Scientists have been observing a change in the climate since the beginning of the 20th century that cannot be attributed to any of the natural influences of the past. Natural and anthropogenic substances and processes perturb the Earth's energy budget, contributing to climate change. In particular, aerosols (particles suspended in air) have long been recognized to be important in processes throughout the atmosphere that affect climate. They directly influence the radiative balance of the Earth's atmosphere, affect cloud formation and properties, and are also key air pollutants that contribute to a variety of respiratory and cardiovascular diseases. Despite their importance, aerosol particles are less well-characterized than greenhouse gases with respect to their sources, temporal and spatial concentration distribution, and physical and chemical properties. This uncertainty is mainly caused by the variable and insufficiently understood sources, formation and transformation processes, and complex composition of atmospheric particles. Instruments that can precisely and accurately measure and characterize the aerosol physical and chemical properties are in great demand. Atmospheric relative humidity (RH) has a crucial impact on the particles' optical properties; the RH dependence of the particle extinction coefficient is an important parameter for radiative forcing and thus climate change modeling. In this work a Humidity-Controlled Cavity Ring-Down (HC-CRD) aerosol optical instrument is described and its ability to measure RH dependent extinction coefficients and related hygroscopicity parameters is characterized. The HC-CRD is capable of simultaneously measuring the aerosol extinction coefficient at three wavelengths (λ = 355, 532, and 1064 nm) and three different RHs (typically 20%, 50%, and 80%). A range of chemicals and their mixtures were used to produce laboratory generated aerosols. Three mixture systems include one inorganic salts mixture system consisting of (NH4)2SO4, NH4HSO4, Na2SO4, NaHSO4 serve as surrogates of the ionic salts found in the atmosphere. Two organic mixture systems were investigated: mixtures of NaCl, D-glucose, sucrose, and glycine are benchmarks for compounds emitted from biomass burning. Finally, mixtures of (NH4)2SO4 (ammonium sulfate, AS) with a series of dicarboxylic acids including malonic acid, adipic acid, and azelaic acid are used as benchmarks to mimic urban pollutants. The extinction coefficients were obtained as a function of RH from the HC-CRD measurements, from which optical growth factors f(RH) and γ(RH) values can be determined to examine their dependence on chemical composition. A volume mixing rule was used to calculate the effective refractive index of the binary substrate mixtures, since both size and composition change during water uptake. The SDA/FMC algorithm developed by O'Neill, et al. 2005 is used to extract the van de Hulst phase shift parameter (Ρeff) from three-wavelength measurements of extinction. The fine mode fraction of extinction (η) and fine mode effective radius (Reff) of laboratory generated aerosol particles can be then determined. An iterative algorithm was developed to retrieve the change in refractive index of particles as function of RH. The calculated Reff of aerosols at different RHs were used to obtain the physical size growth factor (gf), and κ(RH). The size changes as a function of water uptake describe the dependence of aerosol optical properties on chemical composition. This work demonstrates the capability of conducting aerosol optical measurements using HC-CRD to determine the RH dependence of aerosol optical properties. The HC-CRD measurements combined with the SDA/FMC method to retrieve aerosol size for laboratory generated aerosols establish the connection between the optical properties and the aerosol particles' chemical compositions. It also underlines the importance and need for future investigation on the hygroscopic properties of atmospheric aerosols. This work is successfully developed a method that enables using the aerosols optical measurements to predict the compositions; it will greatly contribute to the atmospheric aerosol measurement and global climate modelling.

This thesis characterizes the ambient fine organic carbon aerosol and investigates its sources through the development and deployment of new measurement techniques. The focus is on organic compounds that are soluble in water (WSOC), which comprise a large fraction of the organic aerosol, yet little has been known about its chemical nature. A method was developed for quantitative on-line measurements of WSOC by using a Particle-into-Liquid Sampler (PILS) to capture ambient particles into a flow of purified water, which is then forced through a liquid filter and the carbonaceous content quantified by a Total Organic Carbon (TOC) analyzer. This system allows for a continuous 6 minute (ground-based) or 3 s integrated measurement (airborne) with a limit of detection of 0.1 microgramsC/m3 and uncertainty of 10%. Furthermore, a new quantitative method was developed to group speciate the WSOC. In the first step, WSOC is separated by use of XAD-8 resin into its hydrophilic (WSOCxp) and hydrophobic (WSOCxr) fractions. This separation can be performed on-line by coupling the XAD-8 column with the PILS-TOC or off-line on WSOC extracted from integrated filter samples. If off-line, a second step involving size-exclusion chromatography (SEC) is used to chromatographically separate by organic functional groups the WSOCxp and recovered hydrophobic fraction (WSOCxrr). During this step, the WSOCxp is further separated into aliphatic acids with less than four carbons, neutrals, and bases. The WSOCxrr can be separated into acids and neutrals. Results showing the capabilities of the PILS-TOC both on the ground at the St. Louis Midwest Supersite and when airborne during the New England Air Quality Study/Intercontinental Transport and Chemical Transformation 2004 mission conducted in the northeastern U.S. will be presented. Ambient results from urban sites where a PILS-TOC was coupled with a XAD-8 column will be discussed. Data from the two-step speciation performed on samples collected from urban Atlanta summer and winter, and biomass burning in rural Georgia in a region of prescribed burning are presented. Finally, WSOC measurements obtained in Atlanta and its surrounding regions from both the speciation measurements and PILS-TOC will be used to investigate the sources of WSOC in the southeastern U.S.

The main objective of this study was to investigate the vertical distribution of aerosols over Durban in relation to the vertical stability structure and horizontal transport of air masses. The importance of aerosols in the region is well recognised and recently there have been many international experiments which have focused on aerosol distribution over the subcontinent. Durban is situated at the approximate centre of a giant plume that is known to transport aerosols and trace gases off the east coast of southern Africa and is therefore strategically located for an investigation of the vertical distribution of aerosols. The vertical distribution of aerosols over Durban was measured using a LIDAR (Light Detection And Ranging) system on selected cloud free days in 1997. Backward trajectory modelling was used at selected pressure (standard) levels to determine the origin and transport pathways of aerosols. Six case studies are presented in an attempt to gain insight into the relationship between the vertical distribution of aerosols and absolutely stable layers. The results of the study revealed that the occurrence of absolutely stable layers governs the vertical distribution of aerosols in the troposphere. An absolutely stable layer at ~5km (~500hPa) appears to be the most effective in capping and trapping aerosols in the atmosphere. Below 5km, the atmosphere was charcterised by marked stratification and relatively higher concentration of aerosols. Above 5km, the concentrations were much lower, but generally increased slightly with height. Low aerosol concentrations are observed during post-frontal situations and relatively higher concentrations during anticyclonic conditions. The background to the problem and the objectives of this investigation are elaborated in Chapter 1. A description of the data sets and derived meteorological variables, along with the methodologies applied in this thesis, are given in Chapter 2. A theoretical review of aerosols, including their sources, effects and distribution over the globe and southern Africa, is discussed in Chapter 3. Atmospheric circulation and weather patterns and their relationship to the transport and dispersion of aerosols are described in Chapter 4. The results of the study and an analysis of the major findings are presented in Chapter 5. Finally, Chapter 6 summarises the major findings of this dissertation. / Thesis (M.Sc.)-University of Natal, Durban, 2000.

A Nephelometry camera (NephCam) and Humidity Controlled Cavity Ring-Down Transmissometer (HC-CRDT) were developed for the determination of aerosol optical properties. The NephCams use a reciprocal geometry relative to an integrating nephelometer; a diode laser illuminates a scattering volume orthogonal to a charge coupled device (CCD). The use of a CCD allows for measurement of aerosol scattering in 2 dimensions; scattering coefficients and size information can be extracted. The NephCam's optics were characterized during a set of imaging experiments to optimize the images collected by the camera. An aperture setting of 1.6 was chosen because it allowed for the most light intensity to reach the CCD - albeit with significant vignetting - and also had a constant modular transfer function (MTF) across the image; approximately 0.3. While this MTF value is approaching the minimum usable MTF of 0.2, other aperture settings did not exhibit constant MTF. While the effects of vignetting can be corrected in image post processing, the effects of non-constant MTF cannot. An optical response model was constructed to simulate images collected by the NephCams as a function of particle type and size. Good agreement between modeled and measured images was observed after the effects of contrast on image shape were considered. The image shapes generated by the model also pointed towards the use of polynomial calibration for particle sizes less than 400 nm as a result of multiple charge-to-size effects present from the sizing mechanism of the differential mobility analyzer. Initial calibration of the NephCams using size-selected dry Ammonium sulfate (AS) showed that calibration slopes are a function of particle size which is also in agreement with the model. Calibration slopes decreased as particle size increased to 400 nm; after 400 nm calibration slope oscillated around a common value. This effect is directly related to the forward shift of scattered intensity as particles grow in size and the collection efficiency of the NephCam as particle size increases. The single scattering albedo (SSA) of Nigrosin was calculated using the NephCam; extinction was measured by the HC-CRDT. Good agreement between the SSA and size was noticed for larger particle sizes; particles smaller than 200 nm in diameter over-measured the SSA of Nigrosin because of the multiple charge-to-size effect. In this size regime, light scattering by particles increases much more quickly than absorption; the presence of larger particles causes scattering to be artificially high. The HC-CRDT is a 4 channel, 3 wavelength instrument capable of measuring the extinction coefficients of aerosols at high (> 80%), low (< 10%) and ambient relative humidity. Extinction coefficients as a function of RH were determined for AS, NaNO3, NaCl, and Nigrosin; these particles represent surrogates of the strongly scattering ionic salts and black carbon, respectively. A model was developed to calculate the changes in refractive index and extinction coefficients of these water soluble particles as a function of RH; these particle types were chosen because core-shell morphologies could be avoided. Volume mixing, Maxwell-Garnett and partial molar refraction mixing rules were used to calculate effective refractive indices as a function of water uptake. Particle growth was calculated based upon the Kelvin equation. Measured and modeled results of f(RH) - relative change in extinction between high or ambient RH and dry RH - agree well for all particle types except Nigrosin. This disagreement is thought to stem directly from an incomplete parameter set for Nigrosin; growth parameters were assumed to be identical to NaNO3, density assumed to be 1 g/mL and molecular weight 202 g/mole, which may not be true in reality (different suppliers of Nigrosin quote different molecular weights). The NephCam was not used during these experiments, so the addition of a scattering measurement to better characterize the growth by Nigrosin is necessary. The f(RH) data for NaNO3 showed excellent agreement between measured and modeled data; however particle size information collected by an SMPS does not agree with the theory. This stems from the fact that NaNO3 does not show prompt deliquescence upon drying; instead an amorphous solid forms which exhibits a kinetically limited loss of water.

Indiana University-Purdue University Indianapolis (IUPUI) / Objective: "The purpose of this research is: Study the use of Moderate Resolution Imaging Spectroradiometer (MODIS) data in retrieving the aerosol optical depth (AOD) over Indiana State at high resolution of 500 meters. Examine the potential of using the resulted AOD data as an indicator of particulate air pollution by comparing the satellite derived AOD data with the ground measurements (provided from the continuous air monitors available over the study area). If an association should be found, AOD data would be used to map particulate matter (PM) concentration. Assess current and future ambient concentrations of air pollutants in the State of Indiana using the AOD."

This thesis is based on measurements of aerosol optical and microphysical properties made at inland, coastal and island locations in India. Aerosol vertical distribution measurements have also been made both using surface based and aircraft borne instruments. In addition to these, satellite based measurements (MODIS and OMI) have also been used to estimate regional aerosol radiative forcing over the oceanic regions around India. The measurements at an inland, continental, urban location reveals the large effect of anthropogenic activities on aerosol characteristics at surface and the atmospheric vertical column. A clear seasonality is observed in aerosol optical and microphysical properties as a consequence of modulation by anthropogenic activities and the effect of meteorological parameters like rainfall, winds and boundary layer dynamics. The variability observed at different time scales (from diurnal, weekly, monthly to annual) reveals the importance of anthropogenic and natural processes in modulating the aerosol loading. The estimates of aerosol radiative forcing at surface were as high as ~ 40W m-2. A large discrepancy was observed between the observed and modeled aerosol forcing efficiency (forcing per unit optical depth) at surface. These discrepancies are due to the inadequate representation of aerosol mixing state in models. In addition, the large difference found in the observed forcing between winter and summer could also be influenced due to the presence of elevated aerosols during the summer. Measurements made over coastal and central India shows that a large fraction (75-85%) of aerosol column optical depth was contributed by aerosols located above 1 km. The horizontal gradients were sharp with e-1 scaling distance as small as ~250 km in the well-mixed regions mostly under the influence of local source effects. However, above the atmospheric boundary layer, the gradients were much shallower (~800 to 1200 km). In addition, a large fraction (60-75%) of aerosol was found located above clouds leading to enhanced aerosol absorption. Large spatial gradient in aerosol optical depth and hence radiative impacts between the coastal landmass and the adjacent oceans within a short distance of <300 km (even at an altitude of 3 km) during summer and pre-monsoon is of importance to regional climate. Observations at Minicoy, a remote island in southern Arabian Sea to study the characteristics of transported aerosols reveals variability at daily, weekly, monthly and seasonal time scales associated with changes in precipitation and air mass characteristics. The daily mean Black Carbon (BC) mass mixing ratio varied between as low as ~ 0.2 to 9.0%. The resultant average aerosol atmospheric forcing for the observation period was ~15 W m-2. Trajectory based cluster analysis has shown six distinct advection/transport pathways influencing aerosol characteristics over southern Arabian Sea. The Indo-Gangetic Plain, northern Arabian Sea and west Asia are identified to be the most important source regions having a major impact on aerosols loading over the southern Arabian Sea. The cluster analysis, concentration weighted trajectory (CWT) analysis and the MODIS retrievals show an asymmetry in aerosol characteristics between the Arabian Sea and the Bay of Bengal, with the Arabian Sea characterized by large loading by natural aerosols (eg., dust and sea salt) and the Bay of Bengal characterized by anthropogenic loading (eg., BC). The low value of the BC mass mixing ratio measured at the island (mostly ~ 1 to 1.6%), has major implications for regional radiative forcing. The annually averaged net aerosol atmospheric forcing was as low as ~1.7 W m-2 with highest forcing corresponding to IGP cluster. The single scattering albedo (SSA) which is an important parameter in the estimation of aerosol radiative forcing was retrieved by utilizing a joint OMI-MODIS retrieval methodology. The SSA over the oceanic regions around India shows that the largest absorption (SSA < 0.9) occurs during winter. The largest gradients in AOD and SSA were observed over Arabian Sea during the summer as a result of large dust emissions. The largest forcing observed also was confined to the northern Arabian Sea (~ 37 W m-2) as a result of high aerosol column loading and dust transport. The observed annual mean forcing at Minicoy were comparable to that estimated using satellite measurements, but were much lower than those observed during INDOEX.

The Moderate-resolution Imaging Spectroradiometer (MODIS) onboard NASA’s Terra and Aqua satellites have provided a global distribution of aerosols. The space-based inversion of MODIS measurements requires assumption about the surface and aerosol properties, both are highly heterogeneous in space and time. This thesis has investigated the role of surface reflectance and aerosol properties on the retrieval of aerosols from MODIS over the Indian region. The aerosol properties retrieved by MODIS including total aerosol optical depth (AOD) and aerosol fine mode fraction (AFMF, fractional contribution of fine mode aerosols in the total AOD) were compared with that obtained from Aerosol Robotic Network (AERONET) at Kanpur (26.45◦N,80.35◦E), Indo-Gangetic Basin, northern India. This region is a special region for the study of aerosols as it offers strong aerosol seasonality, where the region is influenced by dust aerosols during pre-monsoon (March to June) and dominated by the fine mode particles in winter (November to February). The MODIS Collection 004 (C004) aerosol products systematically overestimated AOD in the presence of dust and underestimated when fine particles were dominant. The errors in the retrieval of dust AOD were correlated with the apparent reflectance at 2.1 µm, from which the surface reflectance in the visible channels (0.47 µm and 0.66 µm) were estimated using the “dark target” spectral correlation method. The error in the retrieval of AOD were also found to be large in the scattering angle range 120◦150◦, where the scattering properties of the non-spherical dust aerosols differ from that of the assumed spherical particles. AFMF of C004 was found to be highly biased to fine mode at Kanpur. The Collection 005 (C005) aerosol retrieval of the second-generation aerosol algorithm, however, showed improved retrieval of spectral AOD, which is likely to be attributed to the use of updated aerosol models and parameterized surface reflectance. In contrast to the C004 products, fine AOD and fine-model weighting (FMW) of C005 were biased very low at Kanpur and also over the greater Indian land region. This has indicated that the inversion of the space-based MODIS measurements is non-unique in which an improper combination of surface reflectance and aerosol model provide more accurate retrieval of the total aerosol optical depth. The surface reflectance relationships between the visible and shortwave-infrared 2.1 µm channels derived from the actual measurements of the surface reflectance using a spectroradiometer onboard an aircraft over Bangalore (12.95◦N,77.65◦E) in the southern India were found to have higher slope and intercept than that assumed by the MODIS algorithm over the same region. The high spectral correlations between the measured reflectance at longer wavelengths indicated some potential to estimate the surface reflectance at these wavelengths which needs further investigation. An experiment on the retrieval of aerosols carried out with several combinations of aerosol models and visible surface reflectance clearly shown that the surface reflectance in the visible channels assumed in the MODIS aerosol algorithm should be increased from its current parameterization in order to retrieve more accurate total as well as size-segregated aerosol optical properties at Kanpur and also over the greater Indian land region. In addition to the visible channels, inclusion of longer wavelengths in the aerosol inversion would likely improve the accuracy of retrieval over land by resolving the spectral dependence of aerosols. This in turn can help in separating the anthropogenic and natural aerosols in the total aerosol loading.