Effect of Asian Dust Storms on the Ambient SO 2 Concentration over North-East India : A Case Study

Ambient SO2 concentration at a high rain fall site, Shillong (25.67 ̊N, 91.91 ̊E, 1064 m ASL), located in North-East India, was measured during March 2009 and January 2010 with the aim to understand the effect of long range transport of pollutants from North-East Asia on the ambient SO2 levels at this relatively clean site. The concentrations recorded during the former sampling period were very high (Max: 262.3 ppb)—which decayed down gradually towards the end the sampling period—whereas those during the latter sampling period were well within the acceptable limits (Max: 29.7 ppb). This elevated SO2 concentrations during March 2009 is proposed to have association with a major cold air outbreak and an associated cyclone preceding one of the dust storm events reported in China, and a resultant sudden change in wind trajectory leading to the long range transport of pollutants to the sampling site. The argument is formulated on the basis of the back trajectory analysis performed using HYSPLIT for the month of March 2009—the plots clearly showed a drastic change in wind trajectories between 8th and 15th of March 2009 wherein the winds traveled over some of the highly polluted regions such as the Perm region of Russia—and on the results from model runs performed using the global 3-D model of tropospheric chemistry, GEOS-Chem (v8-03-01)—it clearly showed the tropospheric SO2 over Perm region in Russia peaking during Nov, Dec, Jan, Feb and Mar every year, possibly due to central heating. The observation of long range transport of SO2 from the highly industrialized areas of Perm in Russia to North-East India during dust storm events has important implications to the present understanding on its relative contribution to the Asian pollutant outflow to the Pacific during spring as the GEOS-Chem model runs also showed regions in and around Russia with relatively high concentrations of atmospheric NOx, Peroxyacetyl Nitrate, Lumped Peroxypropionyl Nitrate, HNO3, HNO4, C3H8, C2H6, SO4, NH4, Inorganic Sulphur Nitrates and Lumped Alkyl Nitrate.


Introduction
Asian region is fast evolving as one of the major energy consuming regions in the world for a rapidly growing population and emerging economy [1], projected mega cities in Asia as one of the large emission sources of SO 2 .As per IPCC 2007, there has been a regional shift in the emissions of SO 2 from US and Europe to South-East Asia, with implication to shift in atmospheric radiative forcing patterns for these regions [2][3][4].In this scenario, monitoring ambient SO 2 concentration coupled with wind trajectory analysis in the North-East region of India -characterized by high rain fall-has special significance to assessing the role of long range transport from North-East Asia in controlling the pollutant characteristics over this region.With this objective ambient SO 2 concentrations were measured during March 2009 and January 2010 at a high rainfall site in North-East India, Shillong (25.67˚N, 91.91˚E, 1064 m Above Sea Level (ASL)).The sampling site (25.67˚N,91.91˚E, 1064 m ASL) is located about 18 km away from the Shillong city, located on the Shillong Plateau which is an outlier of the plateau of peninsular India and is composed primarily of ancient rocks.The highest point is Shillong peak, at 1961 m ASL located 5 km south of the city of Shillong.It is on the Shillong Plateau, the only major pop-up structure in the northern Indian shield.Due to its latitude and high elevation Shillong has a sub-tropical climate with mild summers and chilly to cold winters.The region is subject to vagaries of the monsoon.The average annual rain fall is about 2200 mm.The monsoon arrives in June and it rains almost until the end of August.Perched at an altitude of 1521 m ASL, the Shillong city stretches for about 6 km on an elevated tract.The city is situated on a plateau bound on the north by the Umiam gorge, on the northwest by the great mass of the Diengiei Hills that rise up to a height of 1852 m ASL, and on the northeast by the hills of the Assam valley.The proximity of this high rainfall site to some of the highly polluted regions in North-East Asia and hence the possibility of long range transport makes this study location strategically important.

Experiment: SO 2 Monitoring System
The experimental set up comprised of a primary UV fluorescence SO 2 Monitor (Thermo-43i Trace Level Enhanced (TLE)), an air compressor for producing zerolevel (zero) gas and a dynamic gas calibrator (Thermo-146i) for preparing diluted SO 2 standard gas.Using pulsed fluorescence technology, the Model 43i-TLE measures the amount of sulfur dioxide in the air with an instrumental detection limit of 50 pptv (300 second averaging time).The UV fluorescence SO 2 monitor was calibrated onsite every 10 days with standard SO 2 gas (2 ppmv with N 2 balance gas, Spectra, USA) and zero air.Five minute averaged SO 2 raw data was accumulated in the SO 2 analyzer which in turn was periodically downloaded and fed to a computer through an ethernet port.Discussions on the performances of similar systems can be seen elsewhere [5][6][7].

The HYSPLIT Trajectory Model
The HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) model, developed by NOAA ARL [8,9], was used for the back trajectory analysis for this work.HYSPLIT uses a modeled vertical velocity scheme; it can depict the vertical motion of the relevant air parcel.The model computes the advection of a single pollutant particle, or simply its trajectory.

General Description
The GEOS-Chem global  [10] and [11].The detailed descriptions of the GEOS-Chem aerosol emission inventories and simulation evaluations are provided in [11] and [12] for sulfur and ammonia, in [13] for dust, and in [14] for recent updates on emission inventories over the China region.The simulations were conducted for January-December 2009 at 4˚ × 5˚ resolutions.They were initialized on 1st January 2009 using GEOS-Chem fields generated by a one year spin-up simulation with 4˚ × 5˚ resolution.

Emission Inventories Included in the Model Runs
In the Model runs, the following emission inventories were used: 1) The EMEP-the Co-operative Programme for Monitoring and Evaluation of the Long-range Transmission of Air Pollutants in Europe-inventory for 2000 [15].
3) The global sulfur emission based on the Emissions Database for Global Atmospheric Research (EDGAR) inventory [17].
The bio-fuel emissions and biogenic emissions also were included in the model runs with the Model of Emissions of Gases and Aerosols from Nature (MEGAN) v2.1 [19] inventory used for the latter.

GAMAP Visualization Toolkit
The Global Atmospheric Model Analysis Package (GA-MAP) (Version 2.15; http://acmg.seas.harvard.edu/gamap/) is used for plotting the output from the GEOS-Chem runs.GAMAP is a self-contained, consistent, and user-friendly software package for reading and visualizing output from Chemical Tracer Models (CTM's) and consists of a suite of routines written in IDL (Interactive Data Language).It can produce line plots, 2D plots, 2D animations, or 3D iso-contour surface plots and can read 2D, 3D or 4D data blocks.
March 2009 and January 2010 at the high rainfall site, Shillong (25.67˚N, 91.91˚E, 1064 m ASL) in North-East India, using the pulsed UV fluorescence analyzer.During the measurements the inlet of the SO 2 analyzer was maintained at an altitude of about 15 m above the ground, while the analyzer was housed in an air conditioned room.The integration time for the analyzer was set at 5 minutes.An Automated Weather Station (AWS) located within a distance of 1/2 km from the sampling site was employed to monitor the various atmospheric parameters such as the wind speed, wind direction, relative humidity, atmospheric pressure etc, throughout the sampling period.

Major Features during March 2009
Figure 1 reveals the temporal changes of the SO 2 concentrations at Shillong, during March 2009.The concentration levels recorded during the initial days of the sampling were very high (Max: 262.3 ppb) which decayed down gradually towards the end the sampling period.
This observed high SO 2 concentrations is explained as due to a transient long range pollutant transport from North-East Asia associated with the cold air outbreak preceding the dust storm events reported in China [20] on 14th March 2009 and in North-East India [21] on 17th March 2009.The former [20], using MODIS 1B data of relevant regions, showed evidences for the dust storm in western Gansu and Neimenggu in China while the latter [21], based on Terra/Aqua MODIS data, reported the dust storm over Guwahati, North-East India.Based on the mesoscale model (MM5) derived wind speed directions at 850 hPa overlaid on sea level pressure on 17th March 2009, [21] found a persistent North-Easterly flow with high wind speed (~6 m/s) over the region resulting in mobilization and lifting of dust particles in to the atmosphere.They analyzed NCEP temperature/relative humidity (RH) anomalies variations and found ~0.6˚C increase in surface air temperature and ~-4% reduction in RH during March 2009, which they said has resulted in dry conditions over the region.They attributed the very high value of Terra MODIS AOD 550 (~1.3) along with lower value of Alpha (~0.78) to coarse mode dust aerosol particles over the region due to dust event occurred on 17th March 2009.
The above argument of long range transport as the mechanism responsible for the observed high SO 2 values at the North-East Indian sampling site is based on the results provided collectively by the back trajectory analysis performed using HYSPLIT for the sampling period and the model runs performed using the Chemical Transport Model GEOS-Chem.The arguments are further supported with the local meteorological parameters measured using nearby Automated Weather Station (AWS).The time series Mineral Dust Optical Depth (MOPD) profiles created using GEOS-Chem (discussed in the following sections) for the month of March 2009 also gave evidence for the dust storm movement reported in China on 14th March 2009.

Source Identification Based on HYSPLIT
Trajectories The 7 days back trajectories beginning from the sampling site were generated (Figure 2) using HYSPLIT for whole the sampling month, with one trajectory plot made each on 1st, 8th, 15th, 22nd and 29th of March 2009.The trajectories computed to identify the source regions were for altitudes 500 m, 1000 m and 4000 m Above Ground Level (AGL).The plots showed clearly that between 8th and 15th of March 2009 there was a drastic change in wind trajectories.During this period the winds which were traveling in the eastward direction suddenly got bend and started traveling towards North-East till it reach ~60˚N and from there it again got bend and started traveling towards the south to reach the sampling site.It was in line with this observed sudden change in the wind trajectory (between 8th and 15th of March 2009), the high SO 2 concentrations were recorded.The back trajectories plotted for the following weeks again showed wind patterns similar to the ones persisted prior 8th March 2009.The observed decay in concentration levels during the days succeeding the transient long range pollutant transport were in line with this normal trajectory pattern.

Source Identification Using GEOS-Chem Model
The GEOS-Chem model (v8-03-01) was run from 1st January 2009 to 31 st December 2009, after spinning up the model fields for one year, and the output averaged for the first half of each month was generated.The results were then plotted using GAMAP to find out the possible 3.1.1.2.1.The Perm City, Russia Perm, projected in this paper as the source region for the high SO 2 containing air parcel detected at the sampling site, is a city in the eastern part of Russia, near the Urals Mountains.It is a major center of heavy industries, chemical industries (varnishes, paints, fertilizers, sulfuric acid, etc.), petrochemical and oil refining industries, defense production, timber and wood processing industries, food industry and so on.This regional area has substantial deposits of natural resources (oil, gas, potassium, magnesium salt, precious and jobbing stones, gold and chromium ores and so on.)which is the base for both the extractive industries and related economic sectors of the region.The region of Perm has one of the richest mineral resource bases in Russia and has many mines.
The Perm region is known for its long frosty, snowy Ural winters.However, the region's severe continental climate causes abrupt seasonal changes, from a cold winter followed by a mild, usually warm spring and then by a hot summer.The cold winter necessitates central heating especially during the months November, December, January, February, and March.The general precipitation trend over Perm region shows lowest precipitation events during the months February and March followed by slightly higher precipitation events in April.

Anomalies Detected in Local Meteorology
The Automated Weather Station (AWS)-located within 500m from the sampling location-provided meteorological parameters such as wind speed & wind direction (Figure 3), relative humidity (Figure 4) and atmospheric pressure (Figure 5) throughout the month of March 2009 except for a small non-operational period from 20 -23rd March 2009.Statistically the wind direction mostly remained South-East with second directional preference for South-West.The wind speed showed a transient increase between 9th and 13th of March 2009 with the maximum speed recorded of 3.2 m/s.The relative humidity (RH) remained more or less on the higher side throughout the sampling period with a small dip recorded between 10th and 12th followed by two spikes each on 13th and 14th.The highest RH value observed for the month was on 17th with the peak relative humidity value recorded of 99%.The Atmospheric pressure also kept unusually fluctuating with a dip observed between 3rd and 7th (lowest value 891 hPa) and a hump observed between 12th and 15th of March 2009 (Max value 903.3 hPa).

Dust Storm Detection Using GEOS-Chem
To visualize and understand the source and the trajectory of the dust storms reported [20] in China on 14th March 2009 and over North-East India [21]

Cold Air Outbreaks and Dust Storms in Asia
It is known that Asian dust storms, especially those of Chinese origin occur during spring [22], when the climate over East Asia reaches its maximum dryness of the year, the surface start warming, and strong winds sweep over the desert areas leading to highly unstable synoptic systems.The percentages of dust storms generally occurring in China during March, April, and May are 20%, 58%, and 22%, respectively [23].
Asian dust storms are always associated with cold air outbreaks, resulting in the Mongolian cyclonic depression and frontal system.It is known that polar front causes mid-latitude jet stream (by the thermal wind relationship) with troughs and ridges to lead to patterns of divergence aloft.This upper air disturbance causes a significant pressure decrease in the lower atmosphere as a result of cyclogenesis, leading to the development of a low pressure system with low central sea level pressures which continue to deepen as time progress to cause cyclone and a resulting dust mobilization.
The reason the sampling was planned during this season at this North-Eastern Indian site was to understand the possible effects of such cold air outbreaks and the associated atmospheric disturbance preceding the Chinese dust storms in altering the atmospheric pollutant transport pattern to North-East India.
To visualize this air disturbance associated with the cold air outbreak occurred during the above sampling period, trajectory matrices were plotted on 8th, 11th, 15th, 22nd and 29th of March 2009 with source at multiple locations (Figure 7).The plots clearly indicated a major atmospheric disturbance between 8th and 15th of March 2009.

Transport from Russian Region: Implication to the Asian Pollutant Outflow
The observation that SO 2 got transported from the highly industrialized areas of Perm in Russia to North-East India during the dust storm event in March 2009 has important implications to its share in the Asian pollutant outflow to the Pacific during spring, every year.To make this picture clear, 7 day back trajectories starting at coordinates (18˚N, 130˚E) and (22˚N, 110˚E), (Figure 8) were plotted which showed the air parcel from Russian region reaching the Pacific during the dust storm days.
Based on these observations, it is projected here that the contribution from this region is non negligible while assessing the impact of Asian pollutant outflow, as the Perm region is characterized by a wide range of industries.
The GEOS-Chem generated SO 2 plots for every month (averaged over the first 15 days) plotted for the whole year 2009 (Figure 9) clearly showed the tropospheric SO 2 over Perm region peaking during November, December, January, February and March every year, possibly because of central heating.The lesser amount of precipitation in the Perm region during the months February and March result in longer atmospheric retention time for these pollutants during these months.The concentration levels go decreasing when there is increase in precipitation events starting from April along with the receding winter to reduce the central heating requirements.
The model runs were also performed to find out the concentration levels of other major pollutants over this region during March 2009.It was found that regions in and around Russia has relatively high concentrations of atmospheric NOx, Peroxyacetyl Nitrate (PAN), Lumped Peroxypropionyl Nitrate (PPN), HNO 3 , HNO 4 , C 3 H 8 , C 2 H 6 , SO 4 , NH 4 , Inorganic Sulphur Nitrates (NIT), and Lumped Alkyl Nitrate (R 4 N 2 ) (Figure 10).
The model runs also showed (figures not included here) that similar to SO 2 , these pollutants also peak during November, December, January, February, and March every year, again possibly has association with central heating and climatology discussed above.But the fact that cold air outbreaks preceding the dust storm events in Asia occur only during March, April and May makes the relative contribution from this region significant only during March, when the atmospheric pollutant peak for the region overlaps with the dust storm months (March, April, May).So it suggest that during the dust storm events in March, the contribution from regions in and around Russia may not be neglected while considering the impact of Asian pollutant outflow to the Pacific, which becomes less significant during cold air outbreaks and the associated dust   The model runs also showed (Figure 11) that the pollutants N 2 O 5 , NH 3 , hydrophilic black carbon, hydrophilic organic carbon, hydrophobic black carbon and hydrophobic organic carbon are present only in smaller levels in the Perm region compared to the other known polluted regions in North East Asia, and so are not expected to contribute significantly to the Asian pollutant outflow.

Major Features during January 2010
Unlike March 2009 the SO 2 concentrations recorded during January 2010 (Figure 13) were well within the acceptable limits, with maximum value going up to 29.7 ppb only and a minimum value recorded of 1.94 ppb.The wind back trajectory analysis (Figure 14) showed that the winds were coming from West Asian region and had no component coming from North-East Asia ruling out the possibility of any long range transport from Russia.So the concentration levels recorded may be taken as the back ground SO 2 values for this North-Eastern Indian site.

SO 2 Time-Series Data (January 2010): Model Versus Observations
In the case of January 2010, the time series output from the model (Figure 15) matched more or less with the observations both magnitude wise and to certain extend shape wise.The diurnal variation observed in the experimental data is reproduced in the model generated time series output.

Conclusions
Ambient SO 2 concentrations at Shillong (25.67˚N, 91.91˚E, 1064 m ASL), a high rain fall site located in North East India, measured during March 2009 and January 2010 showed significant differences in magnitude, with concentrations going to a maximum of 262.3 ppb during the former sampling period-which decayed down gradually towards the end the sampling periodagainst the maximum value recorded during the latter period of 29.7 ppb.This elevated SO 2 concentrations during March 2009 is explained as due to a long range transport associated with the cold air outbreak preceding one of the dust storm events reported in China.This arument of long range transport for the observed high SO 2 g

Figure 1 .
Figure 1.The measured SO 2 time series profile during March 2009 at the sampling site, Shillong.

Figure 2 .
Figure 2. The 7-day back trajectories originating at the sampling site plotted using HYSPLIT for March 2009.SO 2 source regions along the back trajectories obtained between 8th and 15th of March 2009.The plots clearly showed that many of the regions over which the winds traveled during this period are characterized by high concentrations of SO 2 .Out of these the Perm region in Russia has the highest SO 2 concentration in the GEOS-Chem generated plots.

Figure 3 .Figure 4 .Figure 5 .
Figure 3. Wind speed and wind direction during the sampling period, obtained from nearby AWS.
on 17th March 2009, the GEOS-Chem model was run with the inventories discussed previously, and generated the time series Mineral Dust Optical Depth (MOPD) data for the month of March 2009.The plots generated from these data for the days 10th to 20th of March 2009 (Figure 6) clearly showed this formation and the movement of the dust storm.The plots also showed high MOPD values for the Indo-Gangetic Plane also during 13th and 14th March 2009.A second phase of high MOPD values for North-East India started on 17th March 2009 which remained high in the following few days (18th, 19th and 20th March 2009).

Figure 8 .
Figure 8.The 7-day back trajectories showing the air parcel from Russian region reaching the Pacific during the dust storm events.

Figure 9 .
Figure 9. SO 2 profile averaged for the first half of every month for 2009 from GEOS-Chem model runs showing the SO 2 peaking over the Perm region in Russia, during the months November, December, January, February and March.

3. 3 .Figure 12 Figure 12 .Figure 13 .
Figure12shows the time series SO 2 profile obtained for the nearest co-ordinates (26˚N, 90˚E) from the GEOS-Chem model runs for the month of March 2009.The model was not able to pick up the magnitude of this major transport event even though a slight increase in the

Figure 14 .
Figure 14.The 7-day back trajectories originating at the sampling site plotted using HYSPLIT for January 2010.