Size segregated aerosol mass concentration measurements over the Arabian Sea during ICARB

Mass concentration and mass size distribution of total (composite) aerosols near the surface are essential inputs needed in developing aerosol models for radiative forcing estimation as well as to infer the environment and air quality. Using extensive measurements onboard the oceanographic research vessel, Sagar Kanya, during its cruise SK223B in the second phase of the ocean segment of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB), the spatial distribution of the mass concentration and mass size distribution of near-surface aerosols are examined for the first time over the entire Arabian Sea, going as far as 58°E and 22°N, within a span of 26 days. In general, the mass concentrations (M _T ) were found to be low with the mean value for the entire Arabian Sea being 16.7 ± 7 μg m^?3; almost 1/2 of the values reported in some of the earlier campaigns. Coarse mode aerosols contributed, on an average, 58% to the total mass, even though at a few pockets accumulation mode contribution dominated. Spatially, significant variations were observed over central and northern Arabian Sea as well as close to the west coast of India. In central Arabian Sea, even though the M _T was quite low, contribution ofs accumulation aerosols to the total mass concentration was greater than 50%. Effective radius, a parameter important in determining scattering properties of aerosol size distribution, varied between 0.07 and 0.4 μm with a mean value of 0.2 μm. Number size distributions, deduced from the mass size distributions, were approximated to inverse power-law form and the size indices (ν) were estimated. It was found to vary in the range 3.9 to 4.2 with a mean value of 4.0 for the entire oceanic region. Extinction coefficients, estimated using the number-size distributions, were well-correlated with the accumulation mode mass concentration with a correlation coefficient of 0.82.     

Physical and optical characteristics of atmospheric aerosols during ICARB at Manora Peak,Nainital: A sparsely inhabited,high-altitude location in the Himalayas

Collocated measurements of the optical and physical properties of columnar and near-surface aerosols were carried out from Manora Peak, Nainital (a sparsely inhabited, high altitude location, ～2 km above mean sea level, in the Himalayas), during the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) under the Geosphere Biosphere Programme of the Indian Space Research Organization (ISRO-GBP). Under this, observational data of spectral aerosol optical depths (AOD), mass concentration of aerosol black carbon (M _B ), mass concentration (M _T ) and number concentration (N _t ) of composite (total) aerosols near the surface and meteorological parameters were collected during the period February 15 to April 30, 2006. Though very low (<0.1 at 500 nm) AODs were observed during clear days, as much as a four-fold increase was seen on hazy days. The Ångström exponent (α), deduced from the spectral AODs, revealed high values during clear days, while on hazy days α was low; with an overall mean value of 0.69 ± 0.06 for the campaign period. BC mass concentration varied between 0.36 and 2.87 μg m^?3 and contributed in the range 0.7 to 1.8% to the total aerosol mass. Total aerosol number concentration and BC mass concentration showed diurnal variation with a midnight and early morning minimum and a late afternoon maximum; a pattern quite opposite to that seen in low altitude stations. These are attributed to the dynamics of the atmospheric boundary layer.     

Aircraft measurements of aerosol black carbon from a coastal location in the north-east part of peninsular India during ICARB

During the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) over India, high-resolution airborne measurements of the altitude profiles of the mass concentrations (M_B) of aerosol black carbon (BC) were made off Bhubaneswar (BBR, 85.82°E, 20.25°N), over northwest Bay of Bengal, in the altitude region upto 3 km. Such high-resolution measurements of altitude profiles of aerosols are done for the first time over India. The profiles showed a near-steady vertical distribution of MB modulated with two small peaks, one at 800m and the other at ～2000m. High resolution GPS (Global Positioning System) sonde (Vaisala) measurements around the same region onboard the research vessel Sagar Kanya (around the same time of the aircraft sortie) revealed two convectively well mixed layers, one from ground to ～700m with an inversion at the top and the other extends from 1200m to ～2000m with a second inversion at ～2200m and a convectively stable region in the altitude range 700–1200m. The observed peaks in the M_B profile are found to be associated with these temperature inversions. In addition, long-range transport from the Indo-Gangetic Plain (IGP) and deserts lying further to the west also influence the vertical profile of BC. Latitudinal variation of MB showed a remarkable land ocean contrast at the 500m altitude (within the well mixed region) with remarkably lower values over oceans, suggesting the impact of strong sources over the mainland. However, above the ABL (at 1500m), the latitudinal variations were quite weak, and this appears to be resulting from the impact of long-range transport. Comparison of the altitude profiles of M_B over BoB off BBR with those obtained during the earlier occasion over the inland stations of Hyderabad and Kanpur showed similarities above ～500m, with M_B remaining around a steady value of ～1 μg m^?3. However, large differences are seen within the ABL. Even though the observed M_B values are not unusually high, their near constancy in the vertical column will have important implications to radiative forcing.     

Aerosol optical depths at Mohal-Kullu in the northwestern Indian Himalayan high altitude station during ICARB

First time observations of spectral aerosol optical depths (AODs) at Mohal (31.9°N, 77.11°E; altitude 1154 m amsl) in the Kullu valley, located in the northwestern Indian Himalayan region, have been carried out during Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB), as a part of the Indian Space Research Organisation-Geosphere Biosphere Program (ISRO-GBP). AODs at six wavelengths are obtained using Microtops-II Sunphotometer and Ozonometer. The monthly mean values of AOD at 500 nm are found to be 0.27 ± 0.04 and 0.24 ± 0.02 during March and April, 2006 respectively. However, their monthly mean values are 0.33 ± 0.04 at 380 nm and 0.20 ± 0.03 nm at 870 nm during March 2006 and 0.31 ± 0.3 at 380 nm and 0.17 ± 0.2 at 870 nm during April 2006, showing a gradual decrease in AOD with wavelength. The Ångstrom wavelength exponent ‘α’ had a mean value of 0.72 ± 0.05, implying reduced dominance of fine particles. Further, the afternoon AOD values are higher as compared to forenoon values by ～ 33.0% during March and by ～ 9.0% during April 2006 and are attributed to the pollutant lifted up from the valley by the evolving boundary layer. Besides the long-range transportation of aerosol particles by airmass from the Great Sahara and the Thar Desert regions to the observing site, the high values of AODs have also been influenced by biomass burning and frequent incidents of forest fire at local levels.     

Chemical characteristics of PM<Subscript>10</Subscript> aerosols and airmass trajectories over Bay of Bengal and Arabian Sea during ICARB

For the first time, chemical characterization of PM₁₀ aerosols was attempted over the Bay of Bengal (BoB) and Arabian Sea (AS) during the ICARB campaign. Dominance of SO ₄ ^2? , NH ₄ ⁺ and NO ₃ ^? was noticed over both the regions which indicated the presence of ammonium sulphate and ammonium nitrate as major water soluble particles playing a very important role in the radiation budget. It was observed that all the chemical constituents had higher concentrations over Bay of Bengal as compared to Arabian Sea. Higher concentrations were observed near the Indian coast showing influence of landmass indicating that gaseous pollutants like SO₂, NH₃ and NO _x are transported over to the sea regions which consequently contribute to higher SO ₄ ^2? , NH ₄ ⁺ and NO ₃ ^? aerosols respectively. The most polluted region over BoB was 13°?19°N and 70°?90°E while it was near 11°N and 75°E over AS. Although the concentrations were higher over Bay of Bengal for all the chemical constituents of PM₁₀ aerosols, per cent non-sea salt (nss) fraction (with respect to Na) was higher over Arabian Sea. Very low Ca2+ concentration was observed at Arabian Sea which led to higher atmospheric acidity as compared to BoB. Nss SO ₄ ^2? alone contributed 48% of total water soluble fraction over BoB as well as AS. Ratios SO ₄ ^2? /NO _? ³ over both the regions (7.8 and 9 over BoB and AS respectively) were very high as compared to reported values at land sites like Allahabad (0.63) and Kanpur (0.66) which may be due to very low NO.3 over sea regions as compared to land sites. Air trajectory analysis showed four classes: (i) airmass passing through Indian land, (ii) from oceanic region, (iii) northern Arabian Sea and Middle East and (iv) African continent. The highest nss SO ₄ ^2? was observed during airmasses coming from the Indian land side while lowest concentrations were observed when the air was coming from oceanic regions. Moderate concentrations of nss SO2. 4 were observed when air was seen moving from the Middle East and African continent. The pH of rainwater was observed to be in the range of 5.9–6.5 which is lower than the values reported over land sites. Similar feature was reported over the Indian Ocean during INDOEX indicating that marine atmosphere had more free acidity than land atmosphere.     

Characteristics of spectral aerosol optical depths over India during ICARB

Spectral aerosol optical depth (AOD) measurements, carried out regularly from a network of observatories spread over the Indian mainland and adjoining islands in the Bay of Bengal and Arabian Sea, are used to examine the spatio-temporal and spectral variations during the period of ICARB (March to May 2006). The AODs and the derived Ångström parameters showed considerable variations across India during the above period. While at the southern peninsular stations the AODs decreased towards May after a peak in April, in the north Indian regions they increased continuously from March to May. The Ångström coefficients suggested enhanced coarse mode loading in the north Indian regions, compared to southern India. Nevertheless, as months progressed from March to May, the dominance of coarse mode aerosols increased in the columnar aerosol size spectrum over the entire Indian mainland, maintaining the regional distinctiveness. Compared to the above, the island stations showed considerably low AODs, so too the northeastern station Dibrugarh, indicating the prevalence of cleaner environment. Long-range transport of aerosols from tshe adjoining regions leads to remarkable changes in the magnitude of the AODs and their wavelength dependencies during March to May. HYSPLIT back-trajectory analysis shows that enhanced long-range transport of aerosols, particularly from the west Asia and northwest coastal India, contributed significantly to the enhancement of AOD and in the flattening of the spectra over entire regions; if it is the peninsular regions and the island Minicoy are more impacted in April, the north Indian regions including the Indo Gangetic Plain get affected the most during May, with the AODs soaring as high as 1.0 at 500 nm. Over the islands, the Ångström exponent (α) remained significantly lower (～1) over the Arabian Sea compared to Bay of Bengal (BoB) (～1.4) as revealed by the data respectively from Minicoy and Port Blair. Occurrences of higher values of α, showing dominance of accumulation mode aerosols, over BoB are associated well with the advection, above the boundary layer, of fine particles from the east Asian region during March and April. The change in the airmass to marine in May results in a rapid decrease in α over the BoB.     

Influence of circulation parameters on the AOD variations over the Bay of Bengal during ICARB

MODIS (Moderate Resolution Imaging Spectroradiometer) level-3 aerosol data, NCEP (National Centers for Environmental Prediction) reanalysis winds and QuikSCAT ocean surface winds were made use of to examine the role of atmospheric circulation in governing aerosol variations over the Bay of Bengal (BoB) during the first phase of the ICARB (Integrated Campaign for Aerosols, gases and Radiation Budget) campaign (March 18–April 12, 2006). An inter-comparison between MODIS level-3 aerosol optical depth (AOD) data and ship-borne MICROTOPS measurements showed good agreement with correlation 0.92 (p < 0.0001) and a mean MODIS underestimation by 0.01. During the study period, the AOD over BoB showed high values in the northern/north western regions, which reduced towards the central and southern BoB. The wind patterns in lower atmospheric layers (> 850 hPa) indicated that direct transport of aerosols from central India was inhibited by the presence of a high pressure and a divergence over BoB in the lower altitudes. On the other hand, in the upper atmospheric levels, winds from central and northern India stretched south eastwards and converged over BoB with a negative vorticity indicative of a downdraft. These wind patterns pointed to the possibility of aerosol transport from central India to BoB by upper level winds. This mechanism was further confirmed by the significant correlations that AOD variations over BoB showed with aerosol flux convergence and flux vorticity at upper atmospheric levels (600–500 hPa). AOD in central and southern BoB away from continental influences displayed an exponential dependence on the QuikSCAT measured ocean surface wind speed. This study shows that particles transported from central and northern India by upper atmospheric circulations as well as the marine aerosols generated by ocean surface winds contributed to the AOD over the BoB during the first phase of ICARB.     

Volatile properties of atmospheric aerosols during nucleation events at Pune,India

Continuous measurements of aerosol size distributions in the mid-point diameter range 20.5–500 nm were made from October 2005 to March 2006 at Pune (18°32′N, 73°51′E), India using Scanning Mobility Particle Sizer (SMPS). Volatilities of atmospheric aerosols were also measured at 40°, 125°, 175°, 300° and 350°C temperatures with Thermodenuder–SMPS coupled system to determine aerosol volatile fractions. Aerosols in nucleated, CCN and accumulated modes are characterized from the measured percentage of particles volatized at 40°, 125°, 175°, 300° and 350°C temperatures. Averaged monthly aerosol concentration is at its maximum in November and gradually decreases to its minimum at the end of March. The diurnal variations of aerosol concentrations gradually decrease in the night and in early morning hours (0400–0800 hr). However, concentration attains minimum in its variations in the noon (1400–1600 hr) due to higher ventilation factor (product of mixing height and wind speed). The half an hour averaged diurnal variation of aerosol number concentration shows about 5 to 10-fold increase despite the ventilation factor at higher side before 1200 hr. This sudden increase in aerosol concentrations is linked with prevailing conditions for nucleation bursts. The measurement of volatile fraction of ambient aerosols reveals that there are large number of highly volatile particles in the Aitken mode in the morning hours and these volatile fractions of aerosols at temperatures <150°C are of ammonium chloride and ammonium sulfate, acetic and formic acids.     

Radiocarbon measurements of black carbon in aerosols and ocean sediments

Black carbon (BC) is the combustion-altered, solid residue remaining after biomass burning and fossil fuel combustion. Radiocarbon measurements of BC provide information on the residence time of BC in organic carbon pools like soils and sediments, and also provide information on the source of BC by distinguishing between fossil fuel and biomass combustion byproducts. We have optimized dichromate-sulfuric acid oxidation for the measurement of radiocarbon in BC. We also present comparisons of BC ¹⁴C measurements on NIST aerosol SRM 1649a with previously published bulk aromatic ¹⁴C measurements and individual polycyclic aromatic hydrocarbon (PAH) ¹⁴C measurements on the same NIST standard.Dichromate-sulfuric acid oxidation belongs to the chemical class of BC measurement methods, which rely on the resistance of some forms of BC to strong chemical oxidants. Dilute solutions of dichromate-sulfuric acid degrade BC and marine-derived carbon at characteristic rates from which a simple kinetic formula can be used to calculate concentrations of individual components (Wolbach and Anders, 1989). We show that: (1) dichromate-sulfuric acid oxidation allows precise, reproducible ¹⁴C BC measurements; (2) kinetics calculations give more precise BC yield information when performed on a % OC basis (vs. a % mass basis); (3) kinetically calculated BC concentrations are similar regardless of whether the oxidation is performed at 23°C or 50°C; and (4) this method yields ¹⁴C BC results consistent with previously published aromatic ¹⁴C data for an NIST standard.For the purposes of intercomparison, we report % mass and carbon results for two commercially available BC standards. We also report comparative data from a new thermal method applied to SRM 1649a, showing that thermal oxidation of this material also follows the simple kinetic sum of exponentials model, although with different time constants.     

Aerosol mass loading over the marine environment of Arabian Sea during ICARB: Sea-salt and non-sea-salt components

Mass loading and chemical composition of atmospheric aerosols over the Arabian Sea during the pre-monsoon months of April and May have been studied as a part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB). These investigations show large spatial variabilities in total aerosol mass loading as well as that of individual chemical species. The mass loading is found to vary between 3.5 and 69.2 μg m^?3, with higher loadings near the eastern and northern parts of Arabian Sea, which decreases steadily to reach its minimum value in the mid Arabian Sea. The decrease in mass loading from the coast of India towards west is estimated to have a linear gradient of 1.53 μg m^?3/° longitude and an e^?1 scale distance of ～2300 km. SO ₄ ^2? , Cl^? and Na⁺ are found to be the major ionic species present. Apart from these, other dominating watersoluble components of aerosols are NO ₃ ^? (17%) and Ca²⁺ (6%). Over the marine environment of Arabian Sea, the non-sea-salt component dominates accounting to ～76% of the total aerosol mass. The spatial variations of the various ions are examined in the light of prevailing meteorological conditions and airmass back trajectories.     

天津市海岸线现状

海岸线是陆地与水体交界的重要标识,海岸线的位置和类型是海岸带生态环境修复的基础数据.本文以天津市海岸线为研究对象,通过遥感解译与实地验证相结合的方式,对天津市海岸线位置和分类进行了研究,结果显示,目前天津市海岸线长度约366.92 km,其中港口和围海造陆岸线281.78 km、围海养殖岸线22.92 km、海堤岸线25.77 km和人工河口岸线36.45 km,自然岸线数值趋近为零,围海造陆及港口岸线所占比例达到了76.80%.由于围填海等人类活动,岸线较2000年增加了143%.     

Physical properties of aerosols at Maitri,Antarctica

Measurements of the submicron aerosol size distribution made at the Indian Antarctic station, Maitri (70‡45′S, 11‡44′E) from January 10th to February 24th, 1997, are reported. Total aerosol concentrations normally range from 800 to 1200 particles cm⁻³ which are typical values for the coastal stations at Antarctica in summer. Aerosol size distributions are generally trimodal and open-ended with a peak between 75 and 133 nm and two minima at 42 and 420 nm. Size distributions remain almost similar for several hours or even days in absence of any meteorological disturbance. Total aerosol concentration increases by approximately an order of magnitude whenever a low pressure system passes over the station. Based on the evolution of aerosol size-distributions during such aerosol enhancement periods, three types of cases have been identified. The nucleation mode in all three cases has been suggested to result from the photochemical conversion of the DMS emissions transported either by the marine air or by the air from the ice-melt regions around Maitri. Subsidence of midtropospheric air during the weakening of radiative inversion is suggested as a possible source of the nucleation mode particles in the third case. Growth of the nucleation mode particles by condensation, coagulation and/or by cloud processes has been suggested to be responsible for other modes in size distributions.     

reconstruction of the holocene coastline of mesopotamia

At the end of the last glacial period, a major marine transgression inundated the head of the Persian Gulf to a position as much as 400 km inland from the present shoreline. After 6000 B.P., the Shatt al-Arab delta prograded southward to its current position. These events had a profound impact on our knowledge of early human occupation and the development of civilization in the Tigris-Euphrates valley. The former settlements of people living on the pre-6000 B.P. coastal plains are today either under the waters of the Gulf or under sediments of the lower alluvial plain.     

Late Holocene tectonic movements at Akhziv,Mediterranean coastline of northern Israel

The recent discovery of a post-Roman-aged marine calcarenite terrace, at an altitude of 7 to 8 m at the ancient harbor of Akhziv, supports the hypothesis of oscillatory tectonic movements along the coastline of Israel during the late Holocene. The marine to estuarine origin of this terrace is indicated by the presence within it of a biocoenosic, sea-marginal to estuarine, assemblage of well-preserved molluscs consisting of taxa tolerating brackish-water, together with a few fresh-water specimens. The transgressive marine sediments onlap and overlie a 2700-yr-old middle Iron Age (Phoenician) tomb, which was built on a dark clay layer, containing a middle Bronze HB (3750 yr B.P.) settlement. In post-Roman times the coastal zone at this site, both east and west of the present coastline, was first subjected to tectonic subsidence of a few meters, and was then tectonically uplifted to its present altitude.     

Localization of submicron inclusion re-equilibration at healed fractures in host garnet

Microstructures in Permian inclusion-bearing metapegmatite garnets from the Koralpe (Eastern Alps, Austria) reveal re-equilibration by coarsening of abundant submicron-sized inclusions (1 μm–2 nm diameter) at the site of healed brittle cracks. The microstructures developed during Cretaceous eclogite-facies deformation and the related overprinting of the host–inclusion system. Trails of coarsened inclusions (1–10 μm diameter) crosscut the garnet, defining traces of former fractures with occasional en-echelon overlaps. Trails are flanked by 10- to 100-μm-wide ‘bleaching zones’ characterized by the absence of ≤1-μm-sized inclusions in optical and SE images. FEG-microprobe data show that trails and bleaching zones can form isochemically, although some trails exhibit non-isochemical coarsening. Cross-correlation-based EBSD analysis reveals garnet lattice rotation of up to 0.45°, spatially correlated with bleaching zones. The garnet lattice in the center of trails is misoriented around different axes with respect to the lattice either side of the trail. Elevated dislocation density within bleaching zones is confirmed by TEM observations. Dislocations represent a plastic wake formed by crystal plastic deformation at the crack tip. Fracture enhanced diffusion rates in the lattice adjacent to crack planes by introducing dislocations, priming these areas to behave differently to the bulk of the garnet during Cretaceous metamorphism and facilitating localized coarsening of inclusions. Diffusion within the bleaching zone was enhanced by a minimum factor of 10². The partially closed host–inclusion system records the influence of deformation mechanisms on re-equilibration and contributes to understanding of the interaction between deformation and chemical reaction during metamorphism.     

Latitudinal and longitudinal variation in aerosol characteristics from Sun photometer and MODIS over the Bay of Bengal and Arabian Sea during ICARB

Spatial variations in aerosol optical properties as function of latitude and longitude are analysed over the Bay of Bengal and Arabian Sea during ICARB cruise period of March–May 2006 from in situ sun photometer and MODIS (Terra, Aqua) satellite measurements. Monthly mean 550 nm aerosol optical depths (AODs) over the Bay of Bengal and Arabian Sea show an increase from March to May both in spatial extent and magnitude. AODs are found to increase with latitude from 4°N to 20°N over the Bay of Bengal while over Arabian Sea, variations are not significant. Sun photometer and MODIS AODs agree well within ±1σ variation. Bay of Bengal AOD (0.28) is higher than the Arabian Sea (0.24) latitudinally. Aerosol fine mode fraction (FMF) is higher than 0.6 over Bay of Bengal, while FMF in the Arabian Sea is about 0.5. Bay of Bengal α(～1) is higher than the Arabian Sea value of 0.7, suggesting the dominance of fine mode aerosols over Bay of Bengal which is corroborated by higher FMF values over Bay of Bengal. Air back trajectory analyses suggest that aerosols from different source regions contribute differently to the optical characteristics over the Bay of Bengal and Arabian Sea.     

Impact of the clouds of volcanic aerosols in Italy during the last 7 centuries

Intense natural pollution has occurred in the past in Italy corresponding to intense volcanic activity, which appears to have diminished somewhat in recent times. Between 1500 and 1900, Etna, Vesuvius, Vulcano and Stromboli, plus volcanoes outside Italy were very active and there are numerous, well documented episodes of atmospheric acidification which caused widespread damage, especially to the vegetation. Other than the acid rains, volcanic emissions also caused so calleddry fogs which consist of a more or less dense mist composed of foul smelling gases and aerosols, characterized by a reddish color, that could appear and persist when the relative humidity was low as shown by measurements taken on such occasions. This phenomenon appeared most frequently at the beginning of the hot season. In fact, volcanic clouds of gases and aerosols formed especially when the Tyrrhenian sea water was relatively cold giving rise to very stable atmospheric conditions locally and the summer anticyclone meant that the winds were calm. Under such conditions the emissions of Stromboli and Vulcano, especially those emitted at low levels, remained entrapped in the stable layer, which were then transported towards the land reaching Northern Italy at a distance on the order of 10³ km. Thedry fogs could persist for days or weeks. Harvests were seriously damaged and frequently the crops were subsequently attacked by parasites. The leaves of the vegetation became discoloured with numerous punctiform lesions or gangrene at the edges of the leaves. The phenomenon was so frequent that, in texts on agricultural meteorology of the 1800s, a distinction was made between the causticdry fogs which damaged the vegetation anddamp fogs which instead were good for it. The most important episode occurred in 1783 due to the activity of Laki Grímsvötn (Iceland) with the contribution of Italian volcanoes. This dry fog lasted many months and affected the greater part of the northern hemisphere including Europe and Asia, harming people, animals and vegetation. Apart from paroxysmal cases, from the 1300s up to today, some tens ofdry fogs have been noted, all of which have been sufficiently well documented. The frequency of these events culminated between the middle of the 1700s and the middle of the 1800s. There is reason to believe that this well documented phenomenon of the past, with sufficient volcanic activity, could recur on the meso and large scale; with present day activity the emissions continue to cause damage to vegetation, both in the Aspromonte mountains (Calabria, Southem Italy) as well as in other parts of Italy.     

Markov chain model to study the occurrence of pre-monsoon thunderstorms over Bhubaneswar,India

The present work deals with pre-monsoon thunderstorms over Bhubaneswar belonging to the state of Orissa, India. A Markovian approach has been adopted to discern the probabilistic behavior of the time series of the occurrence and non-occurrence of this hazardous weather event by introducing a dichotomy within the time series. After a painstaking analysis through chi-square tests, we have identified serial independence in a few years and first-order two-state Markovian dependence in a few years (2000, 2001, 2004 and 2006). Finally, for the years of first-order two-state Markovian dependence, it has been observed that the probability of occurrence or non-occurrence of thunderstorm gets higher if the state of the previous day is similar to that of the current day. Furthermore, the probability of getting non-thunderstorm day followed by non-thunderstorm day is higher than the probability of getting thunderstorm day followed by thunderstorm day. It has been also observed that the unconditional climatological probability of the occurrence of severe pre-monsoon thunderstorm implied by the Markov chain is closely in agreement with the observed relative frequencies. However, it could be revealed that Markov chain cannot, in general, be suggested as a predictive tool for pre-monsoon thunderstorms under study without investigating the serial dependence inherent in the time series.     

Observations of trace gases and aerosols over the Indian Ocean during the monsoon transition period

Characteristics of trace gases (O₃, CO, CO₂, CH₄ and N₂O) and aerosols (particle size of 2.5 micron) were studied over the Arabian Sea, equatorial Indian Ocean and southwest part of the Bay of Bengal during the monsoon transition period (October–November, 2004). Flow of pollutants is expected from south and southeast Asia during the monsoonal transition period due to the patterns of wind flow which are different from the monsoon period. This is the first detailed report on aerosols and trace gases during the sampled period as the earlier Bay of Bengal Experiment (BOBMEX), Arabian Sea Monsoon Experiment (ARMEX) and Indian Ocean Experiments (INDOEX) were during monsoon seasons. The significant observations during the transition period include: (i) low ozone concentration of the order of 5 ppbv around the equator, (ii) high concentrations of CO₂, CH₄ and N₂O and (iii) variations in PM2.5 of 5–20μg/m³.