Abstract. Eastward air-mass transport from the Asian summer monsoon (ASM) anticyclone
in the upper troposphere and lower stratosphere (UTLS) often involves
eastward-shedding vortices, which can cover most of the Japanese
archipelago. We investigated the aerosol characteristics of these vortices
by analysing data from two lidar systems in Japan, at Tsukuba
(36.1∘ N, 140.1∘ E) and Fukuoka (33.55∘ N,
130.36∘ E), during the summer of 2018. We observed several events
with enhanced particle signals at Tsukuba at 15.5–18 km of altitude (at or
above the local tropopause) during August–September 2018, with a
backscattering ratio of ∼ 1.10 and particle depolarization of
∼ 5 % (i.e. not spherical, but more spherical than ice
crystals). These particle characteristics may be consistent with those of
solid aerosol particles, such as ammonium nitrate. Each event had a
timescale of a few days. During the same study period, we also observed
similar enhanced particle signals in the lower stratosphere at Fukuoka. The
upper troposphere is often covered by cirrus clouds at both lidar sites.
Backward trajectory calculations for these sites for days with enhanced
particle signals in the lower stratosphere and days without indicate that
the former air masses originated within the ASM anticyclone and the latter
more from edge regions. Reanalysis carbon monoxide and satellite
water vapour data indicate that eastward-shedding vortices were involved in
the observed aerosol enhancements. Satellite aerosol data confirm that the
period and latitudinal region were free from the direct influence of
documented volcanic eruptions and high-latitude forest fires. Our results
indicate that the Asian tropopause aerosol layer (ATAL) over the ASM region
extends east towards Japan in association with the eastward-shedding
vortices and that lidar systems in Japan can detect at least the lower-stratospheric portion of the ATAL during periods when the lower stratosphere
is undisturbed by volcanic eruptions and forest fires. The upper-tropospheric portion of the ATAL is either depleted by tropospheric
processes (convection and wet scavenging) during eastward transport or is
obscured by much stronger cirrus cloud signals.