Trans-Boundary Volcanic SO 2 Detected over Pakistan from Satellite Observations during the Time Period 2004 – 2012

This study emphasizes on the amount of trans-boundary Sulfur dioxide (SO2) present in the atmosphere of Pakistan as a consequence of various global volcanic eruptions, by using satellite data. The data products of SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY), Ozone Monitoring Instrument (OMI) and Global Ozone Monitoring Experiment-2 (GOME-2) were used for the time period of 2004–2012. SO2 columns retrieved with Differential Optical Absorption Spectroscopy (DOAS) technique were used to perform both spatial and temporal analyses. The Nabro volcano eruption during 2011 had caused high SO2 columns over East Africa, Middle East and South Asian regions. Daily satellite observations were used to study SO2 plume pathway during this event. Other significant volcanic eruptions and their effects on atmospheric composition of Pakistan are also discussed. Back trajectory analysis is also performed to track the origin of air masses enriched with SO2 column densities detected over Pakistan. Maximum SO2 column densities of 9.4 Dobson Units (DU) were measured over Pakistan caused by Dalafilla volcanic eruption during November 2008.


INTRODUCTION
Atmospheric sulfur dioxide (SO 2 ) results from both natural and anthropogenic sources (Kettle and Andreae, 2000;Halmer et al., 2002;Vijay et al., 2004;Dentener et al., 2006;Lee et al., 2008).Besides anthropogenic activities (fossil fuel combustion, metal smelting and biomass burning etc.), it is produced naturally from volcanic activity, oxidation in the soil, and the oxidation of hydrogen sulfide.Volcanic eruptions are sporadic and major contributor, emitting 7.5-13 Tg SO 2 per year (Andres and Kasgnoc, 1998;Halmer et al., 2002).SO 2 life time varies from few days to several weeks in the troposphere due to its reaction with hydroxyl (OH) and other oxidizing agents (Finlayson-Pitts and Pitts, 2000;Platt and Stutz, 2004) by several homogeneous and heterogeneous processes.The final products are always sulfuric acid and sulfate particles, respectively.In the lower stratosphere lifetime of SO 2 exceeds from several weeks to 2 years (Eisinger and Burrows, 1998;Platt and Stutz, 2004).Tropospheric lifetime of SO 2 allows it to travel larger distances before it is washed-out or transformed into sulfuric acid and/or sulfate particles.Therefore, SO 2 can have impacts on air quality and regional climate.
Sulfur dioxide plays an important role in both tropospheric and stratospheric chemistry, and helps to determine the stratospheric aerosol loading (SPARC, 2006).In the stratosphere, sulfate aerosols provide sites for heterogeneous reactions which can convert chlorine from reservoir to radical forms (Solomon et al., 1996).Sulfate aerosols can also affect air quality and climate by direct and indirect radiative effects (e.g., Graf et al., 1997;Robock, 2000;Zhang et al., 2007).
SO 2 is also one of the criteria pollutants (e.g., Aneja et al., 2001) and a key precursor of acid rain; hazardous to forests and many fresh water ecosystems around the world (Ferrari and Salisbury, 1999).
SO 2 monitoring stations are sparse and monitoring on a global scale through these stations is not possible.Since last few decades, through satellite observations and the advancement of technology, now the atmospheric SO 2 monitoring on large scales has become possible (e.g., Krueger et al., 1995;Eisinger and Burrows, 1998;Khokhar et al., 2005;Carn et al., 2008) with good temporal and spatial coverage.Space borne instruments, for example the Total Ozone Mapping Spectrometer (TOMS), since 1978 in space providing long-term SO 2 record (Krueger et al., 1995), the Global Ozone Monitoring Experiment (GOME), 1996-2002(Eisinger and Burrows, 1998;Khokhar et al., 2005), SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY), 2002-2012 onboard ENVISAT-1 (Bovensmann et al., 1999;Afe et al., 2004;Lee et al., 2008), Ozone Monitoring Instrument (OMI), since 2004-present (Krotkov et al., 2006(Krotkov et al., , 2008)), GOME-2 since 2007-present (Callies et al., 2000;Rix et al., 2012) in addition to hyperspectral Infrared Atmospheric Sounding Interferometer (IASI) launched in 2006 onboard MetOp-A (Clerbaux et al., 2009) have opened new corridors by providing a continuous spatial and temporal analysis of various trace gas and green house gas monitoring around the globe and especially regions with limited or no alternate monitoring facilities.
This study has first time focused on the amount of volcanic SO 2 present in the atmosphere of Pakistan.It emphasizes mainly on spatial and temporal variation in SO 2 column densities over Pakistan during the time period of 2004-2012 as a consequence of trans-boundary SO 2 pollution by using satellite observations.For this purpose SO 2 column densities level-2 data from three satellite instruments: SCIAMACHY, OMI and GOME-2 was used.Differential Optical Absorption Spectroscopy (DOAS) (Platt and Perner, 1979;Van Roozendael, 1999;Khokhar et al., 2005) technique is used to retrieve SO 2 column densities from back scattered UV radiances measured by satellite based nadir-viewing instruments.Both daily and monthly images are used to track the special events of high SO 2 columns detected over Pakistan as consequence of volcanic activity in the neighbouring and remote regions.
Pakistan has no continuous monitoring facility for the monitoring of ambient air quality.Pakistan Environmental Protection Agency (EPA) and Pakistan Council of Scientific and Industrial Research (PCSIR) have the facility of 7 fixed and 3 mobile (Wagon/Trucks with air quality monitoring instruments onboard) air quality monitoring stations with intermittent and rare measurements in few cities of Pakistan.According to report from economic survey of Pakistan 2012-2013, all of the air quality monitoring stations are non functional since year 2010 (ESoP, 2013).
A recent study (Khattak, 2013) has indicated temporal increase of 70-120 per cent (statistically significant) in the measured SO 2 column densities during the time period of 2004-2012 by using SCIAMACHY observations.Such studies are mandatory and play vital role in providing data/information about air quality of a region that has little or no air quality monitoring facilities.

Data
Level-2 data from SCIAMACHY, GOME-2 and OMI instruments was used to evaluate high SO 2 columns during the time period of 2004-2012.Data was downloaded from the Website (http://temis.nl/) of tropospheric emission monitoring internet service (TEMIS) project.All of these satellite instruments have various similar features like they orbit the earth in Sun synchronised orbits and SO 2 column densities are retrieved from UV back scattered radiances measured in nadir view and by same DOAS method.They also differ in various aspects of different overpass time, spatial and spectral resolutions as listed in Table 1.The SCIAMACHY (Bovensmann et al., 1999;Afe et al., 2004) a multi-channel grating spectrometer onboard Environmental Satellite -1 (ENVISAT-1) by European Space Agency's (ESA), was launched on 1 March, 2002.It is functioning since August 2002, with a resolution of 60 × 30 km 2 .The OMI instrument was launched on Earth Observing System (EOS) on 15 July, 2004.It has a spatial resolution of 13 × 24 km 2 at nadir (while 13 km × 128 km for the extreme viewing angles at the edges of the swath) and it provides daily coverage on global basis (Krotkov et al., 2006;Levelt et al., 2006).The GOME-2 instrument was launched on October 2006.It has been onboard the Meteorological Operational satellite-A (MetOp-A), with a resolution of 80 × 40 km 2 (Rix et al., 2009(Rix et al., , 2012)).Details about the retrieval algorithms used to retrieve SO 2 column densities used in this study are not presented here just to avoid the repetition as this has been described in various studies (e.g., Afe et al., 2004;Khokhar, 2006;Krotkov et al., 2006;Levelt et al., 2006;Lee et al., 2009;Yang et al., 2010;Rix et al., 2012;Fioletov et al., 2013, Theys et al., 2013).
The retrieval of the SO 2 slant column is affected largely by strong absorption by ozone (O 3 ), therefore, both SO 2 and O 3 should be taken into account in the retrieval process to avoid/minimize spectral interference.This "interference" 1 at nadir and 13 km ×128 km for the extreme viewing angles at the edges of the swath.
2 OMI covers only wavelengths < 500 nm.when concentrations of SO 2 are low, may produce negative SO 2 slant column values, with an error of the same magnitude.This then represents the SO 2 background level, i.e., the apparent SO 2 absorption in the absence of emissions of SO 2 .This is further dependent on solar zenith angle (SZA) and more prominent for higher SZA due to increased slant light path and larger horizontal ozone concentrations.For low SZA, i.e., at low and mid latitudes, the interference between SO 2 and O 3 absorption results in less negative SCD (low back ground level) and errors.Therefore, emissions of SO 2 (by pollution or volcano eruptions) can be detected against this background.This effect is corrected by applying background correction method used by various studies (e.g., Khokhar et al., 2005;Lee et al., 2009) An air mass factor (AMF) is mandatory for the conversion of the retrieved slant column-integrated along the light path, into a vertical column densities (VCD-generally expressed in Dobson unit -1 DU = 2.69 × 10 16 molecules/cm 2 ).The SO 2 AMF depends strongly on surface albedo, clouds, aerosols optical properties, the vertical distribution of SO 2 , and the ozone column (Khokhar et al., 2005;Thomas et al., 2005;Khokhar et al., 2008;Krotkov et al., 2008;Lee et al., 2009).
In case of volcanic eruption, SO 2 AMF calculation is strongly dependent on the SO 2 plume height.In general, there is no information available on the altitude of the SO 2 cloud, so an assumption must be made.Therefore, VCD were computed for three different assumed plume heights 2 km for Low AMF, 6 km for Middle AMF and 14 km for High AMF, with an assumed plume thickness of 1 km (For further details see documents available at TEMIS website http://sacs.aeronomie.be/info/vcdamf.php).
HYSPLIT trajectory model was used to have first guess of SO 2 cloud height and then VCD were computed by using the respective AMF (middle or High) during days of volcanic activity.Finally, monthly mean VCD were extracted over Pakistan.The retrieved SO 2 VCD over Pakistan from SCIAMACHY observations still experienced some negative values (see Figs. 4,6 and 8) which are mainly due to remaining error caused by imperfect removal of ring effect and/or spectral interference with ozone (e.g., Khokhar et al., 2005Khokhar et al., , 2006;;Wagner et al., 2008;).The SO 2 retrieval error from SCIAMACHY data is larger as compared to that from OMI and GOME-2 data sets.

Spatial Mapping
In order to investigate spatial distribution of SO 2 column densities, ArcGIS 10.1 software was used to extract data over Pakistan from global observations.For the statistical analysis of temporal trends and regression analysis, Partial Autocorrelation Function (PACF) statistical tools were used.

TRANS BOUNDARY VOLCANIC SO 2 POLLUTION OVER PAKISTAN
Temporal record of SO 2 column densities over Pakistan was prepared by plotting monthly average SO 2 variation during the time period of year 2004 to 2012.Analysis presented in Fig. 1 shows enhanced SO 2 column densities observed on various occasions during this time period.
To evaluate the strength of temporal increase in SO 2 columns over Pakistan, a trend analysis was performed by performing linear fits.The blue line represents the trend of SO 2 over Pakistan including the volcanic eruptions, whereas the red line represents the trend excluding the volcanic events.Analysis showed 120 percent increase in SO 2 with volcanic eruptions and 70 percent increase in SO 2 without volcanic eruptions over the time period of 2004 to 2012 (Khattak, 2013).Regression analysis indicated statistically significant data with p-value < 0.03.Further investigations related these high SO 2 column densities detected over Pakistan to various volcanic activities (Red arrows in Fig. 1 and indicated by triangles in Fig. 2) within Pakistan and its neighbouring regions/continents.
In Pakistan, only a historical fissure vent volcano named 'Tor Zawar' was active with minor seismic activity but no gaseous emissions were reported (GVP, 2013).It is located   2 and presented in Fig. 2. It also describes the volcano type and status.According to volcano database of GVP, both in Iran and Afghanistan there are 11 Holocene volcanoes and one historical stratovolcano in India.None of these volcanoes experienced a significant volcanic activity during the past decade, therefore, did not contribute in the observed levels of SO 2 column densities over Pakistan.
Pakistan (33°40′N and 73°10′E) is situated in meteorological zone called subtropical high region (Ahrens, 2009).Around these latitudes, not all of the surface air moves equator ward.Some of air moves toward the poles and deflects toward the east due to Coriolis force, resulting in a more or less westerly air flow called the prevailing westerlies or simply, westerlies (Ahrens, 2009) depending on the location/movement of inter-tropical convergence zone (ITCZ).Therefore, this region is largely affected by the movement of both inter-continental and regional air masses coming from Middle East and African continent.Volcanic eruption occurring in the African region may contribute and affect the atmospheric composition of Pakistan through these westerlies.In the following sections the volcanic eruptions which have substantially contributed to the SO 2 column densities over Pakistan are discussed.

Jebel at Tair Volcano (15.55°N, 41.83°E) Red Sea, Yemen
Jebel at Tair, a stratovolcano is located in the Red sea.According to GVP's monthly report, the eruption started on the afternoon of 30 September, 2007 and continued until December, 2007.The eruption caused large SO 2 plumes, directed lava flows into the ocean and also caused the death of soldiers on the island (GVP, 2013).

Nabro Volcano (13.37°N, 41.70°E) Eritrea, North-Eastern Africa
SO 2 datasets exhibited abnormally high column densities during June, 2011 over Pakistan.It was most significant over the northern and central parts of Pakistan as identified from Fig. 7.The reason behind such high SO 2 columns was investigated.It was found that the Nabro volcano eruption in Eritrea has caused these high SO 2 columns densities over Pakistan.Nabro is a stratovolcano in the north-eastern African nation of Eritrea.The eruption started explosively after a series of earthquakes on the evening of 12 June, 2011 (GPV, 2013) and continued till the mid of July, 2011.It produced the highest SO 2 column ever retrieved from space (Fee et al., 2011;Carn et al., 2012), for instance Clarisse et al. (2012) reported 3700 DU retrieved from daily IASI observations.Nabro volcano eruption spread large amount of SO 2 and ash drifting over much of Eastern Africa, Middle East and South Asian regions.Fig. 7 shows GOME-2 (Figs. 7(A

BACK TRAJECTORY ANALYSIS
In order to verify the trans-boundary SO 2 plume over Pakistan, backward trajectory analysis of air masses (Robinson et al., 2011) was performed to track the origin of air masses enriched with SO 2 concentration detected over Pakistan.Maps consisting of backward trajectory were created for each volcanic event, for the identification of SO 2 source and its pathway through which it entered Pakistan's boundaries.Hybrid Single Particle Lagrangian Integrated Trajectory Model-4 (Draxler and Rolph, 2013;HYSPLIT4-Draxler et al., 2013;Rolph, 2013) is an online available trajectory model.HYSPLIT model used for this purpose is developed by National Oceanic and Atmospheric Administration (NOAA)/Air Resources Laboratory (ARL).The meteorological data used for the analysis was from Metrological variables such as potential temperature, rain fall, mixed layer depth, and relative humidity were computed by HYSPLIT.There is a chance of 15 to 30 percent of error (of the travel distance) associated with the trajectories generated by HYSPLIT model (Stohl, 1998;Draxler and Hess, 2004) and it increases with the increasing distance (Sogacheva et al., 2007).For this study, the back trajectory accuracy is sufficient enough to confirm the pathway of the polluted air masses and to determine the altitude of SO 2 plume reaching to Pakistan.

Jebel at Tair Volcano (15.55°N, 41.83°E) Red Sea, Yemen
Trajectory model was run for the backward trajectory of air masses on 4 October, 2007.The trajectory was calculated 120 hours backwards in time and at height 500, 5000 and 15000 meters above ground level (mAGL).Back trajectory showed that only the air masses originated from Ethiopia has caused SO 2 plume over Pakistan.It also indicated that SO 2 plume over Pakistan was present in the upper troposphere/tropopause region.It entered Pakistan from the west side.Air mass average altitude calculated with HYSPLIT is around 15000 mAGL.The trajectory confirmed the pathway of air masses with the pathway and altitude of SO 2 plume reaching Pakistan as shown in the Fig. 9.

Dalaffilla Volcano (13.79°N, 40.55°E) Ethiopia, North-Eastern Africa
Trajectory model simulations for 5 November, 2008 indicated that air masses with large SO 2 column amounts entered from west of Pakistan as shown in Fig. 10.The trajectory was calculated 72 hours backwards in time and at height 500, 4000 and 9000 mAGL.The backward trajectory analysis indicated that air masses have encountered with SO 2 plume in the vicinity of volcano and followed the SO 2 pathway accurately before reaching Pakistan.Other air masses at 9 km above ground level came across and intersected the air masses started from the Arabian Sea, and entered Pakistan on 5 November, 2008.The average altitude of the plume calculated with HYSPLIT is between 4000 and 9000 mAGL.The pathway and altitude of SO 2 plume reaching to Pakistan confirms the trans-boundary origin.

Nabro Volcano (13.37°N, 41.70°E) Eritrea, Northeast Africa
During 16 and 17 June, 2011 the HYPSLIT simulations indicated that SO 2 plumes were hitting Pakistan's boundaries over Northern areas and Baluchistan.The back trajectory of air masses and altitude of SO 2 plume reaching Pakistan is shown in the Fig. 11.According to GVP, initially the plume from volcanic eruption rose to the height of 9100-13700 m (30,000-45,000 ft) above sea level (ASL).Later on 13 to 14 June the plume was detected at the altitude of 6100-10700 m ASL.The trajectory was calculated 120 hours backwards in time and at height of 400, 5000 and 11000 mAGL on 16 June, 2011.On 17 June, trajectory was calculated 144 hours backwards on 400, 7000 and 9000 mAGL.The back trajectory shows that the air mass originated over Bay of Bengal, crossed the volcanic eruption site and from there carried the SO 2 plume to Pakistan.Air masses carrying SO 2 pollution hit Pakistan over two different locations.The average altitude of these air masses calculated with HYSPLIT was around 9000 mAGL.Main objective of using HYSPLIT was to identify the SO 2 plume altitude and track the pathway of air masses reaching over Pakistan during volcanic activity in the African region.

DISCUSSION
Temporal record of SO 2 over Pakistan was analysed by plotting the monthly averaged SO 2 column from 2004 till 2012, as presented in Fig. 1.The bars show the spatial variation (standard deviation) for each month over Pakistan.Satellite database of SO 2 column amounts over Pakistan indicated high SO 2 column densities over cities and/or regions with high population, traffic density and industrial activities.Additionally, anomalous high SO 2 column densities were observed occasionally over northern, western and central parts of Pakistan.It was investigated and these enhanced levels of SO 2 over Pakistan were caused by some of global volcanic activities.For instance, in June, 2011 unexpected high SO 2 columns were observed over Pakistan caused by the Nabro volcanic eruption.It was very explosive and SO 2 rich volcanic eruption (Fee et al., 2011;Carn et al., 2012).For instance Clarisse et al. (2012) reported total SO 2 mass of 1.5 Tg retrieved from daily IASI observations and Carn et al. (2012) estimated 1 to 2 Tg SO 2 based on OMI and Atmospheric Infrared Sounder (AIRS) data.It has caused not only the stratospheric aerosol load (e.g., Bourassa et al., 2012) but also large amounts of SO 2 pollution was detected over neighbouring regions.It has influenced Pakistan's atmosphere with maximum SO 2 of 7 DU detected on 17  -8.However, the effect is not significant in the case of Jebel at Tair volcano as compared to the Nabro and Dalaffilla volcanoes, but it is still identifiable.Another peak in SO 2 time series around January, 2007 is coincident with Tor Zawar volcano from Pakistan.It is difficult to relate this enhancement with Tor Zawar activity, because, none of three satellite instruments were able to detect the SO 2 emissions from this volcano.Although, volcanic activity has been reported by the GVP, but it cannot be speculated without any scientific and solid evidence.In principal whatever the amount of SO 2 was released that became the part of atmospheric composition of Pakistan.It might be due to the reason that amount of SO 2 released was well below the detection limit and/or background SO 2 levels that satellite instruments were not able to detect it over that region.Other enhanced SO 2 levels over Pakistan are observed during the months of March and April 2010.Similarly, 2010 eruption of Eyjafjallajökull volcano (Rix et al., 2012) from Iceland is coincident with enhanced levels of SO 2 over Pakistan during the months of April-May 2010.It has caused a severe disruption to western and northern Europe air spaces.However, none of the satellite instrument could track volcanic gas plume over Pakistan during Eyjafjallajökull volcanic eruption.
In general, SO 2 column densities showed large seasonality due to its reaction with OH radical.Therefore, OH is the key parameter which determines the life time and seasonal cycle in the atmospheric SO 2 abundances .The SO 2 seasonal cycle may also be slightly dependent on both anthropogenic (coal, petroleum products and biomass burning) and natural (monsoon, snow) activities on regional basis.Fig. 1 exhibited a clear seasonal cycle in SO 2 amounts over Pakistan with maximum in winter and minimum in summer season.SO 2 released in the atmosphere is converted into sulfate particles and sulfuric acid (H 2 SO 4 ) through its reaction with O 3 , hydrogen peroxide (H 2 O 2 ), hydroperoxyl (HO 2 ) and OH radicals.As this is the region of monsoon circulation and such major volcanic eruptions can inject SO 2 into stratosphere directly or through the monsoon circulation (Bourassa et al., 2012).Such long-range transport as discussed in previous sections can be a significant source of stratospheric SO 2 and/or sulfate aerosols.According to (Bourassa et al., 2012 and references therein), stratospheric aerosol can have significant impact on earth radiation budget and cause the cooling on the surface.A single Nabro volcano eruption during June 2011 caused approximately 1.3 terragrams of SO 2 in the upper troposphere (around altitude of 9-14 km) and also enhanced largely the stratospheric sulfate aerosols since Mount Pinatubo eruption (Bourassa et al., 2012).Furthermore, stratospheric sulfate aerosols present at temperate latitudes can play an important role in stratospheric ozone chemistry by providing sites for heterogeneous chemistry (e.g., Solomon et al., 1996).
South Asian region is getting attention of world due to a decent growth in GDP of developing nations (India, Bangladesh and Pakistan) during last decade, and of scientific community due to consequent increase in atmospheric pollution.This region with worst air quality caused by rapid industrialization which has brought about more cars, factories, and more people to burn coal and wood for cooking purposes, is relatively less studied.According to (Ramanathan et al., 2001), brown haze is a severe problem over the tropics.It is mainly due to increased emissions of aerosols and precursor gases like SO 2 that have increased over South Asia by a factor of 3 to 4 since 1970.
According to Pakistan Environmental protection Agency

CONCLUSIONS
This study has first time emphasized on the amount of trans-boundary SO 2 pollution present in the atmosphere of Pakistan as a consequence of global and regional volcanic eruption by using satellite remote sensing data.SCIAMACHY, OMI and GOME-2 observations were used to study the spatial and temporal variation of SO 2 over Pakistan during the time period of year 2004 to 2012.An overall temporal increase of 120 percent with volcanic eruptions and 70 per cent without volcanic eruptions was calculated and statistical significance of data sets has been tested (p-value < 0.03, with confidence interval of 95%).Tor Zawar volcano located in Pakistan, erupted during January 2010 but no SO 2 emissions were reported and detected via satellite observations.Some major volcanic eruptions in the African region during last decade have affected atmosphere of Pakistan.Among them, Dalaffilla volcanic eruption in2008 and Nabro volcano event in 2011 caused the major effects by contributing high SO 2 column densities over the regions of northern areas, Baluchistan and Punjab province.Jebel at Tair volcanic activity during October 2007 has also affected few areas of Baluchistan and Punjab province.However, it contributed lesser amounts of SO 2 column densities as compared to the Nabro and Dalaffilla volcanic eruption events.The Back trajectory analyses performed by HYSPLIT model have confirmed the trans-boundary SO 2 plume pathways and also enabled to identify the plume altitude.Most of the time SO 2 plume was present at high altitude like in upper troposphere/tropopause region.Therefore, in order to have a better assessment of atmospheric composition there is a need to constrain SO 2 emissions from all sources, especially the trans-boundary fraction as a consequent of both intense anthropogenic and natural activities.Furthermore, this study will help different stake holders including policy makers to provide with concrete scientific record of atmospheric SO 2 column amounts over Pakistan in order to devise cost effective but efficient policies related to air quality issues in Pakistan.We are also thankful to Engr.M. Zaheer and Mr. Obaid from IGIS-NUST for providing guidance about ArcGIS.Very special gratitude goes to NUST for providing partial financial support as MS research fund to conduct this study.Our acknowledgement will be incomplete without mentioning the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website (http://www.ready.noaa.gov)used in this study.

Fig. 1 .
Fig. 1.Time series of SO 2 column densities (in Dobson units: 1 DU = 2.69 × 10 16 molecules/cm 2 ) observed by SCIAMACHY instrument over Pakistan during the time period of 2004-2012.The arrows are indicating the month influenced by transboundary SO 2 pollution caused by respective volcanic eruptions.Dashed lines are showing the linear fits for respective datasets with and without volcanic activity.The vertical bars represent the calculated standard deviation for respective month (Figure adopted and modified from Khattak, 2013).

Fig. 2 .
Fig. 2. Location map of various volcanoes located (yellow triangle) within and in the neighbouring countries of Pakistan and in the African region as enlisted in the legend and Table 2. Red triangles indicate the volcanoes which contributed SO 2 column densities over Pakistan.

Fig. 3
presents the daily maps of GOME-2 (Figs.3(A), 3(C)) and OMI(Figs.3(B), 3(D), 3(E) and 3(F)) observations for SO 2 plumes during the eruption of Jebel at Tair.The location of volcano is indicated by a black triangle symbol in each figure.On 1 October, 2007 SO 2 plume was first detected close to the vent and drifted northwest of the volcano.Beside the overpass time (stated in each figure) difference, both instruments OMI and GOME-2 detected the SO 2 plume efficiently.Gray color is indicating the region without satellite observations, as these instruments take more than one day for global coverage, especially, over the regions close to the equator.On the following days SO 2 plume has travelled a distance of 1430 km and 1210 km eastward on 2 and 3 October 2007, respectively.Finally, SO 2 plume reached over Pakistan on 4 October, 2007 affecting mainly the areas of Baluchistan and Punjab provinces as illustrated in Fig.4; exhibiting monthly mean SO 2 column densities over Pakistan retrieved from SCIAMACHY observations.
stratovolcano is located in Ethiopia.It has a twin volcano known as Alu.Both Dalaffilla and Alu volcanoes are part of the same volcanic system.In November 2008, major effusive eruption started in the summit of Dalaffilla volcano.According to GVP reports, the eruption started on 3 November, 2008 and continued till December, 2008.Fig. 5 exhibits the maps of GOME-2 (Fig. 5(A)) and OMI (Figs. 5(B), 5(C) and 5(D)) observation from the day after eruption to the day SO 2 plume has reached Pakistan.A satellite overpass on 4 November, 2008 detected SO 2 plume from Dalaffilla volcano (black triangle in the Fig. 5).It has traveled approximately 1650 km towards North.On following days the plume propagated towards East.On 5 November, 2008 SO 2 plume travelled approximately a distance of 3300 km towards East and entered Pakistan on 5 November, 2008 (Fig. 5(C)).On the following days, SO 2 plumes spread over central Pakistan affecting the areas of Baluchistan and Punjab.Fig. 6 exhibits the monthly mean map of SO 2 from SCIAMACHY observations for the month of November, 2008.It clearly shows the elevated amount of SO 2 over the regions of Baluchistan and Punjab provinces.Maximum SO 2 columns observed were 9.4 DU on 6 November, 2008.
Fig.7.The reason behind such high SO 2 columns was investigated.It was found that the Nabro volcano eruption in Eritrea has caused these high SO 2 columns densities over Pakistan.Nabro is a stratovolcano in the north-eastern African nation of Eritrea.The eruption started explosively after a series of earthquakes on the evening of 12 June, 2011(GPV,  2013)  and continued till the mid of July, 2011.It produced the highest SO 2 column ever retrieved from space(Fee et al., 2011;Carn et al., 2012), for instanceClarisse et al. (2012) reported 3700 DU retrieved from daily IASI observations.Nabro volcano eruption spread large amount of SO 2 and ash drifting over much of Eastern Africa, Middle East and South Asian regions.Fig.7shows GOME-2 (Figs.7(A), 7(C) and 7(E) and OMI (Figs. 7(B), 7(D) and 7(F)) images of SO 2 plumes over East Africa and its transportation pattern towards Middle East and South Asia, especially over Pakistan.Both instruments tracked the SO 2 plumes pathway during 13 to 17 June, 2011 efficiently.On 13 June (day after volcanic eruption) plume is just confined to the north-eastern region of Africa.On 14 June, SO 2 plumes have covered almost 1650 km distance towards Northeast and it is drifted towards the Middle East region.On 15 June, plume has traveled around 5390 km towards East and reached closer to Pakistan.On 16 and 17 June, SO 2 plume split into two narrow plumes and spread over North and central parts of Pakistan.Maximum SO 2 columns measured were 8 DU on 17 June 2011 over central Pakistan.As mentioned earlier the eruption continued till the mid of July 2011 and significant SO 2 amount was detected for almost two weeks over Pakistan.It can be clearly identified from Fig. 8; presenting monthly mean map of SO 2 VCD from SCIAMACHY observations for June, 2011.

Fig. 3 .
Fig. 3. Spatial maps of daily observations of GOME-2 (3(A) and 3(C)) and OMI (3(B), 3(D), 3(E) and 3(F)) instruments tracking the trans-boundary SO 2 column densities after the Jebel at Tair volcanic eruption in October 2007.It has caused some perturbations in the atmospheric SO 2 amount over Pakistan.Black triangles in each figure are indicating the location of Jebel at Tair (15.55°N, 41.83°E) volcano.Both instruments have efficiently monitored the SO 2 plume extent on respective days and compare well with each other.Overpass time in UTC for each orbit and instrument is given in each figure.

Fig. 4 .
Fig. 4. Monthly mean map of SCIAMACHY data for SO 2 column densities observed over Pakistan during the month of October 2007.Enhanced SO 2 column amount can be clearly identified over Baluchistan and Punjab provinces caused by Jebel at Tair volcanic eruption.Negative SO 2 columns exhibited are mainly caused by remaining noise/error due to improper removal of ozone spectral interference and ring effect.

Fig. 5 .
Fig. 5. Spatial maps of daily observations of GOME-2 (5(A)) and OMI (5(B), 5(C) and 5(D)) instruments tracking the trans-boundary SO 2 column densities after the Dalaffilla volcanic eruption in November 2008.It has caused significant perturbations in the atmospheric SO 2 amount over Pakistan.Black triangles in each figure are indicating the location of Dalaffilla Volcano (13.79°N, 40.55°E).Both instruments have efficiently monitored the SO 2 plume extent on respective days and compare well with each other.Overpass time in UTC for each orbit and instrument is given in each figure.Gray color is indicating the region without observations, as these instruments take more than one day for global coverage, especially, over the regions close to the equator.

Fig. 6 .
Fig. 6.Monthly mean map of SCIAMACHY data for SO 2 column densities observed over Pakistan during the month of November 2008.Enhanced SO 2 column amount can be clearly identified over Baluchistan and southern Punjab regions caused by Dalaffilla volcanic eruption.

Fig. 7 .
Fig. 7. Spatial maps of daily observations of GOME-2 (7(A), 7(C), 7(E) and 7(G)) and OMI (7(B), 7(D), 7(F) and 7(H)) instruments tracking the trans-boundary SO 2 column densities after the Nabro volcanic eruption in June 2011.It has caused significant perturbations in the atmospheric SO 2 amount over Pakistan.Black triangles in each figure are indicating the location of Nabro Volcano (13.3°N, 41.7°E).Both instruments have efficiently monitored the SO 2 plume extent on respective days and compare well with each other.Overpass time in UTC for each orbit and instrument is given in each figure.Gray color is indicating the region without satellite observations, as these instruments take more than one day for global coverage, especially, over the regions close to the equator.

Fig. 8 .
Fig. 8. Monthly mean map of SCIAMACHY data for SO 2 column densities observed over Pakistan during the month of June 2011.Enhanced SO 2 column amount can be clearly identified over northern regions of Pakistan caused by Nabro volcanic eruption.

Fig. 11 .
Fig. 11.HYSPLIT backward trajectory analyses performed at 400 (red), 5000 (blue), 11000 (green) mAGL for 16 June 2011 and at 400 (red), 7000 (blue), 9000 (green) mAGL for 17 June, 2011.Each gap represents a 6 hours interval.Trajectories following the SO 2 plume pathways (upper panel figure) are located at 7000, 9000 and 11000 mAGL.During both days, they originated from Bay of Bengal and passed over eastern Africa, headed North and then eastward over Middle Eastern region and finally reached to North of Pakistan.

Table 1 .
Features of the three satellite instruments used in this study.

Table 2 .
Volcanoes in the African region, Pakistan and its neighbouring countries * .Volcanoes within in Pakistan, its neighbouring countries and the African Region with their last known Eruption * Source: Bulletin of Global Volcanism Program.