Nicholas Meskhidze
Description of Current Research
My current research is focused on improved understanding of the effects
of ocean biological productivity on marine clouds and assessment of the
radiative effect of this interaction. Production of sulfate from the oxidation
of dimethyl sulfide (DMS) (i.e., the "CLAW" hypothesis) and primary
emissions of biogenic organic matter from wave breaking have been suggested
as possible mechanisms by which ocean biota can modulate properties of marine
clouds. Our research suggests an alternative
pathway; production of secondary organic aerosol from the oxidation
of phytoplankton produced isoprene can also lead to changes in CCN chemical
composition and number concentration. Using MODIS remotely sensed data for
cloud droplet effective radius, cloud optical depth, sea surface temperature,
Chlorophyll a, and other parameters near large phytoplankton bloom region
we show that compared to the background, cloud droplet number concentration
over the bloom was doubled and cloud effective radius was reduced by 30%.
The resulting change in the short-wave radiative flux at the top-of-the-atmosphere
is -15 W m-2, comparable to the aerosol indirect effect over highly
polluted regions. This is by far the strongest impact of phytoplankton on
clouds observed to date, and is attributed to changes in the size-distribution
and chemical-composition of cloud condensation nuclei (CCN).
I am also interested in a full exploration of the significance of Fe mobilization
processes on a global carbon cycle and climate. Currently I am extending
the basic Fe mobilization mechanism of Meskhidze et al. (2005) to explicitly
include water soluble organic acids and photochemistry. Photoinduced reductive
dissolution is particularly important in relatively less acidic environment
(pH <~4), where current dust-Fe dissolution model may lead to underestimation
of amount of dissolved Fe. This improvement will also allow cloud cycling
of mineral dust, potentially important mechanism for the production of bioavailable
Fe, to be incorporated in the model. When finished, the Fe dissolution module
will be included in 3D chemical transport models (CTM) to conduct the long-term
simulations with different met fields and emission scenarios to better understand
the processes controlling Fe solubilization in ambient aerosols and help
explaining the possible environmental consequences of Fe fertilization of
the oceans. In conjunction with the modeling studies, I am also using remotely
sensed data collected by the MODIS and MISR satellites for exploring the
possible links between aeolian dust, atmospheric trace gases, ocean productivity
and climate.
Southern Ocean Productivity: How can current satellite
data help us to better quantify the climate change in geological past?
Using the analysis of MODIS observed sea surface temperature (SST) and
ocean chlorophyll a content [Chl a] in conjunction with model-derived atmospheric
dust-Fe fluxes we show that the predominate source of Fe for phytoplankton
blooms in the Atlantic sector of the Southern Ocean (ASO) between 40°-60°S
is from the oceanic input not from the atmospheric deposition of mineral
dust. To explain the strong correlation between aeolian dust deposition
and [Chl a] described in literature, we suggest an existence of atmospheric
circulation pattern over Patagonia and ASO region that simultaneously causes
the uplift and transport of mineral dust and an upward supply of nutrient-rich
waters from the deep ocean to the surface waters of Antarctic Circumpolar
Current. Such a mechanism, if it exists, would enhance ocean productivity
in the ASO through upwelling but would still exhibit a high correlation
between [Chl a] and atmospheric dust-Fe flux even if the dust did not contain
significant amounts of bioavailable-Fe and thus played no actual role in
fostering enhanced biological activity in the region. To the extent that
biological activity in this region plays an important role in global C-cycle,
results our study in conjunction with previous analysis of mineral dust
transport (Meskhidze et al., 2005) suggest that mere changes in the dust-Fe
supply (perhaps without significant increase in sulfur sources) may not
exert considerable influence on the drawdown of atmospheric CO2 in the ASO.
Pollution may alter ocean photosynthesis
Sulfur dioxide (SO2), a gas emitted by industrial processes and implicated in acid rain, may promote the formation of nutrients from mineral dust. Based on the analysis of aircraft data collected over heavily polluted areas of China we propose that SO2, when combined with airborne dust plumes, can convert iron in mineral dust into a form that can be assimilated as a nutrient by phytoplankton (Meskhidze et al., 2003).(Double click on figure to see a full-scale image.)
The limited availability of iron restricts primary production in some regions
of the oceans where airborne dust is considered to be the main source. While
only soluble Fe is bioavailable virtually all the Fe found in sands from
arid and semi-arid regions is in a crystalline Fe(III) form. This form of
Fe is insoluble in high pH solutions such as seawater; thus for phytoplankton
to utilize the Fe deposited in mineral dust, some fraction of the Fe must
be dissolved during transport in the atmosphere. We suggest that industrial
SO2 emissions can acidify the airborne dust commonly blown from arid and
semi-arid regions worldwide and promote the solubilization of the Fe contained
in the dust. The dissolved Fe then acts as micronutrient to oceanic organisms
and enhances primary productivity in the oceans. My research showed that
the recipe of adding pollution to mineral dust from East Asia may actually
enhance ocean productivity and, in so doing, draw down atmospheric carbon
dioxide. Thus, China's current plans to reduce sulfur dioxide emissions,
which will have far-reaching benefits for the environment and health of
the people of China, may have the unintended consequence of exacerbating
global warming.