attenuation of solar uv radiation by aerosols during air poixution episodes
S. Kondragunta", Climate and Research Applications Division, Office of Research and Applications,
NESDIS/NOAA
P, M, Udclhofan, Instrtude for Terrestrial and Planetary Atmospheres, Slate University of Hew York, Stony Brook,
NY
K. L, Scheie and S- J, Roadie, Atmospheric Sciences Modeling Division, NOAA/ARL, Research Triangle Park,
NC
B. Holben, Laboratory of Terrestrial Physics, NASA/GSFC, GreenbeLt, MD
1.	INTRODUCTION
Increase in the amount of solar UV radiation
reaching the surface due to decrease in stratospheric
ozone continues to be a major concern (WMO,
1998). However, recent studies show that absorption
and scattering by aerosol# during air pollution
episode decreases the amount of radiation reaching
the surface (Dickerson et al,, 1997; Jacobson, 1998;
Papayannis et al, 1998; Repapis ct al, 1998;
Kondragunta et al , 1999). To examine the role
played by column ozena and aerosols in perturbing
the solar radiation reaching the surface, we analyzed
four yean of spectrally resolved UV radiation
measured by Brewer spectrophotometer at
Gaithersubrg, MD (39.1° N and 77,2° W). Transport
from upwind regions and local pollution result in
severe air pollution episodes at Gaithersburg when
meteorological conditions are favorable. We present
observations of aerosol optical depth and column
ozone (from ground based sun photometers and
satellites) and ground measurements of spectrally
resolved UV flux. We will compare the observed
and computed effects of aerosols an surface UV flux
and discuss the implications.
2.	OBSERVATIONS AND RADIATIVE
TRANSFER CALCULATIONS
Continuous measurements of aerosol optical
depth are made at Greenbclt, MD, 25 miles east of
Gaithersburg, using an automated sun
photometer/sky radiometer at six different
wavelengths (Holben et al., 1998). Aerosol optical
Corresponding author's address; S, Kondragunla,
ORA, NOAA/NESDIS, Room 810,5200 Auth Road,
Camp Springs, MD 20746,
Email: skondragunta@nesdis.no ca. gov
depths at Gaithersburg (where U V flux
measurements are made) were deduced by using a
linear correlation between aerosol optic al depths
measured al Gieenbelt and Gaithersburg for three
months in 1996 (Figure 1). Observations of aerosol
optical depth for non-absorbing aerosols from TOMS
instrument oa Earth Probe satellite were provided by
NASA/GSFC (Tones et al., 1998), Continuous
measurements of spectrally resolved UV flux in the
wavelength range 305 to 365 nm, column ozone,
column sulfur dioxide, and column nitrogen, dioxide
contents are made at GaitherSubrg by the EPA
network of Brewer spectrophotometers. For this
study, we used only UV flux and ozone
measurements which have been corrected for
instrument drifts but not for changes in cosine
response of the instruments. The overall uncertainty
in UV flux measurements is about 10% For ozone
measurements, comparisons with TOMS
measurements indicate a bias of 4-8 D.U (1 5%) over
a period of 4 years.
Radiative transfer model calculations were
performed using the Discrete Ordinate Radiative
Transfer Model (DISORT) developed by Stamnes et
al., 1988. Inputs to this model include surface
albedo, molecular and aerosol optical depth, aerosol
single scattering albedo, asymmetry factor, phase
function, and extra-terrestrial solar inadiance.
Optical properties of aerosols were obtained by
providing retrieved aerosol size distributions from
sky radiometer and a refractive index of 1,45-0.005i
for scattering aerosols as inputs to Mie code
(Wiscombe, 1980; d'Almeida el al,, 1991)
3. RESULTS AND DISCUSSIONS

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Aerosols and column ozone have opposite
effects OB the UV flux reaching the surface. For
example, radiative transfer model calculations show
that a decrease in 40 D.U, of column ozone increases
the erythema! (Diffey weighted) UV flux (DUV) by
14% at 50° solar zenith angle; the effects are larger at
larger solar zenith angles. Similarly, for a fixed
column ozone, increase in aerosol optical depth of
1,0 in the UV decreases the DUV by 21%. To
isolate the effect of aerosols on measured UV flax
from the combined effect of aerosols and ozone, we
analyzed the UV flux at 340 mn where OZiQQC
absorption cross section is negligible. Radiative
transfer calculations and observations show that
aerosols reduce the surface flux at a rate of 80
mWtoVnm per unit aerosol optical depth in the UV
(Figure 2).
Observations show that summer time
aerosol optical depth in the UV (340 mn) in the
eastern US can rmge between 0. land 2.0, with a
mean value of 0.74 for all smoggy days during 1994
to 1998. To leam about potential implications of
aerosol effects on UV flux during pollution events
we now focus an one specific episode in 1997.
Conditions conducive for a pollution event resulted
in a severe multi-day episode during July 08-18
1997; high concentrations of ozone and aerosols were
observed at the surface across the entire eastern US.
Table 1 shows daily average aerosol optical depth at
3 80 mn as measured by TOMS satellite and ground
based sun photometer. The sun photometer
measurements are from Oreenbeh, MD (39.01° N and
76,87® W). The satellite measurements arc averages
around a 1° are from the Greenbelt site.
Julian Day
AOD @ 380
nm (TOMS)
AOD @380
am (Sun
photometer)
193
0.315
0.341
194
0.719
0.750
195
0.921
1.447
1%
NA
1.203
m
NA
0.996
198
NA
0.696
199
NA
0.469
Table 1; Comparison of aerosol optical depth
measurements at 380 nm measured by satellite
(TOMS instrument) and ground instrument (sun
photometer) during the air pollution episode in 1997.
Based on the data shown in Table 1, we
found the JD193 and JD195 ideal for analyzing the
effects of aerosols on UV fluxes because low and
high aerosol optical depths wen; observed on those
two days respectively. Both days were clew (cloud-
free) and had similar column ozone measurements
(318 and 320 DU respectively),
Figure 3 shows observed and DISORT model
calculated UV flax (wans/m*/nm) as a function of
wavelength for JD193 and JD 195 at 50° solar zenith
angle. Observations and model calculations agree
well; observations have a fine structure compared to
modal calculations because of higher spectral
resolution (0.3 run). Both calculations and
observations show a decrease m UV flux up to 19%,
The effect of aerosols on integrated UV flux (295 to
3 65 nm) as a function of solar zenith angles for the
same two days (JD193 and JD195) are shown in
Figure 4. Model calculations are slightly higher than
observations; possibly due to exclusion of SO,
absorption in the model calculations. Aerosol
scattering decreased integrated UV flux by up to 14
to 17% depending on zenith angle.
4.	CONCLUSIONS
Observations and radiative transfer model
calculations show that aerosols attenuate solar UV
radiation at 340 nm reaching the surface at a rate of
80 mWMVjun per unit aerosol optical depth; this
effect is about 17 to 19% depending on wavelength.
Depending on solar zenith angle, aerosols attenuate
the integrated UV flux (295 to 365 am) by up to
17%.
Reduced UV flux near the surface due to
aerosol scattering may decrease the amount of
photochemical processing of pollutants (smog
production) at the surface but increase it aloft. Life
time of photodegradable carcinogens in air-borne and
aquatic particulate matter may mcrease. In future
studies we will focus on quantifying these effects in
relation to health*effects.
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Acknowledgment: We like to thank Omar Tones at
NASA/GSFC for providing satellite retrieved aerosol
optical depth. This work was funded by the
Environmental Protection Agency (EPA)/National
Exposure Research Laboratory (NERL) and
administered by the University Corporation for
Atmospheric Research (LICAR).
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