SPATIO-TEMPORAL DISTRIBUTION OF POLLUTANT TRACE GASES DURING DIWALI OVER INDIA

People effected due to air pollution in India rose by almost 150% during 1990 to 2015. Diwali event is one of the major anthropogenic source contributing to the air pollution. The study focuses on spatial and temporal distribution of trace gases emitted during pre, on and post diwali days and identify areas with high concentration using station measured and satellite derived data during 2008-2017. The ground measured data shows that during diwali days, NO2, SO2, CO & O3 concentration is almost 1.5 to 7 times the NAAQ safety limits over major cities particularly in northern, western and eastern India. Central and southern India experience low to moderate increase in pollution concentration. Spatial distribution over diwali days using satellite data reveal that NO2 values over India are mostly below NAAQ standards, however high range are observed (27-48 μg/m) over Delhi, Punjab, Haryana region (Northern zones), Western, central and Eastern Indo-Gangetic plain and this concentration is seen denser on diwali days compared to pre and post diwali. The observation reveal that SO2 concentration is below safety levels over almost entire country except few cities like Delhi region, part of Gujarat, Tamil Nadu and Kolkata region. CO concentration is at higher level than NAAQ standards over Western, central and Eastern Indo-gangetic plain. The regression shows that the satellite derived values are in close agreement with the ground measured over the diwali days. The analysis conclude that the peak of the pollutants during diwali may not be increasing quite drastically over many parts of the cities but the overall spatial distribution of the pollutants is increasing from ‘moderate’ to ‘moderately high’ range. * Corresponding author


INTRODUCTION 1.1 Background
In recent years, there has been concern about the degradation of air quality and its subsequent effect on human health, and one of the reasons of its increase is due to the use of fireworks during festivals and celebrations.Fireworks during festival events or celebrations are closely linked to the increased levels of pollutants, including particulate matter, SO2, NOx, CO and ozone, amongst others.In India, a number of festivals are celebrated throughout the year; Diwali, the festival of light, is one of the most important.Diwali is celebrated between the prewinter season (October) and the post-monsoon season (November).As the festival of light, firecrackers are often used on the days before and during the festival, resulting in deteriorated air quality.WHO, 2014 reported that fireworks used during diwali create high mass concentrations of particulate matter (PM) with a chemical composition that varies from normal atmospheric conditions and can be harmful to human health (Gautam, et al. 2018).Diwali is celebrated for three consecutive days all over India.It includes lighting lamps and igniting fireworks.The fireworks contain 75% potassium nitrate, 15% carbon (C), and 10% sulphur (S).On igniting the firecrackers, the potassium nitrate present in these fireworks releases pollutant such as SO2, NO2, and PM10, increasing their concentrations and massively degrading the air quality within a short time, which is associated with serious health impacts (Haque & Singh, 2017).The ignition of colour-emitting firecrackers generates a large amount of ozone, even in the absence of sunlight and nitrogen oxides, which can be harmful to children and elderly people particularly with heart and respiratory ailments (Attri, et al., 2001).
Worldwide ambient air pollution accounts for 25% of all deaths and disease from lung cancer, 17% of all deaths and disease from acute lower respiratory infection, 16% of all deaths from stroke, 15% of all deaths and disease from ischaemic heart disease, 8% of all deaths and disease from chronic obstructive pulmonary disease (WHO, 2017).India has the world's highest deaths of 1.81 million due to the air pollution followed by China 1.58 million (Lancet, 2015) estimating a GDP loss of 1.3% in developing countries compared to 0.5% in developed countries.Most of these deaths are caused by heat diseases, stroke, lung cancer, chronic obstructive pulmonary disease and upper respiratory tract infection.
From various studies and reports, the concern for air pollution throughout India is in a very critical position and addition to that Diwali celebration of firecrackers make nothing good for the government to control the various air pollutant level.The extensive use of fireworks was found to be related to short-term variation in air quality.During the festival, Total Suspended particulate (TSP) is almost of the same order as compared to the concentration at an industrial site in all the years.However, the concentrations of PM10, SO2, and NO2 increased 2 to 6 times during the Diwali period when compared to the data reported for an industrial site.Similar trend was observed when the concentrations of pollutants were compared with values obtained for a typical foggy day each year in December.The levels of these pollutants observed during diwali were found to be higher due to adverse meteorological conditions, i.e., decrease in 24 hr average mixing height, temperature, and wind speed.The trend analysis shows that PM10, NO2, and SO2 concentration increased just before diwali and reached to a maximum concentration on the day of the festival and the values gradually decreased after the festival.During diwali days, 24-hourly values of PM10 in 2002-2007 period were 3.6 times higher than prescribed limits of National Ambient Air Quality Standards as well as similarly for NO2 values that was extremely higher in 2004 and 2007.These results indicate that fireworks during the diwali festival affected the ambient air quality adversely due to emission and accumulation of PM2.5, PM10, SO2, and NO2 (Singh et al., 2010).The present study tries to understand the spatio-temporal variability using satellite data and analyze the trend of concentration of air pollutants (NOx, CO, O3 & SO2), over various cities, divided into different zones across India, during pre, on and post diwali period for the 10 years (2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017).

STUDY AREA
In most of the mega cities around the world, the effects of air pollution on human health present a growing problem.The study focussed on India as there is reportedly urban air pollution related problems and are poorly reported and understood.Here area of interest is indirectly dependent on the availability of ground based records but more so over whole India is covered except Chattisgarh due to non-availability of ground data.The cities having the ground data are divided into six zone; these are -Northern Zone, consist of 29 cities from Punjab, UP, Haryana and New Delhi; Southern Zone, consist of 26 cities from Andhra Pradesh, Karnataka, Kerala, Telangana and Tamil Nadu; Eastern Zone, consist of 21 cities from Bihar, Jharkhand, West Bengal and Odisha; Western Zone, consist of 33 cities from Rajasthan, Maharashtra and Gujarat; Central Zone, consist of 8 cities from Madhya Pradesh and North-East Zone, consist of 11 cities from Assam, Nagaland and Mizoram.Figure 1 shows the distribution of 128 cities having ground data for trace gases SO2 and NOx.Data criteria was defined for 13 days during diwali celebration for a 10-year duration i.e. 2008 to 2017.It was divided into three sessions 'pre-diwali' defined as 5 days prior from a day before diwali, 'on-diwali' defined for 3 days i.e. a day before, during and a day after diwali and 'post-diwali' defined as 5 days post day after diwali.As per the data criteria and duration for different air pollutant parameters, the data availability was the major problem arising during data assembling and processing.
Figure 3 shows the spatial distribution of the cities with at least 3 years data available from CPCB.
(a) (b) Figure 3. Cities having at least 3yrs data for (a) SO2, NOx and (b) CO and Ozone

Ground data unit and conversion:
CPCB provided standard unit in µg/m 3 , mg/m 3 and ppb for SO2, NOx and O3 respectively.All the units for various pollutants were standardized to µg/m 3 .

Satellite datasets for air pollutant gases.
3.2.1 Dataset used: OMI -AURA (Ozone Monitoring Instrument) observed Level 3 daily data of 0.25 x 0.25-degree resolution is acquired for NO2 and SO2 concentration distribution while MEERA-2 (Modern-Era Retrospective analysis for Research and Applications version 2) derived level 4 hourly data of 0.625 x 0.5 degree resolution has been taken for surface concentration of CO (in ppbv).AIRS level 3 daily gridded product with 1 x 1 degree spatial resolution at 925 hPa was used for O3.The details on OMI-AURA, MERRA-2 and AIRS can be found elsewhere (https://earthdata.nasa.gov/earthobservation-data).

Satellite data unit conversion:
A conversion formula is applied to convert the satellite measurement unit (molecules/cm 2 ) to its equivalent in CPCB standard (µg/m 3 ) unit.The molar mass is used for converting microgram (µg) to number of molecules, [i.e.One molar mass of NO2 (46.0055 g) contains Avogadro number (6.023×10 23 ) of molecules].To find the number of molecules in unit volume (cm 2 to cm 3 ) a hypothetical condition is assumed in which each measurement corresponds to a height (100m) of the planetary boundary layer and within which NO2 molecules are mixed homogeneously.
The height of 100m is an estimated height from the comparison of surface measured values of NO2 (mean) CPCB stations.Even though the data represents the total tropospheric column density (height~10-13km), most of the NO2 molecules are occupied in the boundary layer (up to 1-2 km), with a rapid drop off at higher altitudes (Boersma et al., 2011).Therefore, molecule/ cm 2 is converted to Dobson Unit (DU) using the formula provided by Support to Aviation Control Service (SACS)-ESA.Following, DU has been converted to µg/m 3 using formula from Ramachandran et al., 2013.The unit has been converted from ppb to µg/m 3 (Boguski, 2008).

Comparison of Ground and satellite data
The methodology followed up in this study is shown in following Figure 4. Trend analysis and investigation on spatial variation of air quality over six zones over India for last 10 year period is done using air pollutant parameter such as SO2, NOx, CO and O3.The trend has generated for both data and had been compared using time series data over India and correlation is obtained by regression analysis for pre, on and post-diwali event for last 10 years i.e. 2008 to 2017.

Spatio-temporal variation of air pollutants across India during Diwali from satellite based data
Spatio-temporal variation of air pollutants SO2, NO2, CO and Ozone were generated for 10 years i.e. 2008 to 2017 using OMI-AURA (for SO2 and NO2), MERRA-2 (for CO) and AIRS (for Ozone).The spatial distribution of the air pollutants have been discussed over pre, on and post diwali days.Figure 9 shows the spatio-temporal annual distribution for NO2 across India using OMI-AURA.The observation shows that CO concentration is at higher side of NAAQ safety standards over Western, central and Eastern indo-gangetic plain region.The same results were also observed from the ground data measurement discussed earlier.The higher value of NOx and CO are attributed majorly due to the combustion and burning of fire crackers and plying of large of number of combustive vehicles in major cities across India, especially Western and Northern India.Figure 12 describes the spatio-temporal distribution for Ozone across India using AIRS.The ozone values found to be under NAAQ safety standard across India and gradually decreases from North to South.It is also evident that the ozone values during the 10 years is relatively increasing over various cities in Northern region.

Comparison of Satellite Data from Ground based data
The satellite derived air pollutant parameters were compared with the ground measured values for almost 35-44 cities across India.For comparison purposes, similar dates of satellite and ground measurements were chosen.Due to lack of continues ground data from 2008 to 2018 during diwali days, most of the data that matches and taken were from 2017 year.Figure 13 , 14, 15 & 16 shows the regression between ground and satellite data for year 2017 of NO2 (spread over 35 cities across India) SO2 (spread over 42 cities across India), CO (spread over 44 cities across India) & Ozone (spread over 39 cities across India) during pre, on and post diwali event here satellite and ground measurement respectively.The regression between ground measured and satellite estimated O3 for 2017 during pre, on and post diwali is depicted in Figure 16.Here satellite and ground measurement are taken for 39 cities across India.The regression shows that the satellite derived values are in close with the ground measured over the diwali days.The slight offset in the agreement may be due to data difference in magnitude as for ground data is acquired at surface level while satellite data is as per tropospheric column.All parameters are showing good correlation between ground and satellite based data and the trend is more or less similar for satellite and ground based data indicating the variability and trends are similar in both cases during pre, on and post diwali days.

CONCLUSION
The analysis from satellite derived, ground values and the overall trend of pollutant trace gases over different cities suggests that the values are gradually high during diwali comparatively low in pre and post-diwali.The precise analysis of satellite derived CO reveals that the trend in peak values across the country has decreased after year 2012-13 during diwali event, although the spatial distribution of CO (in range of 0.2 -0.6 mg/m 3 The trend analysis of air pollutants over one decade conclude that the satellite based data depict better scenario about the distribution of pollutant gases across the country than the ground based information.The ground station data are quite irregular, provide only specific location information, and also the location of the stations are not evenly distributed across the cities.By studying the trend and comparing ground and satellite data we can conclude that the peak of the pollutants during diwali may not be increasing quite drastically over many parts of the cities but the overall spatial distribution of the pollutants is increasing from 'moderate' to 'moderately high' range.The NO2 emitted distribution map indicates & focuses those targeted areas where the crop residue burning is practiced majorly, whereas SO2 distribution map indicates the areas where the burning of crackers is high and O3 distribution map shows the combined resultants of crackers and crop residue burning.The CO emitted distribution map shows those areas where the emissions due to fossils burning or due to the extensive use of oil for lighting pradeep, candles burning during diwali and also from the contribution of increasing vehicular transportation in those days.Hence, with these inferences of this study, the Indogangetic region is most likely to be sensitive for the abundance of these pollutant gases followed by the central and other parts of western India.The availability of moisture due to evaporation from the river and inland water bodies and low wind flow, helps in trapping and increasing the concentration of pollutant gases instead of settling down over the IGP.

Figure 1 .
Figure 1.Distribution of SO2 and NOx across various cities in India.

Figure 2 .
Figure 2. Distribution of CO and O3 across 36 cities in India

Figure 4 .
Figure 4. Flowchart of the Methodology

Figure 5 .
Figure 5. Spatio-temporal variation of annual NOx across different zones.Results shows that NOx values in central and North East zones are within the range with respect to NAAQ safety standards (80 µg/m 3 for 24hrs average) (CPCB, 2015), however Northern, Western and Eastern zones have relatively higher values (range 100-150 µg/m 3 ) with respect to NAAQ safety standards over few cities.To be precise, NOx values are at highest level for pre, on and post diwali days over New Delhi (Northern zone), Ahmedabad, Mumbai and Bhiwadi (Western zone) and Kolkata (Eastern zone), which are major cities contributing the major NOx flux distribution.The temporal distribution for annual SO2 across different zones is depicted in figure 6.

Figure 6 .
Figure 6.Spatio-temporal variation of annual SO2 in different zones.It is revealed that for almost all zones across India, SO2 concentration is under NAAQ safety standards (80 µg/m 3 for 24hrs average), except in case of the Byrasandra, Tavarekere & Madiwala (BTM Layout) Bengaluru (Southern zone) during 2014.Over BTM layout there is a constant deterioration of air quality standard due to rapid increase in population and corresponding fuel combustion due to increase in transport, commercial vehicles, industries and domestic sectors.Particularly during diwali episodic event, the SO2 level has increased by 1.23 times the NAAQ safety standards, while over Kolkata and Raniganj (Eastern zone) shows 2.13 times the NAAQ safety standard in 2014, 2013 and 2012 respectively.During year 2016 diwali episodic event, Thane (Western zone) shows 3.2 times the NAAQ safety standard level, while in 2010, Behmiyat, Meghalaya (North East zone) shows 10 times NAAQ level due to accidental fire broke out in factory on eve of diwali day.Figure7shows zone wise temporal distribution for annual CO concentration distribution across India.

Figure 7 .
Figure 7. Spatio-temporal variation of annual CO in different zones.Over Northern and Western zones the value of CO are higher than NAAQ safety standards i.e. 2 mg/m 3 for 24hrs average due to large number of combustive engines vehicles plying during pre, on and post diwali, whereas Southern zone shows relatively high value CO concentration, precisely in year 2014 where Bengaluru depict 1.5 time NAAQ safety standard and for same year Chennai shows 5 times NAAQ safety standards level.Similarly, Ahmedabad and Nagpur (Western zone) shows 7 times NAAQ level during 2012 and 2013 respectively and again Nagpur shows 3 times during 2010 and same year Mumbai (Western zone) shows 1.5 times NAAQ safety levels.There is no data availability for central and North-East zones during

Figure 8 .
Figure 8. Spatio-temporal variation of annual Ozone in different zones.It is observed that over western zone, the Ozone concentration is high with respect to the NAAQ safety standard (100 µg/m 3 for 24hrs average) during diwali, which is due to large number of combustive engines vehicles plying during pre, on and post diwali days.Particularly ozone values are substantive higher over western zone with cities like Mumbai (1.5 times NAAQ safety level), Ahmedabad (1.6 times NAAQ safety standard), Nagpur (7 times NAAQ safety standard); Northern zone with cities like Lucknow (1.6 times NAAQ safety level), Kanpur (3.7

Figure 9 .Figure 10 .Figure 11 .
Figure 9. Spatio-temporal variation of NO2 over India for pre, on and post diwali event during 2008 to 2017.

Figure 12 .
Figure 12.Spatio-temporal variation of Ozone over India for pre, on and post diwali event during 2008 to 2017.

Figure 13 .
Figure 13.Validation of satellite derived NO2 for Pre, On and Post Diwali 2017.

Figure 14 .
Figure 14.Validation of satellite derived SO2 for Pre, On and Post Diwali 2017.

Figure 15 .
Figure 15.Validation of satellite derived CO for Pre, On and Post Diwali 2017.

Figure 16 .
Figure 16.Validation of satellite derived Ozone for pre, on and post diwali 2017.
).It2008observed that over Northern India, spati2010, 2011ution & peak values in NO2, has increased during diwali days followed by eastern and central region.Basically the distribution of NO2 is noticed to be concentrated over few isolated areas in Punjab, Haryana, Delhi, northern Chhattisgarh, central West Bengal and central Odisha.The areas where seasonal crop residue burning occur during diwali time register quite high values of NO2.One of the major sources from crackers is SO2, and its distribution is observed in dispersed manner throughout the country however, yearly peaks are observed to be noticed mostly in northern and eastern zones.Regions of eastern Madhya Pradesh, Chhattisgarh, Odisha, and Jharkhand are found to be more susceptible for the pollution due to significant presence of SO2.The northern Indian region i.e.Punjab, Haryana, Chandigarh, Delhi followed by the eastern and central Indian region are identified with significant higher concentration of O3.Specifically the kharif crop residue burning combined with the diwali events make O3 concentration level high enough cause the levels to rise and have adverse health impact.There have been few years when the post diwali period experienced more pollutant concentration than during on diwali period such as 2011 forSO2;2008  , 2009  , 2012  , 2013 for CO; for CO;2010, 2011for NO2; 2013 and 2017 for O3.There were certain years where pre diwali period registered high pollution than post diwali period such as 2012 & 2015 for SO2; 2017 for CO; 2016 for NO2; 2010 for O3.These anomalies are mainly due to certain local meteorological factors like precipitation, change in humidity or strong wind or else.