Ambient sulphur dioxide (SO 2) measurements have been performed at a high altitude site in the semi arid region of western India, Gurushikhar, Mt. Abu (24.6°N, 72.7°E, 1680 m ASL), during different sampling periods span over Sep-Dec 2009 and Feb-Mar 2010. A global three dimensional chemical transport Model, GEOS-Chem, (v8-03-01) is employed to generate the SO 2 profile for the entire region for the different sampling months which in turn is used to explain the major features in the measured SO 2 spectra via correlating with HYSPLIT generated wind back trajectories. The mean SO 2 concentrations recorded at the sampling site varied for the different sampling periods (4.3 ppbv in Sep-Oct 2009, 3.4 ppbv in Nov 2009, 3.5 ppbv in Dec 2009, 7.7 ppbv in Feb 2010 and 9.2 ppbv in Mar 2010) which were found to be strongly influenced by long range transport from a source region surrounding 30°N, 75°E—the one projected with the highest SO 2 concentration in the GEOS-Chem generated profiles for the region—lying only a few co-ordinates away. A diurnal cycle of SO 2 concentration exists throughout the sampling periods, with the greatest day-night changes observed during Feb and Mar 2010, barely detectable during Sep-Oct 2009, and intermediate values for Nov and Dec 2009 which are systematically studied using the time series PBL height and OH radical values from the GEOS-Chem model. During the sampling period in Nov 2009, a plume transport to the sampling site also was detected when a major fire erupted at an oil depot in Jaipur (26.92°N, 75.82°E), located few co-ordinates away. Separate runs of the model, performed to study the long range transport effects, show a drop in the SO 2 levels over the sampling region in the absence of transport, throughout the year with Jan to Apr seen to be influenced the lowest by long range transport while Jul and Dec influenced the highest.
The present day increased fossil fuel and coal usage in Asia, to meet the energy needs of a rapidly emerging economy, has raised concerns about the potential threats of sulphur dioxide (SO2) emission from this region [1-5] to the global climate. Further, there exist many recent reports [6-8]—based on direct observations as well as chemical transport modeling studies—of SO2 from Asia out-flowing to the western Pacific and influencing the climatology of this region.
In urban areas it is the fossil fuel combustion which contributes to the majority of the anthropogenically produced SO2, releasing over 90% of the sulphur content in these fuels to the atmosphere as sulphur dioxide [9,10]. With the known prominent roles of SO2 in the chemistry of the global troposphere [11-18] and in the sulphur cycle including its potential to interfere with the oxidizing power of the atmosphere, to cause acid rain (e.g., [19, 20]), and to modify the atmospheric radiative forcing pattern via gas-phase chemistry and particle formation, there is an impending need to monitor systematically the emission and transport pattern SO2 over different region in Asia, with differing environmental conditions.
As a special case, the monitoring of ambient SO2 levels coupled with wind trajectory analysis at a location devoid of local emissions and situated at free tropospheric altitudes (seasonal entry into the free troposphere when the planetary boundary layer (PBL) height is low) can help improve the present understanding of the free tropospheric transport patterns of SO2 from source to remote locations. Keeping this as one of the major goals, ambient SO2 concentrations were measured at a high altitude site in the semi arid region of western India, Gurushikhar, Mt. Abu (24.6˚N 72.7˚E, 1680 m Above Sea Level (ASL)), during Sep-Oct’09, Nov’09, Dec’09, Feb’10 and Mar’10. In this study a global three dimensional chemical transport model, GEOS-Chem, (v8-03- 01) is also employed, to generate the SO2 profiles for the entire region for the different sampling months to identify the major SO2 source regions.
The sampling site for this study, Gurushikhar, Mt. Abu (24.6˚N, 72.7˚E, 1680 m ASL) is a high altitude site in the semi-arid region of western India, having a more or less clean atmosphere with minimal local emissions. Gurushikhar is the highest mountain peak in the southern end of Aravali range of mountains in western India with an annual average rainfall of about 600 - 700 mm precipitating only during SW-monsoon. The Gurushikhar site, because of its high elevation, can enter the free tropospheric zone during the winter and post-monsoon months when the Planetary Boundary Layer (PBL) height is low over the region for low incoming solar radiation (insolation) rates, while during summer it gets accommodated within the PBL. This special characteristic makes the Mt. Abu site suitable for the kind of study mentioned above viz., assessing the free tropospheric transport pattern of SO2 from source to remote locations.
A primary UV fluorescence SO2 Monitor (Thermo—43i Trace Level Enhanced (TLE)) was employed for the monitoring of ambient SO2 levels during the different sampling periods. The performances of similar systems are reported elsewhere [21-23]. Calibrations were performed routinely with a standard SO2 gas (2 ppmv with N2 balance gas, Spectra, USA) and zero air using a dynamic gas calibrator (Thermo—146i). The integration time for the SO2 analyser was set at five minutes and the data was periodically downloaded to a computer for analysis.
The GEOS-Chem global 3-D chemical transport model (v8-03-01; http://acmg.seas.harvard.edu/geos/) employed for this study uses GEOS-5 assimilated meteorological observations—including winds, convective mass fluxes, mixed layer depths, temperature, clouds, precipitation, and surface properties—from NASA Global Modeling and Assimilation Office (GMAO). The GEOS-5 data have a temporal resolution of 6-h (3-hour resolution for surface fields and mixing depths) and horizontal resolution of 0.5˚ latitude × 0.667˚ longitude, with 72 levels in the vertical extending from surface to approximately 0.01 hPa. The GEOS-Chem model (v8-03-01) simulates the Ozone-NOx-VOC-aerosol chemistry [24,25] and elaborative simulation evaluations are provided in [25-28].
In the present work, simulations were carried out for Jan 2009-Apr 2010 at 4˚ × 5˚ resolutions with the runs initialized on 1st Jan 2009 using GEOS-Chem fields generated by a one year spin-up simulation at 4˚ × 5˚ resolution. The emission inventories EMEP [
The GEOS-Chem Model outputs during different runs are plotted using the Global Atmospheric Model Analysis Package (GAMAP) (Version 2.15; http://acmg.seas.harvard.edu/gamap/) which is a selfcontained, consistent, and user-friendly software package written in IDL (Interactive Data Language). The package can read outputs from chemical tracer models (CTM’s) and visualize it via line plots, 2D plots, 2D animations, or 3D iso-contour surface plots.
The wind back trajectory analysis for this work employs the HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) model, developed by NOAA ARL [33, 34] which uses a modelled vertical velocity scheme and can depict the vertical motion of the relevant air parcel. The 7-day back trajectories plotted for altitudes 50, 500 and 3000 m above the ground level (AGL) are used to identify the origin and the transect of the air parcels reaching the sampling site.
Ambient SO2 measurements were performed during Sep-Oct’09, Nov’09, Dec’09, Feb’10 and Mar’10. The GEOS-Chem chemical transport model (v8-03-01) is used to generate the SO2 profile for the entire region surrounding the sampling site for the different sampling months to identify the major source regions. Now by analysing the back trajectories generated using HYSPLIT for the sampling periods—performed at different altitudes to investigate the source and the transport pathways of the air parcel, prior to its arrival to the sampling site— and correlating them with the GEOS-Chem generated SO2 plots showing the source regions, the major features in the measured spectra are explained.
The GEOS-Chem model (v8-03-01) runs were performed for Sep 2009 to Apr 2010, with the inventories discussed previously, to find out the major SO2 source regions in the vicinity of the sampling site. The output averaged for the first half of each sampling month was then plotted using GAMAP and is shown in
The time series SO2 profiles for the nearest co—ordinates to the Mt. Abu site possible with the model (26˚N, 75˚E), for the ground level (0 - 0.3 km AGL), also was generated during the GEOS-Chem runs by turning ON the ND49 diagnostics in the input.geos file. This model generated SO2 time series were then plotted for the sampling periods to make comparison with the experimental results. The data is also used to study the diurnal variabilities and its dependence on different atmospheric parameters such as Planetary Boundary Layer (PBL) height and OH radical concentrations.
The measured SO2 time series profiles had distinctly different features during different sampling periods span over Sep-Oct’09, Nov’09, Dec’09, Feb’10, and Mar’10, and were found to be dictated by the transport patterns and climatology prevailed in the respective months. The major features in the SO2 spectra for different sampling months and its correlation with various atmospheric parameters are discussed below. A comparison of the experimental SO2 time series data with GEOS-Chem model generated one’s also is made for the different sampling periods.
During Sep-Oct 2009, when the winds were mostly from the South-West direction (as shown in
The SO2 profile had minimal diurnal variability (
During Nov 2009 when the wind back trajectories (
Because of the wind pattern specific for this month, only short duration random air parcels arrive at the sampling site at Mt. Abu from the high SO2 region surrounding 30˚N, 75˚E—as shown by the back trajectory analysis (
A diurnal pattern is observed (
The spikes as well as the concentration levels decreased from 12th Nov 2009, as is the case with the GEOS—Chem generated SO2 time series shown also in
Detection of SO2 Plume during a Major Oil Fire Incident A major fire had erupted at an oil depot of Indian Oil Corporation Limited (IOCL) at Jaipur, Rajasthan (26.92˚N, 75.82˚E) on 29th October 2009 at around 7:30 PM (IST). Approximately 60,000 kilolitres of oil were burned in this fire incident which lasted for almost a week till 6th Nov 2009, resulting in a significant rise in the air pollution levels over the region.
Studying the possible long range transport of SO2 containing plumes to the high altitude sampling site at Mt. Abu from the disaster site, located only a few co-ordinates away was another objective of the sampling for this month. Three short duration high intensity spikes were recorded (
(around 2230 hrs) and 5th (around 2120 hrs) of Nov 2009 which was found associated with this major oil fire from back trajectory analysis—which showed air parcels reaching the sampling site from the disaster site (26.92˚N, 75.82˚E), during the above days (
During Dec 2009, the winds—as seen in the back trajectory plots shown in
30˚N, 75˚E to produce many spikes in the measured spectra (
A diurnal pattern is observed during this sampling period (
The GEOS-Chem generated SO2 time series (seen in
The GEOS-Chem based calculations showed (as will be discussed in section 3.5) that it is in the month of December every year, the contribution of long range transported
SO2 over Mt. Abu attains its peak.
The sampling period in Feb 2010 was characterized by high SO2 concentrations (
zone during the winter time explains the non-influence of pollutant carrying winds in the lower layers of the atmosphere on the ambient SO2 levels for this month.
The mean SO2 concentration for this sampling period is 7.7 ppbv. The strong diurnal pattern observed for this sampling period (
For this sampling period, the GEOS-Chem generated SO2 profile (seen in
During the sampling period in Mar 2010 also, relatively high SO2 concentrations were recorded (
The diurnal variation patterns observed in the measured SO2 profiles over Mt. Abu as well as those in the GEOS-Chem generated SO2 time series data for the co-ordinates 26˚N, 75˚E (shown in
The GEOS-Chem generated SO2 profiles (
and continue to remain at this minima till 0600 hrs IST. The spread in the PBL values increases from Jan to Jun with maximum spread for the month of Jun, followed by a decrease. The minimum diurnal variability in the PBL height was observed during Jul to Sep.
Unlike the diurnal variation pattern seen in the PBL heights, where during Jan-May there is a transient increase in the values between 0600 hrs and 0800 hrs, the OH variation during the months Jan-Mar are at a relatively slow pace. The sudden manifold increase in OH levels between 0600 hrs and 0800 hrs IST takes place during May, Jun and Jul when the relative humidity (RH) in the atmosphere is sufficiently high over this region. The GEOS-Chem generated OH radical profiles showed highest concentrations during the months, Jun, Jul, Aug, and Sep followed by a decrease till Dec and then the accenting mode for OH radical levels in the atmosphere start from Jan. The minimum OH concentrations were recorded during Dec when both the insolation as well the atmospheric water content—the two major parameters
controlling the OH formation in the atmosphere—over this region are on the lower side.
As can be seen from the above discussion, the PBL height and the OH radical maxima doesn’t overlap but falls on different months to give the relative influence of these parameters to the SO2 oxidation efficiency a strong seasonality and is discussed in detail below.
It is evident from the above discussion that interplay between seasonal PBL height and the OH concentration variability can modulate the diurnal pattern of the SO2 concentrations. The fact that the rates at which these two parameters vary—both seasonal as well as diurnal are different, leads to a seasonal dependence for the diurnal variability of SO2 levels.
For example, during the months Feb, Mar and Apr the effect put in by the sudden change in PBL height in the morning hours leads to the sudden decrease in the SO2 values between 0600 hrs and 0800 hrs IST, where the OH radical variation is not sufficient to cause much of a change in the SO2 variability pattern. Whereas, the sudden changes in the OH concentrations occurring during Jun-Sep is the major player controlling the SO2 diurnal pattern during these months.
To visualize the above mentioned relative contributions clearly, 3-dimensional graphs were plotted with the GEOS-Chem generated OH concentration along X-axis, PBL height along Y-axis and the SO2 concentration along the Z-axis for different months of the year 2009 (
During Feb 2009, for all values of OH concentration above ~4e+6, an increase in PBL height above ~700 m is sufficient to pull the SO2 values to the minimum for that month. Below an OH level of ~4e+6, a combined effect of OH radical as well as PBL height variation is found to influence the SO2 concentration. Similarly for OH concentrations below ~2e+6, even with the highest PBL height recorded for the month, the SO2 concentration remained high. Compared to Jan 2009, the graph plotted for Feb 2009 has fewer uncorrelated humps for the SO2 values, indicating the possible shift in the transport pattern.
The month, Mar 2009 saw a clear interplay between the PBL and OH values in deciding the diurnal pattern of ambient SO2 over this region. This is one such month where both PBL height and OH levels are significant. As a result, for OH values between ~1e+6 and 4e+6, the increase in PBL height from the minimum of the month to ~1500 m, reduces the SO2 concentration at a steady rate to reach the minimum of the month. For all OH values above ~4e+6, the SO2 concentration drops to the minimum, for PBL heights above ~700 m. Interference from
other players such as transient long range transport is a minimum in this month as evidenced by the absence of any random humps in the SO2 values.
The behaviour observed in Apr, May and Jun were somewhat similar to the one in Mar. The scenario started changing from Jul and became clearly visible by Aug, when the atmospheric OH concentration over the sampling region reached its peak values of the year. In Aug, the OH concentration was sufficient to pull down the SO2 values to the minimum of the month for all PBL values above ~150 m, showing the major role the OH radical play in the diurnal pattern of SO2 for this month. The non observation of any humps in the SO2 values correlate very much with the minimum transient transport expected
for these months (as will be discussed in section 3.5), underlining the argument that the third major player controlling the ambient SO2 levels over this region is the long range transport.
During Sep 2009 and Oct 2009, when the boundary layer heights are on the lower side, the OH radical and the transport are the only major players, resulting in many humps in the SO2 values in addition to the contribution from heterogeneous phase oxidation on water droplets. During Nov and Dec, an overlap of low PBL heights with low OH concentration along with higher transport events (as discussed below) keep the SO2 values on the higher side during most of the time.
In order to understand the possible effects of long range
transport on the measured SO2 values over Mt. Abu, GEOS-Chem model runs were performed with all the parameters keeping identical to the previous runs but now with the Transport mechanism turned OFF. Now the percentage difference in the SO2 concentration at 26˚N, 75˚E—the nearest grid point co-ordinates available with the GEOS-Chem model at 4˚ × 5˚ resolutions in the absence of transport is calculated as follows:
Percent Difference in the SO2 Concentration in the Absence of Transport = (WithOutTransport – FullInput) × 100/FullInput
where:
WithOutTransport → SO2 concentration recorded when the Transport turned OFF
FullInput → SO2 concentration recorded when Transport also was kept ON