Becky Alexander, Department of Atmospheric Sciences
Our interests lie in the interactions of atmospheric chemistry and climate change. We
make use of the oxygen isotopic composition of sulfate and nitrate aerosols from aerosol,
water, snow and ice core samples to study photochemical oxidation pathways on a variety
of time scales. Our research tools consist of isotope measurements and global 3D
chemical transport modeling. For more information on what is going on in our lab, check
out our web site: http://www.atmos.washington.edu/~beckya/. We are also members of
the ISOLAB: http://depts.washington.edu/isolab/index.html.
1) Ice cores provide valuable information on past atmospheric composition, chemistry,
and climate. Atmospheric chemistry is controlled by its "oxidation capacity", defined by
OH radical concentrations. OH concentrations determine the lifetime of many
atmospheric species, including climatically important species such as methane. The
oxygen isotopes of sulfate and nitrate can provide the first conservative proxy for past
OH concentrations, enabling us to study the link between atmospheric chemistry and
climate. Our longest record of the O-isotope composition of sulfate comes from the
Vostok ice core in east Antarctica, extending through the past 125,000 years. The figure
below shows the variability in the O-isotopic composition (17O) of sulfate, along with
Dice measurements (indicative of temperature), through the last glacial-interglacial cycle
from the Vostok (Antarctica) ice core [Alexander et al. 2002]. Sulfate formation through
oxidation of reduced sulfur species by OH is 30-50% more important during the cold
glacial period, than the surrounding interglacial periods, suggesting that glacial OH
concentrations are higher during cold climates.
In collaboration with Eric Steig and Shelley Kunasek, we are currently measuring
samples from the WAIS Divide project (http://waisdivide.unh.edu/). The WAIS (West
Antarctic Ice Sheet) ice core will provide the first complete record of multiple isotope
ratios of nitrate and sulfate covering the last ~100,000 years. The WAIS ice core will be
the highest resolution long ice core obtained from Antarctica. At the UW, we have
developed methods to measure the multiple-isotope composition of small samples of
sulfate and nitrate making continuous high-resolution measurements on ice cores feasible
for the first time. This project will coincide with the International Polar Year (20072008), and contributes to goals of the IPY, which include the fostering of
interdisciplinary research toward enhanced understanding of atmospheric chemistry and
climate in the polar-regions.
2) We are performing sensitivity studies using a coupled global chemistry-climate model,
to determine the sensitivity of the sulfate 17O proxy to oxidant concentrations and
climate change over a full glacial-interglacial cycle, to calibrate the 17O proxy on these
time scales. We use the GEOS-Chem (http://wwwas.harvard.edu/chemistry/trop/geos/index.html ) model [Park et al., 2004], a global threedimensional model of atmospheric composition and chemistry. The figure below shows
monthly mean sulfate 17O (= 17O - 0.5x18O) values. We are coupling the GEOSChem model to the NASA GISS global climate model (GCM) [Schmidt et al., 2006],
using the calculated meteorological fields from the GISS ModelE
(http://www.giss.nasa.gov/tools/modelE/) to drive the GEOS-Chem model in the past.
This coupled model will be used calibrate the 17O proxy, and will eventually be used to
quantitatively interpret our ice core sulfate and nitrate 17O measurements.
3) Through collaboration with Jordi Dachs (IIQAB-CSIC, Barcelona) and Antonio Tovar
(IMEDEA-CSIC, Mallorca), we are involved with the Spanish campaign RODA (English
translation of the acronym RODA = Oceanic Eddies and Atmospheric Deposition in the
Canary Current). Size segregated aerosol samples were collected during two cruises in
the subtropical North Atlantic (west of the Sahara desert) that took place during August
2006 and February 2007. In addition, aerosol samples were collected over the course of
one year in the Canary Islands. We will be measuring the oxygen isotopic composition of
sulfate and nitrate aerosols collected during the RODA campaign. Sulfate and nitrate that
form on alkaline aerosols such as dust have a unique oxygen isotopic composition, which
will allow us to quantify the acidification pathways of dust aerosols. Acidification of
dust has implications for atmospheric chemistry, climate, and marine biogeochemistry.
Complementary measurements by other groups participating in RODA include Fe
speciation to determine the amount of soluble iron in the aerosols (important as a nutrient
for marine phytoplankton), and phytoplankton speciation and abundance, among other
studies. These measurements will allow us to study the impact of atmospheric
acidification of dust aerosols on nutrient deposition and marine primary productivity. We
will be using the GEOS-Chem model to help interpret our results. The below (courtesy
T.D. Fairlie) shows PM2.5 dust concentrations in the model, and highlights the large
Saharan dust source.