Report Mineral dust aerosols, the small soil particles

Report of Top Scientist III class (Prof. Imad)

 

Mineral dust effect on
climate change and weather

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Batjargal Buyantogtokh

 

Arid Land Research Center (ALRC), Tottori
University, Tottori, Japan

[email protected]

 

Introduction

 

Mineral dust aerosols, the small soil
particles suspended in the atmosphere, can effect
on
the atmospheric radiation budget and hydrological
cycle through their radiative, cloud condensation nucleus
effects (Huang et al., 2014). Also mineral
dust effect on boundary layer weather
such as
surface energy exchanges, temperature and wind speed, precipitation (Choobari et al., 2014). Mineral
dust aerosols influence the climate system and cloud microphysics
in multiple ways. They effect the climate system directly by
scattering and partly absorbing shortwave and longwave
radiation, semi-directly by changing the atmospheric cloud
cover through evaporation of cloud droplets (Converting
visible light to thermal energy, dust aerosols can burn clouds to produce a
warming effect on climate, which is
opposite to the ?rst and second indirect effects of aerosols),
and indirectly by acting as cloud and ice
condensation nuclei, which changes the optical properties of
clouds (i.e. the first indirect effect), and may decrease or increase
precipitation formation (i.e. the second
indirect effect). Radiative forcing by mineral dust is associated with changes
in
atmospheric dynamics that may change the vertical
profile of temperature and wind speed, through
which a feedback effect on dust emission can be established (Choobari et al., 2014).

 

Radiative
forcing by dust

 

The climate impacts of mineral dust
aerosols have been identified as direct, semi-direct and indirect effects. They
have an influence on radiative processes directly through absorption and
scattering of shortwave and longwave radiation (Sarra et al., 2013) semi-directly
through changes in atmospheric temperature structure and burning
off of cloud droplets due to absorption of radiation (Helmert et al., 2007), and indirectly through
impact on the optical properties and lifetime of clouds,
and suppression or invigoration of precipitation
processes
(Konwar et al., 2010).

 

Direct
effects of dust aerosols

 

A schematic picture of the interaction of
dust aerosols with different radiative components of
the atmosphere, the direct radiative forcing, is shown in
Fig. 1.

 

Fig
1. A schematic illustration of interaction of mineral dust
aerosols with radiative components of the atmosphere during daytime. Daytime
surface cooling is caused by the fact that shortwave
reflection dominates over longwave trapping by dust aerosols (from Choobari et al., 2014).

 

Shortwave and longwave
absorption by dust aerosols increases the heating rate
of the dust layer. Note that mixed aerosols, which consist of
mineral dust and other particles may have higher (dust
+ soot) and lower (dust + sulfate) absorption
properties
(Garcia et al., 2011). The combination of
absorption and backscattering of shortwave radiation reduces
the incoming solar radiation reaching the surface, and
thereby decreases the heating rate of the atmosphere below
the dust layer. In contrast, by trapping the outgoing longwave
radiation, thermal radiative forcing of dust at the surface
is always positive (Hansell et al., 2010), suggesting that
mineral dust warms the surface during night
when it only interacts with longwave radiation. It should be
noted that direct radiative forcing by dust
aerosols also depends on the albedo of the underlying surface
due to the interaction of mineral dust with reflected solar
radiation (Liao and
Seinfeld, 1998). For example, higher reflection
of solar radiation by bright desert surfaces leads to a
greater absorption of radiation by dust aerosols, thereby increasing
the degree of atmospheric heating. Likewise, absorbing
dust aerosols above low-level clouds increases the
atmospheric heating rate due to high reflectivity of clouds (Podgorny and
Ramanathan, 2001).

 

Semi-direct
effects of dust aerosols

 

The term semi-direct effect was describe the effects
of the absorption of radiation by aerosols on clouds (Ackerman et al., 2000). Dust aerosols embedded
within clouds absorb radiation which leads to a reduction of relative humidity,
enhancement of cloud evaporation (i.e. the cloud burning effect; Huang
et al., 2006), and therefore a reduction
in cloud cover. This suggests that there is a negative correlation between absorbing
aerosols that are
embedded
within clouds and cloud development. Such negative correlation is more pronounced
for larger dust
aerosols
as they are more absorptive. The reduced cloud cover is associated with warming of the
ground surface, in
contrast
to the general daytime surface cooling effect of mineral dust aerosols. Note that the
semi-direct effect of mineral dust is sensitive to the position of the dust
layer relative
to clouds. If the aerosol layer is located below clouds, heating
within the dust layer can enhance convection and thus cloud cover. The warming effect of
absorbing dust
aerosols
above the cloud top, on the other hand, stabilizes the underlying layer, so that vertical
development of clouds is inhibited, while their horizontal development may be increased (Koch and Del Genio, 2010).

.

 

Indirect Radiative Effect

 

Dust aerosols can also
change the energy ?uxes in the Earth-atmosphere system by modifying cloud macrophysical
and microphysical properties, such as the cloud liquid water content, cloud
fraction, cloud top temperature, droplet number concentration, cloud particle
size, and so on (Huang et al., 2006a)  as shown in Figure 2.

 

Figure 2. A schematic depiction of the indirect
radiative effect of

dust aerosols (from Huang et al., 2014).

 

Fundamental Physical Processes:

 

Aerosols are necessary
ingredients for cloud formation for their roles as condensation nuclei (CN) in
forming cloud droplets and ice crystals. Dust aerosols are one of the most
common types of ice nuclei (IN), and emerging evidence suggests that mineral
particles can reach high into the upper troposphere to serve as IN for cirrus
and mixed-phase cloud formation. Dust may also interact with sea salt,
anthropogenic pollutants, and secondary organic aerosol, forming particles that
consist of a “core” of insoluble mineral dust with coatings of soluble
material. Dust particles with a soluble coating are typically very ef?cient
cloud condensation nuclei (CCN), often maintaining their activity as IN. In microphysical
process, the ability of an aerosol particle to take up water and subsequently
activate cloud condensation is determined by its size and composition. It has
been suggested that the size of CCN is more important than its chemical
composition. Larger particles are more readily activated than smaller particles
because they require a lower critical super saturation. While CCN generally
increases with CN for dusty events, the activation ratio tends to decrease
sharply with increasing CN, implying that dust particles do not increase CCN
concentration freely, despite mixing with other anthropogenic aerosols.
Moreover, due to their large size, dust particles can act as giant CCN that can
form ef?cient collector drops and initiate the onset of drizzle and precipitation.
When more aerosol particles are competing for the uptake of a ?xed amount of
liquid water content, the resulting cloud droplets should be more but smaller,
which have a larger total surface area and higher cloud albedo (cloud albedo
effect). Therefore, a dust-polluted cloud re?ects more solar radiation back to
space, resulting in a negative RF at the top of the atmosphere (TOA) (Huang et al. 2014).

 

Dust-Cloud-Precipitation
Interactions

 

Qian et al.2009 analyzed
the precipitation data collected across China over nearly half a century and found
a persistent pattern of decrease in the occurrence of drizzle and light rain
but of increase in the occurrence of heavy rain. Recent studies have suggested
that these trends of precipitations were probably the results of aerosol
effects, which could inhibit the rainfall in stratus clouds but invigorate the
convective cloud to produce heavy rain.

 

Decrease
of
light rain:

 

Aerosol indirect and
semidirect effects are expected to reduce cloud particle size and even burn
clouds, thus would suppress warm rain processes in stratocumulus clouds, small
cumulus, topographic clouds, and so on (Andreae et
al., 2004). These hypotheses have been tested by results from both
observations and model simulations. Dai et al. 2008 observed that as more
aerosols entered clouds, precipitation was suppressed in the higher altitude of
the clouds near the high altitude of Mounts Hua and Xi’an, China. Yin and Chen
2007 simulated the effects of mineral dust particles on the development of
cloud microphysics and precipitation over northern China through a
two-dimensional spectral-resolving cloud model and found that the heating
caused by increasing dust aerosol loadings would suppress precipitation when
dust particles acted as CCN and IN simultaneously during the development of a
cloud. This is because the enhancement of CCN is nearly overwhelmed by the
strong suppressing effect of IN.

Higher aerosol
concentrations result in more cloud droplet embryos competing for available
water vapor. Changes in the number and size distribution of cloud droplets
create thermodynamic and microphysical feedbacks that have the potential to
change cloud evolution and properties. A signi?cant feature of
dust-cloud-precipitation interactions over the arid and semiarid areas is that
it creates a positive feedback loop. The feedback loop begins with a decrease
in rainfall and results in de?cit in soil moisture. This leads to an increase
in the occurrence of dust storms. Consequently, dust aerosols in the atmosphere
warm clouds, increase the evaporation of cloud droplets and further reduce the
cloud water path (the semidirect effect). This decreases the low cloud cover
and water vapor amount, leading to less rainfall. The occurrence of dust storms
would then increase, which could lead to even less rainfall. Figure 3 shows a
schematic diagram of this feedback loop.

Figure 3. A schematic depiction of the feedback
mechanism

between precipitation and dust storm (from Huang et al., 2014).

 

Increase of heavy rain:

 

When it comes to deep
clouds or storms, the stories about aerosol-cloud-precipitation interactions
are completely different. Aerosol invigoration effect would enhance
precipitation. Microphysical processes associated with dust-cloud interactions
also affect cloud hydrometeor pro?le and cause phase change, which, in turn,
alter cloud dynamics and thermodynamics through latent heat release. Vertical
precipitation pro?le can re?ect the combined effects of dynamic, thermodynamic,
and microphysical processes in cloud system. Koren et al. 2012 examined the
relationship between aerosol abundance and rain rate. The authors found that,
for a range of conditions, increase in aerosol abundance was associated with
local intensi?cation of rain rate as detected by the microwave radiometer on
board the Tropical Rainfall Measuring Mission satellite. This relationship was
apparent over both ocean and land and in the tropics, subtropics, and mid latitudes.

 

Dust effects on wind
speed and temperature

 

Wind speed:

 

The increased stability
of the atmosphere during daytime related to changes in the vertical profiles of
temperature reduces the vertical exchange of horizontal momentum, leading to a
decrease in near-surface wind speed, but its increase in layers aloft. The
regionally averaged (15–30°S, 102–150°E) vertical profile of aerosol extinction
coefficient and the direct shortwave radiative effect by mineral dust on the
vertical profile of wind speed at local noon for a severe dust event over
Australia on 22–23 September 2009 were simulated by the Weather Research and
Forecasting with Chemistry (WRF/Chem) regional model, incorporating the dust
transport (DUSTRAN) module (Shaw et al., 2008)
and the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC; Zaveri et al., 2008). The aerosol
extinction coefficient was simulated to decrease with height as gravity tends
to settle dust particles. Wind speed was predicted to be reduced by mineral
dust from the surface to near the boundary layer top, and increased within the
upper boundary layer and lower free atmosphere.

In contrast to an
overall reduction of near-surface wind speed during the day, the surface
warming effect of dust aerosols during night potentially weakens the surface inversion,
resulting in an increase of near-surface wind speed. As dust particles are
picked up when the surface wind exceeds particular wind thresholds, a feedback
effect on dust emission can be established.

 

Surface
temperature:

 

Vertical distribution
of aerosols has a great impact on heating rate pro?le, which provides more
detail about dust aerosol DRE on different atmospheric levels than RF. Over the
Taklimakan Desert, dust aerosols can heat the atmosphere by up to 1, 2, and 3Kd-1
(daily mean values) under light, moderate, and heavy dust conditions (Huang et al., 2009). When a dust layer touches the
ground and lasts for several days, an increase in surface temperature can
occur, even during daytime. In the event of an elevated dust layer, there is a
decrease in surface temperature. These temperature changes caused by dust DRE
may result in horizontal temperature gradient, which can modify near-surface
winds. Since surface wind threshold determines the uptake of dust from the
surface, a feedback on total emission ?ux could be established.

 

 

Conclusions

 

 

1.      Over the arid and semiarid regions, dust aerosols can
a cooling effect at the surface, and a warming effect within the atmosphere.
The largest forcing values are located over or near dust sources.

2.      Dust aerosols could modify cloud macrophysical and
microphysical properties in many subtle ways. The ?rst and second indirect
effects are generally negative forcing on climate, while the invigoration
effect and semidirect effect are positive. The quanti?cation of those opposite
effects is still highly uncertain.

3.      Dust aerosols can suppress or enhance precipitation,
depending on many factors, such as the vertical distribution of dust aerosols,
humidity in the atmosphere, and cloud type. The amount of rain increases with aerosol
concentration in deep clouds that have high liquid water content but decreases
in clouds with low liquid water content.

4.      Also dust can effect boundary layer weather such as
wind speed and temperature.

 

 

 

 

Reference

 

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