Table Of ContentABSTRACT
YU, SHAOCAI. A Study of Direct and Cloud-Mediated Radiative Forcing of
Climate Due to Aerosols.
(Under the direction of Professor V.K. Saxena)
The Intergovemmental Panel on Climate Change (IPCC) has reported that in the
southeastern US and eastern China, the general greenhouse warming due to
anthropogenic gaseous emissions is dominated by the cooling effect of anthropogenic
aerosols. To verify this model prediction in eastern China and southeastern US, we
analyzed regional patterns of climate changes at 72 stations in eastern China during 1951-
94 (44 years), and at 52 stations in the southeastern US during 1949-94 (46 years) to
detect the fingerprint of aerosol radiative forcing. It was found that the mean rates of
change of annual mean daily, maximum, minimum temperatures and diurnal temperature
range (DTR) in eastern China were 0.8, -0.2, 1.8, and -2.0 C/100 years respectively, while
the mean rates of change of annual mean daily, maximum, minimum temperatures and
DTR in the southeastern US were -0.2, -0.6, 0.2, and -0.8 C/100 years, respectively. This
indicates that the high rate of increase in annual mean minimum temperature in eastern
China results in a slightly warming trend of daily temperature, while the high rate of
decrease in annual mean maximum temperature and low rate of increase in annual mean
minimum temperature lead to the cooling trend of daily temperature in the southeastern
US. We found that the warming from the longwave forcing due to both greenhouse gases
and aerosols was completely counteracted by the shortwave aerosol forcing in the
southeastern US in the past 46 years. A slightly overall warming trend in eastern China is
evident; winters have become milder. This finding is explained by hypothesizing that
increasing energy usage during the past 44 years has resulted in more coal and biomass
burning, thus increasing the emission of absorbing soot and organic aerosols in eastern
China. Such emissions, in addition to well-known Asia dust and greenhouse gases, may
be responsible for the winter warming trend in eastern China that we have reported here.
The sensitivity of aerosol radiative properties to aerosol composition, size
distribution, relative humidity (RH) is examined for the following aerosol systems:
inorganic and organic ions (C1-, Br-, NO3-, SO4 2-, Na +, NH4 +, K+, Ca2+, Mg 2+, HCOO-,
CH3COO-C,H3CH2COOC-,H3COCOOO-,OCCOO 2-, MSA-1); water-insoluble inorganic
and organic compounds (elemental carbon, n-alkanes, SiO2, A1203, Fe203 and other
organic compounds). The partial molar refraction method was used to calculate the real
part of the refractive index. It was found that the asymmetry factor increased by -48%
with the real part varying from 1.40 to 1.65, and the single scattering albedo decreased by
24% with the imaginary part varying from -0.005 to -0.1. The asymmetry factor
increased by 5.4 times with the geometric standard deviation varying from 1.2 to 3.0.
The radiation transmission is very sensitive to the change in size distribution; other
factors are not as significant.
To determine the aerosol direct radiative forcing (ADRF), the aerosol optical depth
(AOD) values at the three operational wavelengths (415, 500 and 673 nm) were
determined at a regionally representative site, namely, Mt. Gibbs (35.78 o N, 82.29 oW,
elevation 2006 m) in Mt. Mitchell State Park, NC, and a site located in an adjacent valley
(Black Mountain, 35.66 oN, 82.38 oW, elevation 951 m) in the southeastern US. The two
sites are separated horizontally by 10 km and vertically by 1 km. It was found that the
representative total AOD values at 500 nm at the valley site for highly polluted (HP),
marine (M) and continental (C) air masses were 0.68+0.33, 0.29+0.19 and 0.10+ 0.04,
respectively. A search-graph method was used to retrieve the columnar size distribution
(number concentration N, effective radius ro_-and geometric standard deviation Og) from
the optical depth observations at three operational wavelengths. The ground albedo,
single scattering albedo and imaginary part of the refractive index were calculated using a
mathematically unique procedure involving a Mie code and a radiative transfer code in
conjunction with the retrieved aerosol size distribution, AOD, and diffuse-direct
irradiance ratio. It was found that N, roft-and Og were in the ranges of 10 to 1.7x104 cm -3,
0.09 to 0.68 gm and 1.12 to 2.95, respectively. The asymmetry factor and single
scattering albedo were in the ranges of 0.63 to 0.75 and 0.74 to 0.97 respectively. The
ground albedo for the forested terrain and imaginary part of refractive index were found
to be in the ranges of 0.06 to 0.29 and 0.005 to 0.051 respectively. On the basis of these
aerosol radiative properties obtained at the research sites and computations using the
Column Radiation Model (CRM) of National Center of Atmospheric Research (NCAR)
CommunityClimateModel(CCM3),it wasfoundthattheaveragecloud-free24-hour
ADRFvalueswere-13_+8-,8_+3-,33_+1W6 m-2formarine,continentala, ndpollutedair
massesr,espectively.Ontheassumptionthatthefractionalcoverageofcloudsis 0.61,it
wasestimatedthattheannualmeanADRFwas7_+W2 m-2inthesoutheasterUnS.
Thereviewwithrespecttothecurrentknowledgeoforganicacidsshowsthataerosol
formateandacetateconcentrationrsangefrom0.02to5.3nmol/m3andfrom0.03to12.4
nmol/m3respectivelya,ndthatbetween34%to77%offormateandbetween21%to66%
of acetatearepresentinthefinefractionof aerosols. It wasfoundthatalthoughmost
(98-99%)of thesevolatile organic acidswere presentin the gas phase,their
concentrationisntheaerosolparticlesweresufficienttomakethemagoodcandidatefor
cloudcondensationnuclei(CCN).It ishypothesizetdhatorganicacidsareatleastoneof
theprimarysourcesof CCNin theatmosphereduetotheirubiquitouspresencein the
tropospheree,speciallyoverthecontinentaflorestedareas.
Theresultsof ourmeasuremenatstPalmerStation,Antarcticashowthatthedaily
averageCCNconcentrationast0.3%and1%supersaturationrsangedfrom0.3to 160
cm-3 and from 4 to 168 cm -3, respectively, during the period from 17 January to 26
February, 1994. New evidence for substantial and definitive CCN enhancement near and
within cloud has been observed at Mt. Mitchell, North Carolina. The results show that
the average monthly CCN concentrations were 460_+217, 386_+286, 429_+228 and 238_+
134 cm -3for in-cloud, overcast, clear and rainy conditions, respectively. The typical CCN
spectra show that there were a lot of small CCN produced and the ion concentrations
(especially H+ and SO42-) were very high during the CCN enhancement period. The
significantly positive correlation between black carbon (BC) and CCN at 1%
supersaturation indicates that a percentage of the BC measured at the site may be in the
form of an internal mixture and participated in the formation of CCN.
A SIUDY OF D_cyr AN][) =_OUD_MED|ATF:D RADIATIVE FORCING OF
CL|MATE DEE TO AEROSOLS
Raki_
1999
Appr_ov_¢l By_
,7
,',,Ii:/,_<>__J
Dr, Viitv;)d K. S_a # _ [ :a.19}'.
ii
DEDICATION
This research is dedicated to my father and mother, who have provided
me with love and support throughout my life. I only wish that my father
and mother could be here to share in my achievements, but I know that they
are proud of me, a country boy, to be a Doctor of Philosophy in the USA.
This research is also dedicated to my wife for her moral support and
patience.
iii
BIOGRAPHY
Shaocai Yu was born in Yongan, Fujian, China, on 7 March, 1964, to his parents
Zhimei Yu and Guangxin Liao. After graduation from Yongan No. 1 High School in
1981, Shaocai applied to and was accepted for Fall admission to Peking University,
Beijing, China. He started studies there in September of 1981. He received his Bachelor
of Science degree in Chemistry in 1985 from Peking University. As he was near the top
of his class, his graduate entrance exam requirement was waived, and he was
subsequently admitted to the Environmental Science Center in the Graduate School of
Peking University. He received a Master of Science degree in Atmospheric Chemistry in
June of 1988 from Peking University. From July of 1988 to July of 1994, he worked as
an environmental engineer in Xiamen Municipal Research Institute and Monitoring
Station of Environmental Protection, Xiamen, China. In August 1994, he entered the
graduate school at North Carolina State University, and continued his education in
Atmospheric Sciences. After completion of his another M.S. degree in Atmospheric
Sciences in May 1996, he went on to pursue a Ph.D. degree in Atmospheric Sciences at
North Carolina State University.
iv
ACKNOWLEDGMENTS
I would like to express my deep thanks to Professor V. K. Saxena, chairman of my
advisory committee, for his expertise, guidance and encouragement throughout this
research. Thanks are also due to all of my advisory committee members, Drs. J.J.
DeLuisi, S.P. Arya, and G.F. Watson for their contribution to my education. I consider
myself fortunate to have conducted my research under their direction. I am also grateful
to Dr. G.K. Yue from NASA for providing helpful comments and advice both in
scientific discussion and on a personal level.
I am particularly thankful to members of the Cloud-Aerosol Interaction
Laboratory (CALL), both past and present, especially, Surabi Menon, Brian Wenny, John
Anderson, Shannon Schafer, Chad Bahrmann, Matt Seybold, Sun Im and Bill Barnard.
The help from Brian Wenny, his resourcefulness, patience and kindness have made life
and research much easier. The friendship of Mel DeFeo and her family will always be
remembered. I would like to thank Dr. I. V. Petropavlovskikh, and Dr. S. Madronich
from NCAR for their help in using the UV-Visible Radiative Transfer Model, and Dr.
C.S. Zender from NCAR for his help in using the Column Radiation Model (CRM) of
NCAR CCM3.
This research was partly supported through the Southeast Regional Center for the
National Institute for Global Environmental Change, in Tuscaloosa, Alabama, by the
United States Department of Energy under cooperative agreement No. DE-FCO3-
90ER61010, and partly by the NASA's Mission to Planet Earth (MTPE) under Contract
No. NAS1-18944 from Langley Research Center, Hampton, VA and US EPA's STAR
(Science to Achieve Results) grant No. R-825248.
TABLE OF CONTENTS
LIST OF TABLES ................................................................................... x
LIST OF FIGURES ............................................................................... xiv
1. AEROSOL-CLOUD-CLIMATE INTERACTIONS ....................................... 1
Introduction: Statement of the Problem ............................................. 1
1.1.
1.2. Objectives of this Study ................................................................ 7
1.3. Results of this Study .................................................................... 9
1.4. Importance of this Study ............................................................... 14
1.5. References .............................................................................. 15
PART I: A STUDY OF AEROSOL DIRECT RADIATIVE FORCING ................... 19
SEARCHING FOR A REGIONAL FINGERPRINT OF AEROSOL RADIATIVE
.
FORCING IN THE SOUTHEASTERN US ................................................. 20
2.1. Abstract .................................................................................... 21
2.2. Introduction ................................................................................ 22
2.3. Methodology and Database .............................................................. 23
2.3.1. Database ............................................................................. 23
2.3.2. Methodology ........................................................................ 23
2.4. Results and Discussion .................................................................. 24
2.4.1. Regional Patterns of Climate Change in the Southeast ....................... 24
2.4.2. Fingerprints of Aerosol Radiative Forcing ...................................... 26
2.5. Concluding Remarks ..................................................................... 29
2.6. References ................................................................................. 30
vi
ON DETECTING THE SIGNATURE OF REGIONAL AEROSOL RADIATIVE
.
FORCING IN EASTERN CHINA .......................................................... 37
3.1. Abstract .................................................................................... 38
3.2. Introduction ................................................................................ 39
3.3. Methodology and Database .............................................................. 40
3.3.1. Database ............................................................................. 40
3.3.2. Methodology ........................................................................ 40
3.4. Results and Discussion ................................................................... 42
3.4.1. Regional Patterns of Climate Change in Eastern China ....................... 42
3.4.2. The Trends of Stratospheric Volcanic Aerosols in Eastern China .......... 43
3.4.3. Signature of Aerosol Radiative Forcing in Eastern China ................... 44
3.4.4. Comparison of Eastern China and Southeastern US .......................... 48
3.5. Conclusions ............................................................................... 50
3.6. References ................................................................................ 51
AN EVALUATION OF CHEMICAL AND SIZE EFFECTS ON RADIATIVE
.
PROPERTIES OF MULTI-COMPONENT AEROSOLS ............................... 63
4.1. Abstract .................................................................................... 64
4.2. Introduction ............................................................................... 65
4.3. Model Formulation ...................................................................... 66
4.3.1. Atmospheric Aerosol Composition and Size Distribution .................. 66
4.3.2. Parameterization of the Influence of Relative Humidity ..................... 68
4.4. Results and Discussion .................................................................. 69
4.4.1. Refractive Index Calculation ..................................................... 68
4.4.2. The Sensitivity to Relative Humidity ........................................... 72
4.4.3. The Sensitivity to Refractive Index ............................................. 73
4.4.4. The Sensitivity to Size Distribution ............................................. 74
4.4.5. The Sensitivity of Wavelength Dependence of Radiative Properties ...... 75
4.4.6. Radiation Transmission .......................................................... 76
vii
4.5.Conclusion.s................................................................................ 78
4.6.Reference..s................................................................................ 78
A STUDY OF THE AEROSOL RADIATIVE PROPERTIES NEEDED TO
.
COMPUTE DIRECT AEROSOL FORCING IN THE SOUTHEASTERN US ....... 94
5.1. Abstract ..................................................................................... 95
5.2. Introduction ................................................................................ 96
5.3. Instrumentation and Database ........................................................... 97
5.4. Results and Discussion .................................................................. 99
5.4.1. Characteristics of Aerosol Optical Depth (AOD) .............................. 99
5.4.2. Characteristics of Diffuse-to-Direct Solar Irradiance Ratio ................. 102
5.4.3. Retrieval of Aerosol Columnar Size Distribution and Radiative Properties
from Optical Depth at the Three Operational Wavelengths ................ 104
5.4.4. Determination of Single Scattering Albedo and Ground Albedo .......... 112
5.5. Concluding Remarks .................................................................... 117
5.6. References ................................................................................ 118
AEROSOL DIRECT RADIATIVE FORCING IN THE SOUTHEASTERN US:
.
ESTIMATES FROM THE MEASUREMENT AND MODEL RESULTS .......... 133
6.1. Abstract ................................................................................... 134
6.2. Introduction .............................................................................. 135
6.3. Aerosol Radiative Properties in the Southeastern US ............................... 136
6.4. Calculation of Aerosol Direct Radiative Forcing ..................................... 138
6.5. Conclusions .............................................................................. 141
6.6. References ................................................................................ 141
PART II: A STUDY OF CLOUD CONDENSATION NUCLEI (CCN) ................... 151
