Atmospheric Fine Mode Particulates at Eastern Himalaya, India: Role of Meteorology, Long-Range Transport and Local Anthropogenic Sources

A study on atmospheric fine (less than 2.0 μm) particulate matter was performed for the first time over the eastern Himalayas in India in order to investigate the formation and sources in two different seasons, pre-monsoon (Mar–May) and winter (Dec–Feb), for three consecutive years, 2008, 2009 and 2010. Fine mode aerosols were further segregated into three modes: ultrafine (less than 0.32 μm), superfine (0.32–1.0 μm) and fine (1.0–2.0 μm). The study was carried out at a high altitude hill station Darjeeling (2200 m asl) in the eastern Himalayas, India, using an aerosol spectrometer (GRIMM) to measure aerosol number concentrations. It was found that the ultrafine and superfine aerosol (less than 1.0 μm) concentrations were higher in the pre-monsoon period compared to winter, whereas the concentrations of fine mode aerosols were comparable during these two seasons. Meteorological conditions like sunny/non-cloudy days with higher radiative fluxes and lower humidity favored the formation of ultrafine aerosols from their precursor gases, and the initiation of this formation were observed during 0900–1000 hours LT. Superfine aerosols were mainly found to be aerosols transported from long distances during the pre-monsoon period, and showed peak levels during late morning till late afternoon. In contrast, higher concentrations were seen at night in winter. The fine mode aerosols were mainly locally generated anthropogenic aerosols, mostly emitted from vehicular sources during the pre-monsoon period, and from biomass burning during winter, with sharp diurnal peaks during morning and evening. The long-term study on size segregated aerosol concentrations carried out in this work is of considerable importance, and can be helpful for the validation of several regional and global aerosol models, and for other climate related studies that focus on the Himalayan region.


INTRODUCTION
The study of atmospheric aerosols especially in fine mode is gaining importance in scientific community as numerous studies have demonstrated their association with healthrelated (Pekkanen et al., 1997;Oberdorster et al., 1990;Laden et al., 2000) and climate change (Tegen et al., 1996;Ramanathan et al., 2001) issues.Further, the study of aerosol over Himalayan region is of paramount interest as the ecology of the Himalaya is under serious threat from various forms of pollutants (Bostrom, 2002).The increase in the loading of atmospheric aerosols over the Himalaya is a matter of concern, since most of the glaciers in the region have been retreating since1850 (Mayewski et al., 1979) with increasing melting rates.The rising anthropogenic interferences for rapid urbanization and development in the Himalaya not only affect the immediate landscape environment, but also the atmospheric environment which is becoming an increasing concern (Momin et al., 1999).
The regional and synoptic characteristics of size-segregated fine mode aerosol and their significance are required to be investigated (Moorthy et al., 1998(Moorthy et al., , 2007(Moorthy et al., , 2008)), while this kind of study has been done at fewer stations over Himalayan regions of India (Sellegri et al., 2010).The anthropogenic activities such as increasing vehicular traffic due to increased tourism-related activities, biomass burning and fuel wood burning for cooking and heating are the causes of concern for most of the Himalayan high altitude hill stations in India which apparently look like pollution-free regions as situated far away from the Indian mega-cities.Most of the studies on physical and chemical characterization of aerosols have been made over western and north-western Himalaya but as far as the eastern Himalaya is concerned, ours was the first study (Chatterjee et al., 2010) on a thorough chemical characterization of aerosol at a high altitude station, Darjeeling.
The fine mode aerosols (< 2.5 μm) can be classified as accumulation mode (< 1 μm), Aitken mode (0.1-0.01 μm) and nucleation mode (0.01-0.001 μm) (Satheesh et al., 2004).Nucleation, Aitken and accumulation mode particles could be collectively called as ultra and superfine particles.These particles penetrate into lungs and produce many respiratory problems and may even act as nano antibiotic to kill human cells (Sharma et al., 2011 and references therein).Although various studies have been carried out on the characterization of total fine mode aerosols, the size segregated fine mode aerosol characteristics specially the formation of ultra fine particles through nucleation of their gaseous precursors is not well documented in India especially over eastern Himalayan region.New particle formation is a complex process that depends on the nature of gaseous precursor species, which differ according to the environment; on meteorological factors such as UV-radiation, temperature, and relative humidity; and on boundary layer dynamics.
The present study has been carried out over a high altitude hill station, Darjeeling at eastern Himalaya in India on a long-term characterization of size segregated fine particles.This has been performed during two important seasons, premonsoon (March-May i.e., before the onset of Asian summer monsoon) and winter (December-February), over three consecutive years 2008, 2009 and 2010.Our earlier study (Chatterjee et al., 2010) carried out over the same location has shown that during premonsoon, aerosols are transported from arid and semi-arid regions of India and beyond to the Himalaya driven by the westerly pre-monsoon winds.Due to enhanced convection and the steep pressure gradient across the Himalayan-Gangetic region, these aerosols are transported to the higher altitudes.During winter, northeasterly winds from sub-continents bring anthropogenic aerosols over the Himalayan region.In addition to that, massive biomass burning during winter also plays a role in loading of anthropogenic aerosols over eastern Himalaya.These distinctly different seasonal behaviors of aerosol produce consequent signature on the various physicochemical properties of aerosols.
The seasonal and diurnal variations of fine, superfine and ultra fine aerosols have been performed for three years with the objectives that 1) how the local anthropogenic activities (mainly biomass burning and tourist activities) contribute to the loading of fine mode (1.0-2.0 μm) aerosols, 2) how the long-range transport is efficient for the enhancement of superfine aerosols (0.32-1.0 μm) mainly during premonsoon and 3) what are the controlling factors for the formation of ultrafine particles (less than 0.32 μm) over this high altitude eastern Himalayan region.
Thus this study is intended to provide a unique longterm database on fine, superfine and ultra fine particles over an urban environment in eastern Himalaya.This long-term study will surely help in evolving regional climatological models over Himalaya.

SITE DESCRIPTION AND PREVAILING METEOROLOGY
Darjeeling, a hill station in eastern India, with a population of ~100,000 persons, is one of the most popular tourist destinations in the entire world.A map showing the geographical location of Darjeeling is given in Fig. 1.The overall areas of the Darjeeling district and Darjeeling Township are about 1200 sq.km and 11.44 sq.km, respectively.Darjeeling Township is located at an average altitude of ~2000 m above mean sea level (amsl) and surrounded by different types of topography of the lowereastern-Himalayas.The southern region comprises the marshy low-lying area at an average height of ~100-300 m amsl.The apex is formed by the Phalut ridge (altitude 3800 m amsl) at the border between Nepal and India.The eastern frontier lies along two rivers, locally called Tista and Rangeet.The map of the experimental site is given in our earlier paper (Chatterjee et al., 2010).
The micro-meteorological parameters like temperature (°C), wind speed (m/s) and relative humidity (%) have been shown in Fig. 2 for premonsoon and winter seasons during the entire study period.The data have been averaged based on day (6 AM-5 PM) and night (6 PM-5 AM) time for premonsoon and winter.During premonsoon, the temperature varied from 11.2 to 16. 5°C, 11.6 to 16.1°C and 11.8 to 16.8°C in 2008, 2009 and 2010 respectively.The average day and night-time temperature was found to be the highest in 2010 followed by 2008 and 2009 as shown in Fig. 1.During winter, the temperature varied from 2.2 to 8.6°C, 1.5 to 7.8°C and 2.1 to 9.3°C in 2008, 2009 and 2010 respectively.The average day-time temperature was found to be the highest in 2010 followed by 2008 and 2009 whereas the night-time temperature was found to gradually decrease from 2008 to 2010.The difference between maximum and minimum temperature during winter (~6-7°C) was thus found to be higher than that in premonsoon (~5°C).During premonsoon, the surface wind speed during day-time was found to be much higher than that during night-time whereas during winter, significant differences were not observed between the day-time and night-time wind speed.During premonsoon, the average wind speed during day-time varied from 2.6 to 2.9 m/s and that during night-time varied from 1.1 to 1.4 m/s over the years 2008, 2009 and 2010.During winter, the average wind speed during day-time varied between 1.1 and 1.3 m/s whereas during night-time it varied between 0.8 and 1.0 m/s over the years 2008, 2009 and 2010.During premonsoon, the relative humidity was found to be much lower than that during winter in all the years.However, both in winter and premonsoon, the day-time humidity was found to be much lower than that in nighttime.The average day-time solar radiative flux was found to vary between 360 and 435 watt/m 2 in premonsoon whereas in winter, it varied between 232 and 290 watt/m 2 .During both the seasons, premonsoon and winter, the mean radiative flux was found to be the highest in 2010 followed by 2008 and 2009.During premonsoon, the wind pattern was mainly westerly and north-westerly whereas it was mainly easterly and north-easterly during winter (not shown in figure).

Aerosol Number Concentration
For the measurement of aerosol number concentrations for different size modes, a portable optical particle counter (Aerosol Spectrometer, Grimm Series 1.108, Germany) was used.It measures the number of particles per unit volume of air using light-scattering technology.Particle concentration  is measured in an optical size range of 0.30-20 μm in 15 channels and with concentration range of 1-2,000,000 particles/lit for count distribution mode.Spectrometer sensitivity is 1 particle/lit and instrument reproducibility is ± 2%.Ambient air is drawn into the unit via an internal volume-controlled pump at a rate of 1.2 lit/min.At the start of each measurement, the instrument initiates a system selftest and zero calibration check.The instrument was fitted with a stainless steel tube utilized as the spectrometer inlet.
The measured real-time number concentration data are transferred at 1-minute intervals to a data storage card.Measured data are then downloaded from the storage card via the Grimm 1177 program on number concentration mode.The responses of light-scattering aerosol monitors are influenced by aerosol parameters such as the refractive index, particle shape, density and size.Therefore, to acquire accurate quantitative measurements of aerosol distribution, the instrument was calibrated time to time with an equivalent method with the target dust under the same environmental conditions as it was utilized to evaluate the number concentration.
We have not studied aerosol concentrations for all the size ranges (for all channels in the instrument) but we have studied aerosol concentrations of the size below 2.0 μm as our study was mainly focused on fine mode particles.Thus we have studied aerosol concentrations of three different size modes namely i) ultrafine, ii) superfine and iii) fine mode for our study.Aerosols of size less than 0.32 μm or 320 nm have been considered as ultrafine mode.The particles of size 0.32-1.0μm have been considered as superfine mode and the particles of size 1.0-2.0μm have been considered as fine mode.Ultrafine mode aerosol concentration has been designated as PM UF , superfine mode aerosol concentration has been designated as PM SF and fine mode aerosol concentration has been designated as PM F in this study.

Meteorological Parameters
The meteorological parameters were recorded with a well-calibrated automatic weather station of Campbell Scientific (Canada) and all the data were run by Logger Net which is Campbell Scientifics' full featured software that support CR-Basic.The weather station was run continuously and the data were recorded at the interval of five minutes throughout the year covering all the sampling events.The weather station was equipped with a tower and all the sensors of wind speed and its direction, pressure, relative humidity (RH), temperature and rainfall were fitted with that tower at a height of 15 m from the ground level.

Seasonal Variation of Aerosols
The seasonal variation of ultrafine (PM UF ), superfine (PM SF ) and fine mode (PM F ) aerosols have been shown in Figs.3(a

Aerosols in Ultrafine Mode
The average concentrations of PM UF in two different seasons have been shown in Fig. 3(a) for three different years.It can clearly be seen that in each year, the particle concentrations during premonsoon were higher than winter.The concentrations were found to vary between 14500 to 29300 cm -3 with an average of 24350 cm -3 during premonsoon and 9200 to 14800 cm -3 with an average of 13050 cm -3 during winter over the years 2008, 2009 and 2010.

Factors Controlling Ultrafine Aerosol Formation
There are several factors involved which affect the particle formation characteristics in ultrafine mode.Meteorological conditions can favour or suppress the formation of the particles.Higher solar radiation promotes the photochemical oxidation process and increases OH radicals in the atmosphere.OH radicals are the key atmospheric constituents for the oxidation of trace gases like SO 2 to form the secondary particles.Higher solar radiation also increases the vertical mixing of the atmosphere which in turn dilutes the condensation sink (Kulmala et al., 2001).Due to increased vertical mixing, boundary layer air could reach at Darjeeling supplying precursor gases necessary for the particle formation.Increased relative humidity suppresses nucleation due to the wet scavenging of particles andprecursor gases and increased condensation sink by growing particles (Hamed et al., 2007).The meteorological conditions were favourable for the particle formation over Darjeeling during premonsoon.The solar radiation was found to be intense during premonsoon and this season was comparatively drier (lower RH) than the other seasons (Fig. 2) which allowed the particles and precursor gases to be not scavenged by the wet deposition and inhibiting the condensation sink to increase by the particle hygroscopic growth.Reduced solar radiation during winter might cause the lack of oxidizing agents inhibiting the secondary particle formation.Also the reduced vertical mixing of the lower atmosphere at lower temperature during winter might cause the depletion of the precursor gases and increase the condensation sink.A high background of larger particulate matter also acts as a condensation sink for the nucleating vapours and initially formed nm-sized particles (Neitola et al., 2011).We observed higher fine mode aerosol (PM 2.5 ) concentration than coarse mode (PM of diameter larger than 2.5 microns) aerosol over the same study location in the year of 2005 (Chatterjee et al., 2010) and 2008 (Chatterjee et al., 2012) throughout the entire dry season.During premonsoon, fine mode aerosol concentration was found to be maximum and much higher than coarse mode aerosol.Those studies have shown that long-range transport significantly contributed to the higher fine particle loading over Darjeeling during premonsoon.
These long-range transported air masses thus seem to contain a high amount of nucleating vapours and precursor gases for the new particle formation.Thus premonsoon is the favourable season for the formation of ultrafine aerosols (PM UF ) over Darjeeling from every aspect.

Comparison with Other High-Altitude Stations
The PM UF concentration over Darjeeling was found to be higher than the ultrafine particle concentration (1-20 nm) over Kothi, another high-altitude (2530 masl) Himalayan station in India as observed by Sharma et al. (2011).They observed that particle concentration on an average lied below 20,000 cm -3 .The PM UF concentration over Darjeeling was also much higher than Mukteshwar, situated almost at the same altitude (2180 masl) at north-eastern Himalaya where Komppula et al. (2009) observed the annual average concentration of ultrafine particles (10-800 nm) of 3480 cm -3 .Selegri et al. (2010) measured the total particle concentration (10 nm-32 µm) of 860 cm -3 at a high altitude Himalayan site, NCOP (5079 masl) in Nepal which was also quite lower than PM UF concentration measured in this study.It was also higher than the ultrafine particle concentration (3280 cm -3 ) measured over a high altitude station Mount Waliguan (3816 masl) in inland China (Kivekas et al., 2009).In the present study, the PM UF concentrations was also found to be higher than ultrafine particle (1-10 nm) concentration (5000 to 15000 cm -3 ) measured over a flat hill station like Malampuzha in Kerala, India by Varikoden et al. (2008).The PMUF concentration over Darjeeling was also found to be higher than the ultrafine particle concentrations measured in some cities in Europe like Helsinki, Stockholm and Augsburg where annual averaged ultrafine particle concentration was around 10,000 cm -3 (Aalto et al., 2005).In Europe, particle concentrations were found to be ubiquitously lower at different mountain sites.At Jungfraujoch, the annual average number concentration of particles (10-750 nm) is 900 cm −3 (Weingartner et al., 1999), while at the puy de Dôme (1465 ma.s.l.), the yearly average concentration is 3080 cm −3 .The PM UF concentration over Darjeeling (present study) was thus found to be much higher than that in other Himalayan stations and some hill stations in Europe.The higher particle concentration over Darjeeling compared to some stations having comparable altitude could be due to the fact that we have measured the ultrafine particle concentration in the higher range (0-320 nm) compared to the other stations like Kothi (1-20 nm) and Malampuzha (1-10 nm).The higher concentration over Darjeeling compared to the stations with higher altitude could be due to the fact that boundary layer air is sampled more frequently over Darjeeling than the other stations where air is sampled mostly from free troposphere and boundary layer air could not reach at those stations.This could be clearly understood from the very low concentration of total particles (10 nm-32 µm) at NCOP, Nepal compared to Darjeeling and other stations as discussed above.

Aerosols in Superfine Mode
The PM SF concentrations were found to be much higher during premonsoon compared to winter (Fig. 3(b)).It varied between 3000 to 8500 cm -3 during premonsoon and 1290 to 3900 cm -3 during winter over the entire study period.The significantly higher PM SF concentration during premonsoon than the other seasons could be attributed to the long range transport of aerosols reaching over eastern Himalayan region.Accumulation mode particles are the evidence of long range transport which is generally observed over Himalaya during premonsoon (Sellegri et al., 2010).We found good correlations between PM SF concentrations and wind speed during premonsoon in all the years (R 2 = 0.86, 0.91 and 0.84 in 2008, 2009 and 2010 respectively).This could be attributed to the wind driven transported aerosols mixed with dust and other anthropogenic components from the long distant source regions.This has been elaborated in the section below.

Long-Range Transport and Local Vehicular Activities
The transport of dust aerosols from arid and semi-arid regions of India including Thar Desert and even from west Asia driven by the premonsoon westerlies, not only influence the plains, but due to enhanced convection, are vertically advected to the higher altitudes against the foothills of the Himalayas (Gautam et al., 2009).During premonsoon, enhanced convection and steep pressure gradient steer aerosols aloft to the high elevated stations and dust-rich aerosols can "climb" the slopes of the Himalayas (Gautam et al., 2009).It has been shown by Carrico et al. (2003) that the long range transport of dust aerosols from west Asia reach over Nepal Himalayan regions.The increasing dustiness has also been reported from other indirect studies using isotopically inferred temperatures from ice-cores in the Himalayan-Tibetian plateau (Thomson et al., 2000).Our earlier study (Chatterjee et al., 2010) also identified two distinct source regions; Thar deserts and Arabian deserts for the long range transport of dust aerosols reaching over Darjeeling during premonsoon.It was found that 45% dust loading was from Thar deserts and 32% was from Arabian deserts during premonsoon.
Aerosol solar extinction from TOMS is a valuable indicator of the total columnar aerosol loading.To better understand the high dust aerosol loading over eastern Himalayas, we analyzed the TOMS aerosol index (AI) data during premonsoon and a representative figure has been given as Fig. 4 for the months of April and May in the year 2008.The high loading of dust aerosol over Thar deserts and its transport to Gangetic-Himalayan region through Indo-Gangetic Plain (IGP) is clearly seen from the figure.The increasing trend of aerosol loading over IGP has been indicated by earlier studies using TOMS satellite measurements (Habib et al., 2006).
During premonsoon, dust aerosols were found to be transported from arid and semi-arid regions of western India and beyond and passed over Indo Gangetic Plain (IGP), the most polluted belt in India before reaching at the eastern part of Himalaya.Thus there is a high possibility for the airmass to pick up anthropogenic aerosols from IGP during its overpass.These anthropogenic aerosols could get mixed with dust aerosols and finally reached over eastern Himalaya and enhanced the concentration of PM SF .Thus high Premonsoon season is also the peak tourist season over Darjeeling with the very high influx of tourists.Thus there was a probability that locally generated anthropogenic aerosols (mainly from vehicular emission) during premonsoon could enhance the superfine particle concentration.During the entire study period, the year 2009 faced a political agitation over Darjeeling with the minimum influx of tourists during premonsoon.But, still, we observed higher superfine particle concentrations during premonsoon compared to winter in 2009.This indicates that long-range transport played a major role compared to local sources during premonsoon.
In order to investigate the contribution of long distant sources in enhancing PM SF during premonsoon, we have divided the sampling days as associated and not associated to the long-range transport.To do this, we have computed 3 day isentropic air-mass back trajectories, arriving at an altitude of 2400 m above mean sea level (~200 m above ground) over Darjeeling for all the days on which aerosols were measured, using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model (http://www.arl.naa.ov/ready/hysplit4.html).Isentropic trajectories are considered, as they account for the adiabatic vertical motions of air parcels enroute and are less sensitive to the uncertainties in the basic meteorological data.What we have found is that, during premonsoon, ~75% of the sampling days were associated to the long range transport (mainly from western and north-western part of India and beyond) and ~25% were associated to the other sources like local and shortdistant/regional sources over the entire study period.The average concentration of PM SF during premonsoon and associated to long range transport was found to be 7655 cm -3 and that associated to other sources (local, short distant/ rgional) was found to be 5190 cm -3 .Thus we can say that, long range transport enhanced the concentration of PM SF by ~48%.This could be taken as the percentage contribution from long range transport, roughly.
Thus it can be inferred that the high increase in superfine particle concentration during premonsoon compared to winter was mainly due to the long range transport of aerosols (driven by premonsoon westerlies) rather than the local anthropogenic sources.However, during winter, the anthropogenic sources could play the role in the loading of superfine particles.

Aerosols in Fine Mode
Fine mode aerosol, unlike the other two modes discussed above, showed comparable concentrations during premonsoon and winter (Fig. 3(c)).However, premonsoon showed slightly higher concentration than winter.This feature was observed in 2008 and 2010 whereas in 2009, premonsoon showed much lower concentration than that in winter.During premonsoon, the vehicular emission was found to be the major anthropogenic source of PM F in addition to the other stationary sources.As stated earlier that there was a political agitation over Darjeeling in 2009 for which the minimum tourist influx was observed during premonsoon in this year.This probably resulted to the lowering of PM F concentration during premonsoon in 2009 compared to 2008 and 2010.On the other hand, the major anthropogenic source of PM F during winter is the massive biomass burning which however remained constant through out the entire study period.Higher loading of fine mode K + and SO 4 2-, the two important markers of the biomass burning was observed during winter over the same location in our earlier study carried out in 2005 (Chatterjee et al., 2010).Also, very frequent and persistent thermal inversion and fog situations at ground level caused a considerable amount of aerosol to accumulate in the lower layers of the atmosphere during winter.

DIURNAL VARIATION OF AEROSOL
Figs. 5(a)-5(f) shows the diurnal variations of the particle concentrations of different sizes in different seasons.

Ultrafine and Superfine Aerosols: Effect of Valley Wind and Local Anthropogenic Activities
It is seen from the figure that PM UF shows higher concentrations during morning hours and the initiation of its increase took place at ~0900 hrs LT during premonsoon whereas during winter it took place at ~1000 hrs LT.These correspond to the shift in wind direction at the station from a weak down-slope northeasterly breeze (prevailing at night) to a strong up-slope south and southwesterly wind (prevailing during the day).Thus the onset of PM UF increase coincides with the advection of valley air masses to the measurement station by up-slope valley wind.These valley air masses could bring the precursor gases necessary for the formation of ultrafine aerosols.Sampling days have been divided into sunny/non-cloudy clear skies and cloudy skies (based on solar irradiance data) in order to understand the effect of solar irradiance on ultrafine particle formation at this station.We observed that particle formation was inhibited by cloudy conditions and favored by sunny conditions.The frequency of particle formation under sunny condition was found to be as high as 0.66 whereas under cloudy condition it was as low as 0.09 during the period 0900-1000 hrs LT at this station.The studies on the effect of other parameters like condensation sink etc. on the particle formation were beyond our scope during this study.
The diurnal variation of PM SF showed peak during late morning hours until late afternoon during premonsoon whereas no such significant diurnal changes have been observed during winter.However winter shows a slight increase of superfine particle concentrations during 1900-2300 hrs LT.The peak during premonsoon could be due to the particles transported from the long distances (as discussed earlier) and also from the lower altitude plain land regions driven by the strong up-slope valley wind.The higher temperature and higher solar irradiance during late morning to afternoon in premonsoon (Fig. 2) favored the convective activity which in turn increased the boundary layer height.Thus the top of the boundary layer could have the probability to reach the sampling station and thus our sampling station could collect the aerosol particles within this layer.This feature has not been observed during winter.The particles emitted from the biomass burning could play the role in the slight increase of the superfine particle concentration at night (1900-2300 hrs LT) during winter.
The superfine particles could get transported directly from the low land regions driven by valley wind whereas ultrafine particles do not get transported by the valley wind and only the precursor gases could get transported and finally the formation of ultrafine particles from those precursor gases depends on the meteorological parameters like higher solar radiation.The day-time peak in PM SF concentration in premonsoon was thus observed even at non-sunny cloudy days, unlike PM UF .Hence we can say that PM UF is the phenomena of formation over Darjeeling depending on meteorology and this formation is triggered by the valley wind and not transported by the valley wind.On the other hand, PM SF is the phenomena of transport from long distances and low land stations at the foothill of the Himalaya.

Fine Mode Aerosols: Local Anthropogenic Activities and Boundary Layer Dynamics
In all seasons, fine mode aerosol showed two prominent peaks during morning and evening hours.The morning peak was found during 0800-1100 hrs LT whereas the evening peak was found during 1700-1900 hrs LT irrespective of the seasons.The morning and evening peaks could be attributed to the peak traffic hours.After the morning peak, the aerosol concentration decreased and found to be minimum during 1300-1500 hrs LT.It again peaked up from around 1600 hrs LT reaching maximum around 1700-1900 hrs LT.These diurnal variations of the particle concentrations are consistent with dynamics of the atmospheric boundary layer.There is negative temperature gradient in atmosphere up to end of troposphere where it gets inverted.Due to this negative temperature gradient, polluted air upwards up to temperature inversion layer present at the end of troposphere whose height keep on rising with the advancing of day (Lal, 1997).This height is characteristic of place and other atmospheric conditions like snow, clouds, sunshine, humidity, temperature of environment, etc.By late afternoon, this inversion layer rises to its maximum height.After sunset, cold surface of the earth leads to lowering of this layer and with advancing of night the nocturnal inversion layer reaches near the earth's surface.The rise of particle concentration in morning rush hours is due to increased vehicular rush and low height of inversion layer.As the day advances, the inversion layer deforms and rises to greater heights till evenings provided day is warm while traffic rush also becomes slightly low by afternoon.Both these factors lower the particle density by afternoons.By the evening, the earth's surface starts becoming cold and so the mixing layer drifts downwards slowly.The traffic rush slightly increases as people from office, school, tourist spots etc., return.Both these factors again slightly raise the particle concentration during evening.But after the evening peak, premonsoon and winter showed different behaviour.During winter, the aerosol concentration after the evening peak was found to decrease slowly and steadily and the night-time aerosol concentration was found to be higher than that in premonsoon.During winter, massive biomass burning at night could play the significant role in the higher loading of fine mode aerosol than premonsoon.

CONCLUSION
Close examination of the above analysis yields the following results: 1.The ultrafine and superfine particles (less than 1.0 µm) showed higher concentrations during premonsoon compared to winter over the years 2008, 2009 and 2010.
On the other hand, the fine mode aerosol (1.0-2.0 µm) concentrations were found to be comparable during premonsoon and winter.The ultrafine aerosol concentration over Darjeeling at eastern Himalaya was found to be much higher than the other Himalayan stations in India and Nepal and also than the several high altitude stations in Europe.2. The ultrafine aerosol (0-0.32 µm) formation over Darjeeling was governed by several micro-meteorological parameters.The sunny and non-cloudy days with lower relative humidity and higher solar radiative fluxes were found to favor the formation whereas the cloudy days were found to inhibit the formation of these particles.The initiation of the formation of these particles was observed during 0900-1000 hours LT during the study period.3. The superfine aerosols (0.32-1.0 µm) were mainly the long-range transported aerosols from distant sources.During premonsoon, the sharp diurnal variation of superfine particles was observed with the peak during late morning till late afternoon.High wind speed favored the loading of these particles in the atmosphere by upslope valley wind.Along with these particles, the upslope valley wind might have carried precursor gases necessary for the ultrafine particle formation, but finally the formation depended on the meteorological conditions.Thus it can be concluded that ultrafine aerosols were triggered by the valley wind and not transported by the valley wind whereas superfine particles were mainly the transport phenomena.4. Fine mode aerosols were mainly the locally generated anthropogenic aerosols.During premonsoon, these particles were emitted mainly from the vehicular sources (due to tourist influx in premonsoon, being peak tourist season) and during winter, these particles were emitted mainly from the biomass burning.The diurnal variation of fine mode aerosols showed two prominent peaks during peak office hours in morning and evening.In addition to this, the minimum concentration during afternoon and slightly higher night-time concentrations during winter were due to the thermodynamic conditions and the dynamics of the planetary boundary layer.
Overall, our study provides the first long-term analysis of fine and ultrafine aerosol concentrations over eastern Himalaya which could be used to validate the predictions of regional and global aerosol and climate models.This would enable us to check whether and to what extent these models would match with the observed seasonal and diurnal cycles of aerosols especially over the high altitude Himalayan regions.This, in turn, will help us to improve and reduce the uncertainties in those aerosol and climate model projections.The study will also help to better understand its implication on aerosol dispersion, because particle size distributions bear a fingerprint of the related source and removal processes.

ACKNOWLEDGEMENT
Authors would like to thank Science and Engineering Council, Department of Science and Technology, Government of India for supporting the study under IRHPA (Intensification of Research in High Priority Areas) scheme.Thanks are also due to Mrs Yasodhara Yadav and Mr Bhaskar Roy for their consistent support during sampling and several analytical works related to this study.Authors would also like to thank Mr D.K. Roy for his overall logistic support.

Fig. 1 .
Fig. 1.Map showing geographical location of Darjeeling at eastern Himalaya, India.The sampling station has been shown in red circles.

Fig. 4 .
Fig. 4. Aerosol Index from TOMS over India during (a) April and (b) May in 2008.