Initial Multi-Year Study of Aerosol Loading, Optical Properties, and Direct Radiative Forcing from Four U.S. Regions
J.P. Sherman1, E. Andrews2,3, and A. Jefferson2,3
1 Department of Physics and Astronomy, Appalachian State University, Boone, NC 28608, 828-262-2438, Email: [email protected]
2 National Oceanic and Atmospheric Administration, Earth Systems Research Laboratory, Boulder, CO 80305, USA
3. University of Colorado, CIRES, Boulder, CO 80305, USA
Methods
Abstract
Satellite-based estimates of aerosol direct radiative forcing (DRF)
depend upon ground-based observational networks such as NASA
AERONET and NOAA-ESRL, which provide long-term,
continuous measurements of column-averaged and near-surface
aerosol loading and intensive aerosol optical properties (AOPs)
possessing well-characterized uncertainties. There are currently
four co-located AERONET / ESRL sites in the U.S. -Bondville, IL
(BND), Southern Great Plains, OK (SGP), Appalachian State
University in Boone, NC (APP), and Trinidad Head, CA (THD).
Each site is situated away from local pollution sources and the four
regions are home to distinctly different aerosol types and aerosol
loading. Seasonal and regional variability of AOPs and aerosol
optical thickness (AOT) measured at the four sites are presented, in
addition to their influences on top-of-atmosphere aerosol direct
radiative forcing (TOA DRF). Though the individual AOPs vary
significantly with region and season, the direct radiative forcing
efficiency (DRFE) is to first order relatively uniform with region
and season. The high seasonal variability in AOT leads to
significantly more negative forcing (cooling effect) at all sites
during summer months, especially at APP (representative of the
southeastern U.S.). This is consistent with the highly seasonal
production of secondary organic aerosols observed in the SE U.S.
region. Aerosol loading is highest at BND (representative of
industrial and agricultural Midwest) during non-summer months.
AOPs at the coastal THD site exhibit the least seasonal variability
and AOT is the lowest at THD during all seasons
Introduction
• Background and Significance: Multi-year aircraft-based in situ
measurements of the vertical distributions of aerosol loading
and optical properties conducted above three continental U.S.
regions (Sheridan, 2012, Andrews, 2004, Taubman, 2006)
revealed that median values of intensive aerosol optical
properties measured at low RH≤40% show little variability in
the lowest ~2km of atmosphere This suggests that regionallyrepresentative near-surface aerosol intensive properties
(measured continuously at ESRL sites) combined with either colocated AERONET AOT or satellite AOT measurements can be
used to improve aerosol DRF estimates on regional scales
Table 1: Sites and Datasets Comprising this Study
Site/region
APP (SE U.S.)
Latitude
36.210 N
Longitude
Elevation (m) ASL
81.690
Major Aerosol
Sources
Secondary organic Industrial,
aerosols (especially agricultural
during summer)
Start/End dates used
(NOAA data)
Start/End dates used
(AERONET data)
6/1/09-10/31/12
7/28/10-11/30/11
RESEARCH POSTER PRESENTATION DESIGN © 2011
www.PosterPresentations.com
W
1080
BND (Industrial
Midwest)
40.050 N
SGP (Great Plains)
THD (Pacific coastal)
36.600 N
41.050 N
88.370
97.480
124.150
W
230
2.
W
W
315
105
Sea salts, some longrange dust and
pollutant transport
1/1/05-5/31/12
Agricultural,(dust,
burning) with some
point sources (oil
refineries, etc)
1/1/05-5/31/12
4/1/05-5/31/12
1/1/05-10/22/11
1/1/05/7/11/11
2/17/05-8/31/11
• Measurement and data processing techniques and data archiving
follow standard AERONET (Holben 1998) and NOAA-ESRL
protocols (Delene and Ogren, 2002)
• Hourly-averaged values of lower tropospheric aerosol light
extinction coefficient and intensive optical properties of
aerosols with aerodynamic diameters Dp<10µm are presented.
All shown values are at λ=550nm and for RH≤40% (i.e.
neglecting hygroscopic growth)
• AERONET level 2.0 AOT at 500nm is presented. No attempt
was made in this initial study to correct AOT values to 550nm
• Single-scattering albedo (SSA )and upscatter fraction ( related
to asymmetry parameter(g) in the Henyey-Greenstein phase
function approximation as described in Andrews, 2006) are used
to calculate TOA DRFE. Scattering angstrom exponent (A0sc) is
also shown as a measure of size distribution, with larger values
(smaller values) corresponding to smaller (larger) particles.
• To first order, clear-sky TOA DRF can be approximated as the
product of AOT and direct radiative forcing efficiency (DRFE).
DRFE is fairly insensitive to AOT and depends primarily on
intensive optical properties (SSA and phase function), along
with surface albedo and solar angle (Delene and Ogren, 2002)
• Presented DRFE values for all sites use globally-averaged
values of broadband surface reflectivity (0.15), atmospheric
transmission (0.76), cloud fraction (0.6), and daylight fraction
(0.50) in the 1st-order DRFE calculations (Delene and Ogren,
2002). Common values for all non-aerosol parameters facilitates
DRFE comparisons based solely on aerosol intensive optical
properties (SSA and g)
Results and Discussion (continued)
B. Seasonal Variability in Aerosol Optical Properties and Loading
• Despite large variations in aerosol types, aerosol DRFE is (to 1st
approximation) relatively uniform at the 4 U.S. sites (albeit
slightly more negative at THD) and AOT is the dominant source of
DRF variability
• Aerosol cooling effects (driven by the high AOT) at the top-ofatmosphere are largest in summer and smallest in winter, with the
seasonal difference being largest in the SE U.S. region
• Subsequent study will incorporate (a) TOA and surface DRF
calculations made using SBDART radiative transfer code; (b)
vertical dependence of aerosol light extinction (from lidars), (c)
humidity-dependence of light scattering; (d) DRF closure studies
utilizing (a), along with co-located solar pyranometers and
AERONET DRF retrievals.
A. Regional Variability in Aerosol Optical Properties and Loading
Table 2: Regional Variability in Annual Mean Aerosol Optical Properties: Mean ±
standard deviation for the entire data period are shown. The number in parentheses is the
number of hours (or measurements, in the case of AOT) used in computing statistics.
BND
SGP
THD
Extinction Coefficient (Mm-1)
38.70±30.56
(29036)
48.2±37.8
(64992)
31.9±26.1
27.8±22.7
(64992)
SSA
0.901±0.054
0.915±0.052
0.900±0.104
0.962±0.034
Asymmetry Parameter
0.549±0.065
0.575±0.063
0.581±0.102
0.619±0.037
Scattering Angstrom Exponent
2.04±0.282
2.02±0.309
1.61±0.551
0.834±0.537
TOA DRFE (W/m2 per unit AOT)
-24.7±3.47
-24.7±3.33
-25.1±5.90
-26.3±2.07
AOT
0.119±0.111
(8324)
0.163±0.146
(22544)
0.125±0.085
(31748)
0.096±0.089
(18079)
• BND possesses the largest annual-averaged lower atmospheric
aerosol loading (extinction coefficient) and column-averaged
loading (AOT)
• THD is home to lowest aerosol loading and to the largest (lowest
A0sc and highest g) and most reflective aerosols (highest SSA)
• Annual mean TOA DRFE values are similar for all regions,
consistent with findings of Delene and Ogren (2002)
• AOPs in all regions possess high temporal variability, indicating
that mean values do not accurately characterize regional aerosols
• DRFE variations are relatively small at all sites (relative to AOT
variations) and median DRFE values are of comparable
magnitude. The significantly higher summer AOT at the three
continental sites leads to a much larger negative TOA DRF (cooling
effect) in these regions
• APP possesses largest summertime lower tropospheric and columnaveraged aerosol loading and the largest seasonal fluctuations,
consistent with high warm season SOA production (Goldstein,
2008). BND is home to the largest loading during nearly all nonsummer months.
• THD is home to the largest aerosols (highest g and lowest A0sc)
possessing the highest albedo during all seasons.
• Aerosols measured at APP exhibit the largest seasonal variability in
SSA and size-related parameters and tend to be larger and more
reflective than the other continental sites during summer and
smaller and darker than all other sites during non-summer months.
This and the fact that most measured aerosol light scattering at APP
(~85-90% for most months) is due to sub-1µm aerosols (not shown)
is consistent with aged SOA during summer months. This has been
verified using episodic aerosol mass spectrometer data.
Conclusions and Future Work
Results and Discussion
APP
Results and Discussion (continued)
References
Andrews, E., P.J. Sheridan, J.A. Ogren, and R. Ferrare. In situ aerosol profiles over the Southern
Great Plains cloud and radiation test bed site: 1. Aerosol optical properties, J. Geophys. Res, 109,
D06208, doi:10.1029/2003JD004025(2004)
Andrews, E., Comparison of methods for deriving aerosol asymmetry parameter. J. Geophys. Res.
111, D05S04, doi: 10/29/2004JD005734 (2006).
Delene, D.J., and J.A. Ogren. Variability of aerosol optical properties at four North American
surface monitoring sites. J. Atmos. Sci. 59, 1135-1150 (2002)
Holben, B.N., et al. AERONET—a federated instrument network and data archive for aerosol
characterization. Remote Sensing of the Environment 66, 1‐16 (1998).
Sheridan P.J., E. Andrews, J. A. Ogren, J. L. Tackett, and D. M. Winker. Vertical profiles of aerosol
optical properties over Central Illinois and comparison with surface and satellite measurements.
Atmos. Chem. Phys. Discuss., 12, 17187–17244 (2012)
Taubman, B.F., J.C. Hains , A.M. Thompson, L.T. Marufu, B.G. Doddridge, J.W. Stehr, C.A. Piety,
and R.R. Dickerson. Aircraft Vertical Profiles of Trace Gas and Aerosol Pollution over the MidAtlantic U.S.: Statistics and Meteorological Cluster Analysis, J. Geophys. Res.,
10.1029/2005JD006196 (2006)
Acknowledgements
Figure(s) 1: Seasonal Variability in Aerosol Loading and Intensive Optical
Properties: The box limits are 25th and 75th percentiles. The black circle is the median
value. Whiskers represent the 5th and 95th percentiles. The color code for all figures is
as follows: APP=black, BND=blue, SGP=green, and THD=red.
The following have made significant contributions leading to the generated dataset: (1)
Patrick Sheridan and John Ogren (NOAA-ESRL); (2) AERONET site PIs Brent Holben
(BND and THD) and Rick Waggoner (PI); (3) APP NOAA-ESRL station
instrumentation specialist Yong Zhou; (4) Brett Taubman and several very dedicated
APP students for their role in daily maintenance (4) APP machinist Dana Greene and
electronics technician Mike Hughes