Radiatively-driven processes in forest fire and desert dust plumes

The absorption of solar radiation by atmospheric aerosol particles
is important for the climate effects of aerosols. Absorption by
aerosol particles heats atmospheric layers, even though the net
effect for the entire atmospheric column may still be a cooling.
Most experimental studies on absorbing aerosols so far focussed
mainly on the aerosol properties and did not consider the influence
of the aerosols on the thermodynamic structure of the atmosphere.
In this study, data from two international aircraft field
experiments, the Intercontinental Transport of Ozone and Precursors
study (ITOP) 2004 and the Saharan Mineral Dust Experiment (SAMUM)
2006 are investigated. The ITOP data were collected before the work
on this thesis started, while the logistics and the instrument
preparation of the SAMUM campaign, the weather forecast during
SAMUM and the in-situ aerosol measurements during SAMUM were done
within this thesis. The experimental data are used to explore the
impact of layers containing absorbing forest fire and desert dust
aerosol particles on the atmospheric stability and the implications
of a changed stability on the development of the aerosol
microphysical and optical properties during long-range transport.
For the first time, vertical profiles of the Richardson number Ri
are used to assess the stability and mixing in forest fire and
desert dust plumes. Also for the first time, the conclusions drawn
from the observations of forest fire and desert dust aerosol, at
first glance apparently quite different aerosol types, are
discussed from a common perspective. Two mechanisms, the
self-stabilising and the sealed ageing effect, acting in both
forest fire and desert dust aerosol layers, are proposed to explain
the characteristic temperature structure as well as the aerosol
properties observed in lofted forest fire and desert dust plumes.
The proposed effects impact on the ageing of particles within the
plumes and reduce the plume dilution, therefore extending the plume
lifetime. This study combines experimental data, modelling of
optical parameters and calculated heating rates to assess the role
of forest fire and desert dust plumes. The microphysical, optical
and chemical properties of forest fire and desert dust aerosol, and
their vertical distribution, were measured with multiple
instruments on the DLR Falcon 20-E5 research aircraft during ITOP
and SAMUM. Aerosol size information and absorption data were
analysed with respect to the aerosol mixing state, effective
diameter and parameterisation of forest fire and dust size
distributions. Altogether, about 90 size distributions for
particles from different sources were extracted from multiple
instruments and parameterised with multi-modal log-normal
distributions. Subsequently, the optical properties were calculated
for the different aerosol layers and compared with other
independent measurements of the optical properties like the
extinction coefficient determined with a High Spectral Resolution
Lidar. The aerosol optical properties serve as the basis for the
radiative transfer calculations with libRadtran (library for
radiative transfer). Finally, the aerosol microphysical and optical
properties, the meteorological data and the heating rates are
examined to investigate the proposed self-stabilising and sealed
ageing effects. The investigation of numerous forest fire and
desert dust plumes in this study revealed characteristic aerosol
properties: the aged (age: 4-13 days) forest fire aerosol is
characterised by the absence of a nucleation mode, a depleted
Aitken mode and an enhanced accumulation mode. In addition, more
than 80% of the particles in the Aitken mode and nearly all
particles in the accumulation mode of the forest fire plumes are
internally mixed with a solid core. The desert dust aerosol
exhibits two size regimes of different mixing states: below 0.5 µm,
particles have a non-volatile core and a volatile coating; larger
particles above 0.5 µm consist of non-volatile components and
contain absorbing material. After regional-scale transport from the
Sahara to South-western Europe, the volatile fraction in the dust
plume did not significantly increase. The lofted forest fire plumes
were found during ITOP at altitudes between 3 and 9 km above sea
level (ASL), while the lofted desert dust plumes were found during
SAMUM between 1 and 6 km ASL. The transition of the aerosol plumes
to the free tropospheric background above and below the plumes was
remarkably sharp and characterised by strong inversions. Within a
height range of 200-300 m, the particle concentrations decreased by
more than one order of magnitude. The results of plume dilution
were evident only in the upper part of the lofted forest fire and
desert dust plumes. The daily mean heating rates in the forest fire
and desert dust plumes showed maximum values of ~0.2 K day-1 and
~0.24 K day-1, respectively. Vertical profiles of the heating rate
suggest that the processes caused by the interaction between the
aerosol particles and the solar radiation stabilise the plume
itself and decelerate plume dilution. Apparently, the aerosol in
such plumes ages in an almost “closed” system, where suppressed
entrainment of condensable gases from the surface inhibits particle
nucleation and the formation of coated particles inside the plume.
The processes described tend to extend the lifetime of the layer
allowing the transport over long distances.