PMEL Programs and Plans
Accomplishments in FY 99 and Plans for FY 00
Atmospheric Chemistry Publications
Figures (a) Climatic effects of tropospheric aerosol, and (b) locations of PMEL aerosol field studies.
Atmospheric Chemistry Program
Accomplishments in FY 99
The Atmospheric Chemistry Program at PMEL is a measurement-based program designed to
improve the accuracy of estimates of climate forcing by tropospheric aerosol particles.
Specific goals of the program are to
i) determine the physical, chemical, and meteorological processes that
control the shape and magnitude of the aerosol number size distribution, aerosol
chemical composition as a function of particle size, and aerosol light scattering and
absorption,
ii) determine the spatial and temporal variability of these parameters, and
iii) compile a data base of aerosol parameters essential to the estimation of
aerosol radiative forcing that encompasses a wide range of geographical regions.
This information is needed to detect regional and global climate change, to attribute
that change to anthropogenic aerosols, and to improve the prediction of future climate
changes for various radiative forcing scenarios.
PMEL plays a lead role in the planning and execution of the
Aerosol Characterization Experiments (ACE) of the International Global Atmospheric Chemistry Project (IGAC).
ACE 1 took place in the remote
marine atmosphere south of Australia in order to characterize aerosol properties
in a minimally polluted environment. The ACE 1 Special Sections of the Journal of Geophysical Research were
published in FY 98 and FY 99 with papers describing the chemical, physical, radiative,
and cloud nucleating properties of aerosols over the remote ocean and the controlling
processes. These data currently are being used to develop aerosol process models.
ACE 2 focused on the radiative
effects and processes controlling anthropogenic aerosols from Europe and desert
dust from the Africa as they were transported over the North Atlantic Ocean.
The experiment, which took place in June/July 1997, involved over 250 research
scientists from Europe and the United States. It included 60 coordinated aircraft
missions with six aircraft, one ship, five satellites, and ground stations on
Tenerife, Portugal and Madeira. NOAA-PMEL coordinated the shipboard measurements
aboard the Ukranian Research Vessel, Professor Vodyanitskiy. The initial results
from ACE 2 have been summarized in 48 research articles that have been submitted
to Tellus for a special issue that will appear
in early 2000. Highlights of the NOAA results include:
(1) The background submicron aerosol measured over the Atlantic Ocean during ACE 2 was more
abundant (number and volume) and appeared to be more aged than that measured over the Southern
Ocean during ACE 1. The submicron aerosol number size distributions in the air masses that
passed over Northern Europe, the Mediterranean, and coastal Portugal were distinctly different
from each other and the background aerosol. The differences can be attributed to the age of
the air mass and the degree of cloud processing. (2) Larger sulfate aerosol concentrations
were measured in the ACE 2 region than the ACE 1 region during periods of both continental and
marine flow. Concentrations during marine flow were about 4 times larger during ACE 2 than
during ACE 1. Continental concentrations during ACE 2 were an order of magnitude larger than
ACE 2 marine concentrations. The higher concentrations during marine flow most likely were a
result of a more continentally-impacted North Atlantic compared to the Southern Ocean and the
longer aerosol lifetimes in the ACE 2 region. Submicron and supermicron sea salt concentrations
were similar during ACE 1 and ACE 2. (3) During ACE 1 sea salt controlled the optical
properties of both the sub- and supermicron aerosol. Sea salt had a relatively smaller
influence on aerosol optical properties during ACE 2 because of the large concentrations
of submicron continental (mainly sulfate) aerosol. The smaller role of sea salt during ACE 2
was observed in several measured aerosol optical properties. The spectral dependence of light
scattering by particles indicated the strong influence of smaller fine mode rather than larger
coarse mode particles during ACE 2. The single scattering albedo indicated the presence of a
more absorbing aerosol than sea salt during ACE 2. (4) The amount of carbon-containing aerosol
and the identity of the carbon species are large unknowns that contribute to the uncertainty
in estimates of aerosol radiative forcing. A previous IGAC experiment in the Western Atlantic
(TARFOX) found sulfate to total carbon ratios of 1.6 +/- 0.7 at altitudes below 300 m. Shipboard
measurements during ACE 2 revealed a ratio of 2.9 +/- 1.3. The average sulfate concentrations
from the two regions were comparable but the total carbon concentration during TARFOX was
larger. This type of data helps us start to understand differences in the aerosol chemical
composition for different ocean regions.
Data from PMEL atmospheric chemistry cruises in the Pacific and Southern Oceans were compiled
and summarized to show that, for the entire central Pacific from 55°N to 70°S, sea salt dominates
the aerosol mass concentration in the marine boundary layer with a significant fraction occurring
in the submicron size range. Because of the high scattering efficiency of submicron sea salt
and its relatively long lifetime, sea salt is major contributor to scattering by the aerosol
in marine regions. It was estimated that in the tropics, outside of the ITCZ, sea salt can
account for 80 to 90% of the aerosol optical depth. These results
were reported by Quinn
and Coffman (1999).
The PMEL atmospheric chemistry group spent 106 days at sea during FY 99 aboard
the Ronald H. Brown participating in the AEROSOLS, INDOEX and
NAURU projects. Our participation in these projects
enabled us to collect an extensive data set of aerosol properties in different air masses
included background marine, desert dust, biomass burning, and North American, African and
Asian urban plumes.
Atmospheric Chemistry Program
Plans for FY 00
- Finish Aerosol99/INDOEX data analysis and prepare manuscripts for publication in the JGR Special Sections.
- Continue long-term monitoring of aerosol chemical composition at the NOAA Aerosol Regional Monitoring Network of stations at Barrow, AK, Bondville, IL, and Southern Great Plains, OK.
- Continue the organization of ACE Asia planned for March/April 2001
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