Abstract : This is a study concerning comparisons between the Dubovik Aerosol optical depth (AOD) retrievals from AEROCAN (ARONET) stations and AOD estimates from simulations provided by a chemical transport model (GEOS-Chem : Goddard Earth Observing System Chemistry). The AOD products associated with the Dubovik product are divided into total, fine and coarse mode components. The retrieval period is from January 2009 to January 2013 for 5 Arctic stations (Barrow, Alaska; Resolute Bay, Nunavut; 0PAL and PEARL (Eureka), Nunavut; and Thule, Greenland). We also employed AOD retrievals from 10 other mid-latitude Canadian stations for comparisons with the Arctic stations. The results of our investigation were submitted to Atmosphere-Ocean. To briefly summarize those results, the model generally but not always tended to underestimate the (monthly) averaged AOD and its components. We found that the subdivision into fine and coarse mode components could provide unique signatures of particular events (Asian dust) and that the means of characterizing the statistics (log-normal frequency distributions versus normal distributions) was an attribute that was common to both the retrievals and the model. / Résumé : Cette étude compare des épaisseurs optiques d’aérosols (AOD) à 5 stations arctiques d’AEROCAN (AERONET), obtenues d’une part à l’aide de l’algorithme d'inversion de Dubovik appliqué à des mesures in situ, et d’autre part du modèle de transport chimique (GEOS-Chem : Goddard Earth Observing Système Chemistry). Les produits d’AOD associés à l’algorithme d’inversion sont divisés en composantes totales, fines et grossières. Pour chacune des 5 stations (Barrow, Alaska, Resolute Bay, au Nunavut, 0PAL et PEARL (Eureka), Nunavut, et Thulé, au Groenland), la période de récupération est de janvier 2009 à janvier 2013. Nous avons également utilisé les mesureurs d’AOD de dix autres stations canadiennes de latitudes moyennes, à des fins de comparaison. Les résultats de l’étude ont été soumis à la revue Atmosphere-Ocean. Pour résumer brièvement ces résultats, le modèle a généralement, mais pas toujours, eu tendance à sous-estimer l'AOD moyenne et de ses composantes. Nous avons constaté que la subdivision en composantes fine et grossière pourrait fournir des signatures uniques d'événements particuliers (poussière asiatique) et que les moyens de caractériser des statistiques (les distributions de fréquence log-normale versus les distributions normales) était un attribut qui était commun aux deux les mesureurs et le modèle.

Aerosol are solid or liquid particles suspended in the atmosphere ranging in size from 10 3 to 102 m. Aerosol influence both the regional and global climate of the earth by its direct and indirect effect. Role of atmospheric aerosols on the radiative forcing of atmosphere is a matter of serious research for past few decades and still it remains highly uncertain as acknowledged by Intergovernmental Panel on Climate Change. Heterogeneous nature of aerosol both spatially and temporally makes it more complex in estimating radiative forcing compared to that of greenhouse gases. Compounding to the existing difficulty in determining the climate effects, changing aerosol concentration and nature of the aerosol further increases the complexity in determining its effects in both regional and global climate. Increasing aerosol loading is emerging to be an issue of major concern over several regions. The first step towards achieving this goal is by determining the trends in the physical and optical properties of aerosol over the globe. Main objective of the thesis is the determination of the recent trends in aerosol loading over the globe and then to focus specifically on the properties of aerosol over an urban site in southern India. Specific objectives are (a) to determine the trend in aerosol physical and optical properties over the globe using AERONET surface observations (b) to characterize the chemical properties of water soluble inorganic aerosol over an urban site in Southern India, Bangalore (c) to have a better estimation of aerosol radiative forcing over Bangalore with measured aerosol chemical concentration, black carbon concentration and aerosol optical depth (AOD). To quantify the recent trends in aerosol loading over the globe, we have used the surface observations from AERONET and the study provided the first step in giving a global picture of the recent trends in the fundamental optical and physical property of aerosol. Trend analysis showed a significant spatial inhomogenity, and Asian continent clearly showed an increasing trend in AOD compared to other continents. Solar village (24.9oN, 46.4oE) of Saudi Arabia showed a maximum with a value of 0.04/yr and Bac -Giang (21oN, 106oE) of Vietnam showed the minimum value of -0.04/yr. North American study region included 18 sites in which eastern US (E.US) exhibited a decreasing trend while the scenario in western US (W.US) is different with more of sites with increasing AOD trend. Single scattering albedo (SSA) trend in W. US showed a decreasing trend irrespective of the AOD trend. Study sites in South America include Cordoba -CETT (31.5oNS, 64.5 oN W) Alta Floresta (9.8oS, 56.1 o W), Riobranco (9.9oNS, 67.8o W) and Soa Poulo (23.5oS, 46.7o W).Except Riobranco which has a positive trend in AOD, all other sites exhibited a statistically signi cant negative trend. Over Australia, there is an inclination towards increasing AOD in sites and all the three sites in Australia exhibit a statistically significant increasing trend in SSA. According to the recent trends in AOD over African region, there exists a significant decrease in AOD compared to that reported for few years earlier, showing the high temporal in homogeneity and need for continuous observation of aerosol over the regions. European study region included 15 stations, among them only 3 sites showed an increasing trend in AOD, remaining 12 sites showed a significant decreasing trend in aerosol loading over the period of study. SSA was also observed to be decreasing over most of the European sites, even with a decreasing AOD over most of the sites. A Comparison study carried out to determine the relation of population growth rate and aerosol loading, and it revealed that the increasing AOD trend not always coincided with the sites having high population growth rate. Having determined the trends in AOD and other aerosol parameters over the globe and seeing an alarmingly increasing trend over most of the Asian sites, especially over Indian region, we have then focused over work on the aerosol properties of one of the rapidly growing urban location in southern India, Bangalore. While physical properties of aerosol have been extensively studied over Bangalore, chemical characteristics are still an unexplored area. Extensive information on aerosol chemical composition is not available over Indian region except for a few locations based on campaign mode. Even available data is of very coarse temporal resolution, since hours or full day sampling is needed to gather enough samples for chemical analysis. High temporal resolution data of aerosol chemical characteristics, especially for all season is completely lacking over Indian subcontinent. Among aerosol, water soluble aerosol form an important component in particulate matter, since it can change its size, composition, can easily mix with other aerosols and can act as cloud condensation nuclei, based on its hygroscopic nature. Present study provided the rst time results from a high temporal resolution water soluble inorganic aerosol chemical data over Indian region, which is first step towards estimating aerosol climate impacts more accurately. Water soluble inorganic aerosol ions over Bangalore namely, sulphate, nitrate, chloride, potassium, calcium, magnesium, sodium and ammonium are measured using Particle Into Liquid Sampler Ion Chromatograph (PILS-IC). PILS is an online sampling technique for quantitatively measuring the chemical concentration of ion in water soluble aerosol particles. PILS IC used in the present study is developed in Georgia Institute of Technology. Instrument samples ambient air at a flow rate of 16.7Lmin 1. Particles below PM 2.5 micron are collected for the analysis using cyclone impactor. Two annular glass denuders are used to remove inorganic gases which else will interfere with the aerosol ion concentration. Ambient air which is deprived of the inorganic gases is then mixed with steam vapours at 150oC, eventually high supersaturated atmosphere is produced with rapid adiabatic mixing of steam and ambient air. High supersaturated air allows droplets to grow enough to be collected by inertial impaction onto a quartz impactor plate. Entire PILS condensation unit is kept at a slight tilt of 15o, to remove all condensate through drain tube connected to the end of the PILS condensate body. Condensed liquid sample is collected from the impaction chamber and known concentration LiF is allowed to mix with the collected sample at a constant rate. LiF known as carrier liquid is added to know the dilution occurring to the collected sample. Sample with carrier liquid is then collected to a debubbler and is supplied to the IC through peristaltic tubings for determining the ion chemical concentration. Seasonal variation of mass concentration of water soluble aerosol species and the influence of long range transport is carried out using HYSPILT back trajectory analysis. Marine air mass from Arabian Sea dominated the air parcel reaching the site for both SW monsoon and summer. Continental air mass dominated the site during both NE monsoon and winter with slight contribution from marine atmosphere. Source characteristics of sulphate, potassium, calcium and magnesium ions are carried out based on sea salt (ss) and non sea salt (nss) origin and it is observed that the nss contribution is dominant over the site for all these ions except magnesium where ss component comparatively dominates the source. SO24 and NO3 form the dominant anions while NH+4 makes the dominant cation species. Monthly variation of the ratio of ammonium to nss -sulphate is carried out to determine the possible cation -anion relation existing between these two major ions. During later winter and summer months ammonium bi sulphate is found to be the existing chemical form and ammonium sulphate during other seasons. High temporal resolution data enabled us to study the diurnal variation of aerosol ions and it is influenced by various mechanisms from boundary layer to local emissions. Optical properties of aerosols depend upon the size and the relative abun-dance of each components. It is usual practice to assume default aerosol chemical composition in radiative transfer models due to unavailability of data, which can lead to errors in forcing estimates. Incorporating realistic aerosol chemical composition in models is essential to reduce the uncertainty in aerosol radiative forcing. Hence we have included measured aerosol chemical compositions, black carbon and AOD to improve the determination of radiative forcing of aerosol. OPAC and SBDART models were used for estimating the aerosol radiative forcing over Bangalore. We have used mainly four components namely, soot, water soluble, sea salt and dust. Except dust all are other components are measured over the site and formed a constrain for the calculation. Dust concentration was altered so that the OPAC AOD matched the measured AOD within 5%. Mineral dust shows the highest contribution in AOD among the four components, however water soluble and soot even being less is mass concentration compared to mineral dust, has significant impact on the AOD. This clearly indicate the influence of both water soluble and soot aerosol over the regional climate of the site. Sea Salt exhibited low AOD compared to other three constituents. The results presented in the thesis highlights the importance of varying trends in the aerosol properties and its effects on a global picture and speci - cally over an urban site in Indian region , we explored the temporal variations of water soluble inorganic aerosol ions and its effects on regional climate. Hence the thesis addressed some of the unexplored areas in aerosol science. This study also suggests the need of continuous observation of aerosol over both spatial and temporal scale, which is essential to estimate their effects on earth's climate.

In this thesis, we have presented a systematic analysis of the change of cloud properties due to variation in aerosol concentration over Indian region using satellite observations, and Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem) simulations. The Tropical Rainfall Measurement Mission (TRMM) based Microwave Imager (TMI) estimates (2A12) have been used to compare and contrast the characteristics of cloud liquid water and ice over the Indian land region and the surrounding oceans, during the pre-monsoon (May) and monsoon (June–September) seasons. Based on the spatial homogeneity of rainfall, we have selected five regions for our study (three over ocean, two over land). In general, we find that the mean cloud liquid water and cloud ice content of land and oceanic regions are different, with the ocean regions showing higher amount of CLW. A comparison across the ocean regions suggests that the cloud liquid water over the or graphically influenced Arabian Sea (close to the Indian west coast) behaves differently from the cloud liquid water over a trapped ocean (Bay of Bengal) or an open ocean (Equatorial Indian Ocean). Specifically, the Arabian Sea region shows higher liquid water for a lower range of rainfall, whereas the Bay of Bengal and the Equatorial Indian Ocean show higher liquid water for a higher range of rainfall. Apart from geographic differences, we also documented seasonal differences by comparing cloud liquid water profiles between monsoon and pre-monsoon periods, as well as between early and peak phases of the monsoon. We find that the cloud liquid water during the lean periods of rainfall (May or June) is higher than during the peak and late monsoon season (July-September) for raining clouds over central India. However, this is not true over the ocean. As active and break phases are important signatures of the monsoon progression, we also analyzed the differences in cloud liquid water during various phases of the monsoon, namely, active, break, active-to-break (a2b) and break-to-active (b2a) transition phases. We find that the cloud liquid water content during the b2a transition phase is significantly higher than that during the a2b transition phase over central India. We speculate that this could be attributed to higher amount of aerosol loading over this region during the break phase. We lend credence to this aerosol-liquid water/rain association by comparing the central Indian cloud liquid water with Southeast Asia (where the aerosol loading is significantly smaller) and find that in the latter region, there are no significant differences in cloud liquid water during the different phases of their monsoon. The second part of our study involves evaluating the ability of the Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem) to simulate the observed variation of cloud liquid water and rain efficiency. We have used no chemistry option, and the model was run with constant aerosol concentration. The model simulations (at 4.5 km resolution) are done for the month of June–July 2004 since this period was particularly favorable for the study of an active–break cycle of the monsoon. We first evaluate the sensitivity of the model to different parameterizations (microphysical, boundary layer, land surface) on the simulation of rain over central India and Bay of Bengal. This is done to identify an “optimal” combination of parameterizations which reproduces the best correlation with observed rain over these regions. In this default configuration (control run), where the aerosol concentration is kept constant throughout the simulation period, the model is not able to reproduce the observed variations of cloud liquid water during the different phases of an active-break cycle. To this end, we proceeded to modify the model by developing an aerosol-rain relation, using Aerosol Robotic Network (AERONET) and TRMM 3B42 data that realistically captures the variation of aerosol with rain. It is worth highlighting here that our goal was to primarily isolate the indirect effect of aerosols in determining the observed changes in cloud liquid water (CLW) during the active-break phases of the Indian monsoon, without getting into the complexity of a full chemistry model such as that incorporated in WRF-Chem. Moreover, the proposed modification (modified run) is necessitated by the lack of realistic emission estimates over the Indian region as well as the presence of inherent biases in monsoon simulation in WRF. The main differences we find between the modified and control simulations is in the mean as well as spatial variability of CLW. We find that the proposed modification (i.e., rate of change of aerosol concentration as a function of rain rate) leads to a realistic variation in the CLW during the active-break cycle of Indian monsoon. Specifically, the peak value of CLW in the b2a (a2b) phase is larger (smaller) in the modified as compared to the control run. These results indicate a stronger change in CLW amount in the upper levels between the two transition phases in the modified scheme as compared to the control simulation. More significantly, we also observe a change in sign at the lower levels of the atmosphere, i.e., from a strong positive difference in the control run to a negative difference in the modified simulation, similar to that observed. Additionally, we investigated the impact of the proposed modification, via CLW changes, on cloud coverage, size of clouds and their spatial variability. We find that the transformation of optically thin clouds to thick clouds during the break phase was associated with larger cloud size in modified compared to the control simulation. Moreover, the higher rate of decay of the spatial variability of CLW with grid resolution, using the modified scheme, suggests that clusters of larger clouds are more in the modified compared to control simulation. Taken together, the interactive aerosol loading proposed in this thesis yields model simulations that better mimic the observed CLW variability between the transition phases.

Epidemiological research around the world has shown that exposure of urban populations to fine microparticles (PM2.5) suspended in air from, among other things, car combustion, is responsible for many cases of lung and cardiovascular disease and even mortality. However, most of these studies examine urban centers as ensembles without considering that population exposure to microparticles is not homogeneous across an urban space. For example, individuals living near major arterial roads are much more exposed to microparticles than others living in low traffic neighborhoods. Unfortunately, ground stations measuring PM2.5 are few and far between to generate accurate microparticle concentration maps at fine scales. One way to spatialize information on microparticle concentrations is to introduce remotely sensed images that allows to calculate an optical parameter of aerosols, their optical depth. The use of medium-to-fine-resolution images is not common in this area. So, we wanted to look at their potential. Tests with hyperspectral and multispectral images at these resolutions have shown that optical depth can be estimated with enough accuracy. The AODFinder software developed for this purpose performs well. Unfortunately, the small sample of AOD values and PM2.5 concentration measurements did not allow us to conclude on the possibility of using AOD as a proxy for PM2.5 and thus on the possibility of refining microparticle monitoring at the local level. / Des recherches épidémiologiques à travers le monde ont mis en évidence que l’exposition des populations urbaines aux microparticules fines (PM2.5) en suspension dans l’air provenant, entre autres, de la combustion automobile, est à l’origine des nombreux cas des maladies pulmonaires et cardiovasculaires et même des cas de mortalité. Cependant, la plupart de ces études examinent les centres urbains comme des ensembles sans tenir compte que l’exposition des populations aux microparticules n’est pas homogène à travers un espace urbain. À titre d’exemple, les individus demeurant à proximité de grandes artères routières sont beaucoup plus exposés aux microparticules que d’autres demeurant dans des quartiers de faible circulation. Malheureusement, les stations terrestres de mesure des PM2.5 sont peu nombreuses pour permettre de générer des cartes de concentration des microparticules précises à des échelles fines. Un moyen pour spatialiser l’information sur les concentrations des microparticules est d’introduire l’imagerie de télédétection qui permet de calculer un paramètre optique des aérosols, leur profondeur optique. L’utilisation des images à résolution moyenne à fine n’est pas chose courante dans ce domaine. Ainsi nous avons voulu examiner leur potentiel. Les tests avec des images hyperspectrale et multispectrale à ces résolutions ont montré que la profondeur optique peut être estimer avec suffisamment de précision. Le logiciel AODFinder développé à cette fin se comporte bien. Malheureusement le faible échantillon des valeurs de AOD et des mesures des concentrations des PM2.5 ne nous a pas permis de se prononcer sur la possibilité d’utiliser le AOD comme proxy des PM2.5 et ainsi sur la possibilité de raffiner le monitoring des microparticules à l’échelle locale.

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