Observational evidence of particle hygroscopic growth in the upper troposphere–lower stratosphere (UTLS) over the Tibetan Plateau
We measured the vertical profiles of backscatter ratio (BSR) using the
balloon-borne, lightweight Compact Optical Backscatter AerosoL Detector
(COBALD) instruments above Linzhi, located in the southeastern Tibetan
Plateau, in the summer of 2014. An enhanced aerosol layer in the upper
troposphere–lower stratosphere (UTLS), with BSR (455 nm) > 1.1 and
BSR (940 nm) > 1.4, was observed. The color index (CI) of the
enhanced aerosol layer, defined as the ratio of aerosol backscatter ratios
(ABSRs) at wavelengths of 940 and 455 nm, varied from 4 to 8, indicating
the prevalence of fine particles with a mode radius of less than 0.1 µm. We
find that unlike the very small particles (mode radius smaller than 0.04 µm) at low relative humidity (RHi < 40 %), the relatively
large particles in the aerosol layer were generally very hydrophilic as
their size increased dramatically with relative humidity. This result
indicates that water vapor can play a very important role in increasing the
size of fine particles in the UTLS over the Tibetan Plateau. Our
observations provide observation-based evidence supporting the idea that aerosol
particle hygroscopic growth is an important factor influencing the radiative
properties of the Asian Tropopause Aerosol Layer (ATAL) during the Asian
summer monsoon.
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