolarization ratios measured with lidar

Abstract

The contribution of dust and non-dust aerosols to the lidar observed total backscattering was estimated from the linear particle depolarization ratios (δp) under the assumption of externally mixed aerosols. During intensive observation period, the contribution rate of the non-dust aerosol optical depth (AOD, τ) to total AOD increased from 30% on 22 October to 82% on 30 October, which implies that large amount of non-dust particles was transported along with Asian dust storm. Using independent aerosol optics and profiles for dust and non-dust, for the first time, instantaneous aerosol direct radiative forcing (ADRF) of the Asian dust plumes on atmosphere-surface system was quantified. The average ADRF by the dust particles was −31.5 ± 16.1 (−66.3 ± 20.2) W m−2 at the surface, −16.5 ± 8.8 (−11.2 ± 9.2) W m−2 at the TOA, respectively. The forcing efficiency, defined as the aerosol forcing per unit τ (532 nm), for dust (non-dust) particles was −124.6 ± 12.2 (−209.4 ± 59.1) W m−2/τ532 at the surface and −64.9 ± 7.8 (−35.1 ± 28.0) W m−2/τ532 at the TOA, respectively. Though the contribution of non-dust optical depth to total τ was smaller than the contribution of optical depth from dust to total optical depth, the non-dust particles contributed larger to the radiative forcing at the surface than the dust radiative forcing. This result demonstrates that increased Asian dust radiative forcing can be largely attributed to the presence of non-dust particles mixed into Asian dust layers rather than the radiative forcing by pure dust particles. We also found that non-dust particles play a significant role in stabilizing the dust layer by increasing the radiative heating rates within dust layers.