Evaluation of urban ozone in the Brahmaputra River Valley

Journal Pre-proof Evaluation of Urban Ozone in the Brahmaputra River Valley U.C. Dumka, A.S. Gautam, S. Tiwari, D.S. Mahar, S.D. Attri, R.K. Chakrabarty, P. Permita, Philip K. Hopke PII:

S1309-1042(19)30530-6

DOI:

https://doi.org/10.1016/j.apr.2019.12.013

Reference:

APR 710

To appear in:

Atmospheric Pollution Research

Received Date: 3 October 2019 Revised Date:

11 December 2019

Accepted Date: 11 December 2019

Please cite this article as: Dumka, U.C., Gautam, A.S., Tiwari :, S., Mahar, D.S., Attri, S.D., Chakrabarty, R.K., Permita, P., Hopke, P.K., Evaluation of Urban Ozone in the Brahmaputra River Valley, Atmospheric Pollution Research, https://doi.org/10.1016/j.apr.2019.12.013. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved.

Author Contributions: U. C. D., A.S. Gautam, S. Tiwari and P. H. Hopke organized the research and wrote the manuscript. A.S. Gautam was involved in analysis and discussion. R. K. Chakrabarty, P. Permita was involved in the discussion and provide the valuable input in finalizing the manuscript. D.S. Mahar, S. D. Attri and Ritu Hooda was also involved in the measurements and data analysis part. All authors contributed significantly in compiling the manuscript.

1

Evaluation of Urban Ozone in the Brahmaputra River Valley

2

U.C. Dumka1*¸A.S. Gautam2,S. Tiwari3, D.S. Mahar2, S.D. Attri4,R. K. Chakrabarty,5, P.

3

Permita,5, Philip K. Hopke6, 7, Ritu Hooda8

4

1

Aryabhatta Research Institute of Observational Sciences, Nainital – 263 001, India

5

2

H.N.B. Garhwal University, Srinagar, Uttarakhand-246174, India

6

3

Indian Institute Meteorological Department, New Delhi Branch, 110060, India

7

4

India Meteorological Department, New Delhi, 110060, India

8

5

9

USA

10

Center for Aerosol Science and Engineering, Washington University in St. Louis, St. Louis, MO - 63130,

6

Center for Air Resources Engineering and Science, Clarkson University, Potsdam, NY-13699-

11

5708, USA

12

7

13

Dentistry, Rochester, NY 14642 USA

14

8

Department of Public Health Sciences, University of Rochester School of Medicine and Department of Zoology, Maharshi Dayanand University, Rohtak, Haryana, India

15 16

Research Highlights

17

• Relatively low ozone concentrations were measured in Guwahati, India.

18

• Ozone concentrations in Guwahati were dominated by local NOx emissions.

19

• A high mean NHMCs/NOx ratio (9.45) suggests high downwind O3 values.

20 21

Abstract

22

Ozone (O3), carbon monoxide (CO), nitrogen oxides (NOx=NO+NO2), methane (CH4) and non-

23

methane hydrocarbons (NMHCs) were measured in Guwahati in the Brahmaputra River Valley

24

(BRV) of northeastern India from 1stJanuary 2013 to 30thJune 2014. The current analysis shows

25

that the annual mean concentrations of O3, CO, CO2, NOx, CH4, and NMHC in Guwahati were

26

10.5 (± 6.30 ppb), 0.5 (± 0.2 ppm), 411.7 (± 37.5 ppm), 19.6 (± 11.6 ppb), 2045(± 417ppb) and

27

173( ± 93 ppb), respectively. The pattern of O3concentrations in Guwahati indicated it was

28

strongly influenced by local NOx sources with an adjacent national highway being the likely

*

Correspondence to: U. C. Dumka (E-mail: [email protected]; [email protected]); Suresh

Tiwari (E-mail: [email protected]); P. K. Hopke (E-mail: [email protected])

1

29

major source. Thus, the measured O3 values were reduced by local titration. Diel, weekly, and

30

seasonal patterns for each of these gaseous species showed the strong influence of local sources.

31

The mean ventilation coefficient was greater than >6000 m2s during the day in the pre-monsoon

32

season indicating pollutant dispersion. The seasonally average solar radiation (SR) fluxes

33

decreased in the order: pre-monsoon (180 W m-2), monsoon (176 W m-2), post-monsoon (148 W

34

m-2) and winter (110 W m-2).The corresponding mean O3 concentrations were 14.8, 7.5, 6.4, and

35

10.1 ppb, respectively. In the pre-monsoon season, an impact of SR on the photochemical

36

formation of O3was observed. High O3 winter concentrations were observed likely driven by

37

local biomass burning providing reactive VOCs that contributed to ozone formation.

38 39

Keywords: Ozone, northeastern India, pollutant gases, NOx titration, Brahmaputra River Valley,

40

Nonmethane hydrocarbons (NMHCs)

41 42

1. Introduction

43

Atmospheric ozone is a secondary air pollutant produced by photochemical reactions with

44

primary pollutants. High concentrations of surface O3 affect crop productivity, damage

45

ecosystems and adversely affect human health(Anenberg et al. 2009; Travagli et al., 2010; Zhang

46

et al., 2014; Guo et al. 2015; Ghude et al., 2014; 2016a and b; Wang et al., 2017).Ozone

47

contributes to global

48

(https://www.epa.gov/ghgemissions/understanding-global-warming-potentials). During the past

49

three decades, increasing atmospheric pollution in developing countries like India and China

50

driven by rapid expansion of urban, industrial, vehicular emissions, biomass burning, etc.isa

51

major concern and the impacts extend from local to global scales (Yadav et al. 2016; Wang et al.,

52

2017).

53

Ozone is formed through complex chemical reactions involving oxides of nitrogen (NOx),

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volatile organic compounds (VOCs), and sunlight (Seinfeld and Pandis, 2016). The

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photochemical production rate of O3 depends on the ambient VOCs and NOx concentrations and

56

the solar radiation intensity (Seinfeld and Pandis, 2016). NOx lifetimes vary from an hour to

57

days depending on the photochemical environment (Seinfeld and Pandis, 2016). In addition to

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biogenic emissions, VOCs sources include vehicular emissions, liquefied petroleum gas use, use

59

of solvents, gas leaks, chemical factory emissions, anaerobic wastewater treatment, etc. (Ling et

60

al., 2011). Ozone plays a crucial role in the oxidizing capacity of the atmosphere and contributes

warming as one of the most important greenhouse gases

2

61

to the formation of highly reactive hydroxyl radicals (Sharma et al., 2016).In the tropics

62

including the Indian subcontinent, photochemical O3 production is important because of the high

63

solar fluxes, atmospheric water content, and emitted nitrogen oxides (Andreae and Crutzen,

64

1997).

65

The lifetime of O3 near the surface varies from hours to weeks (Fleming et al., 2011). Although

66

O3 has a relatively short atmospheric lifetime, O3 precursors can be transported over long

67

distances (Brankov et al., 2003). The formation rates of O3 are proportional to temperature and

68

solar intensity (Nishanth et al., 2014). Naja and Lal (2002) reported that the tropospheric O3

69

concentrations are rising in the lower atmosphere in the northern hemisphere. Tropospheric O3 is

70

increasing by 0.5 to 2% yr−1in the Northern Hemisphere due to increasing anthropogenic

71

activities (Vingarzan, 2004).O3is also a greenhouse gas and thus, contributes to global radiative

72

forcing (+ 0.35 W/m2) in the troposphere (Akimoto, 2003; Solomon et al., 2007).

73

In India, surface O3 concentrations have been reported for various areas including urban (Delhi:

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Jain et al., 2005; Pune: Beig et al., 2007; Kanpur: Gaur et al., 2014; Udaipur: Yadav et al., 2016),

75

rural (Anantapur: Ahammed et al., 2006), coastal (Thumba: Nair et al., 2011; and Kannur:

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Nishanth et al., 2012), and in the Himalayan region (Ali et al., 2004; Kumar et al., 2010; Ojha et

77

al., 2012; Sarangi et al., 2014; Bhardwaj et al., 2018).O3concentrations vary depending upon

78

climatic conditions and emission sources. Lelieveld et al. (2001) suggested that extreme solar

79

radiation (SR; W m-2), elevated atmospheric water content, and high NOx, enhance

80

photochemical production of O3in the tropics especially on the Indian subcontinent. The major

81

sources of NOx in India are emissions from fossil fuel combustion in mobile and stationary

82

sources (David and Nair, 2011). In rural areas, a major source of CH4, CO, and volatile organic

83

compounds (VOCs) is the burning of biofuel/biomass (Khare, 2012) and these emissions lead to

84

increased surface ozone.

85

Further, non-methane hydrocarbons (NMHCs) are ozone-producing precursors through hydroxyl

86

radical initiated oxidation and subsequent reactions with NO to yield NO2 without the

87

consumption of O3 (Duan et al., 2008; Jia et al., 2016). Methane (CH4) is also an important

88

greenhouse gas (IPCC, 2013) and has a significant influence on the O3 concentration in the

89

background troposphere (West and Fiore, 2005; West et al., 2006). Rice fields are major methane

90

sources contributing ~15–20% of the total global anthropogenic CH4 emissions (Aulakh et al.,

91

2001). The world’s annual rice production must increase from 518 to 760 million tons in 1990 to

92

2020 to meet the needs of the growing population (Ma et al., 2008). Thus, the total atmospheric 3

93

CH4 emissions from paddy cultivation will likely increase in the future. Carbon monoxide (CO)

94

also is involved in controlling background troposphere O3 through its reaction with hydroxyl

95

radicals (Pochanart et al., 2003). Its major sources include motor vehicles and biomass burning.

96

In the present study, O3, CO (carbon monoxide), CO2 (carbon di-oxide), NOx (oxides of

97

nitrogen), CH4 (methane), and NMHCs (non-methane hydrocarbons) were measured from

98

January 2013 to June 2014 in Guwahati in Brahmaputra River Valley (BRV) region in eastern

99

India. This area is different from other parts of India because of the influence of Ganga Basin

100

and the Bay of Bengal. The BRV is a highly polluted, highly populated region with a unique

101

topography, where major sources of pollution include local traffic, natural emissions (dust

102

storms, lightning and sea-salt), seasonal open fires (agricultural and forest), open waste burning,

103

residential emissions from cooking, heating, emissions from small, medium and heavy industries

104

including power generation, emissions from diesel generator sets, brick kiln emissions and long-

105

range transport from other polluted regions. The study presents the variability of measured

106

pollutants including O3and its precursors including CO, CO2, NOx, CH4, and NMHCs. The main

107

objectives of the present study were to determine the seasonal, day of week, and diel

108

characteristics of O3concentrations, to identify the major source emissions of ozone precursors

109

especially CH4 and NMHCs, and to study the relationships between the meteorological variables,

110

ozone, and its precursors in a tropical setting.

111 112

2. Measurement Location and Methods

113

Hourly concentrations of O3and related gases were measured on the premises of Guwahati

114

(GUW) University (26° 9'7.88"N, 91°39'24.89"E; 55 meters above sea level) in the State of

115

Assam, located in the Brahmaputra Valley Region (BVR) (Figure 1). The sampling site lies

116

between the BVR and Shillong Plateau region with an area of ~216 km2 surrounded by the

117

Nilachal, Narakasur, and Chitrachal Hills to the west, south, and north, respectively. Complete

118

details regarding the observational site were provided by Tiwari et al., (2016a; 2017). The area

119

around the observational site has a high population density of 4,393 persons per km2

120

(https://www.citypopulation.de/php/india-assam.php?cityid=1840607000). Green tea plants and

121

tropical rainforestssurround the city. The area has a mild sub-tropical climate with warm (pre-

122

monsoon: March to May), rainfall (monsoon: June to September), post-monsoon (October to

123

November) and winter (December to February) periods. Sharma et al. (2009) reported a high

124

annual average rainfall (~1800 mm) with ~90% of rain occurring during the monsoon period 4

125

with the highest deposition in July/August. Near the sampling site, a national highway (NH) runs

126

toward Shillong, Meghalaya and contributes pollutant species. The Assam Science Technology

127

and Environment Council (ASTEC) reported ~400,000 registered (both personal and

128

commercial) vehicles without any emission clearance certificatesenter the city every day such

129

that they emit huge quantities of pollutants (ASTEC, 2011).

130

The gaseous species, O3, CO, CO2, NOx, CH4, and NMHC were monitored on the roof of the

131

Department of Environmental Science building of Guwahati University at a height of ~10

132

metersabove ground level from 1stJanuary 2013 to 30thJune 2014.The measurements were made

133

with standard regulatory compliance monitors. The O3 was measured with an Eco tech model EC

134

9810 with auto ranging from 0-0.05 ppm to 0-20 ppm and a detection limit of 0.5 ppb. CO was

135

measured with an Eco tech model Serious 30 with auto ranging from 0 to 200 ppm CO with a

136

detection limit of 40 ppb. The zero and span calibration of CO were performed regularly using a

137

standard calibration mixture (Yadav et al., 2014). NOx was measured using an Eco tech model

138

EC 9841 chemiluminescence analyzer with auto ranging from 0 to 1000 ppm with a resolution of

139

0.01 ppm. The measured NOx may have included other oxidized reactive nitrogen (N) that can

140

be converted by the molybdenum catalyst (Ran et al., 2011).CH4 and NMHC were monitored

141

with a Synspec model Alpha 115.Unlike other systems that determine NMHC by difference, this

142

unit uses gas chromatography to separate the methane from longer chain hydrocarbons and thus,

143

provides separate analyses of these two species.

144

The local surface meteorological parameters (wind speed: WS in m s-1, wind direction: WD in 0,

145

temperature: Temp in 0C, relative humidity: RH in % and rainfall: RF in mm) were measured in

146

Guwahati with an automatic weather monitoring system (Envirotech Instruments, WS10:

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Anemometer; WD10: Potentiometric; Temp: RT10-Thermistor; RH: RT10- Capacitance; LI-

148

200SA: Photo Cell for solar radiation (SR in W m-2).The data were measured at a 10 m above the

149

ground level at 5-minute intervals and averaged to hourly values.

150 151

3. Results and Discussion

152

3.1. General Meteorological Conditions

153

The mean values of the meteorological parameters for each of the four major seasons (Pathak et

154

al., 2014) measured in Guwahati are presented in Table 1.Large seasonal differences were

155

observed reflecting the changing boundary layer conditions. The annual surface meanwind speed

156

(WS) was 1.12 ± 0.73 m s-1and varied from 0.1 to 6.1 m s-1 with the highest mean values (2.2 ± 5

157

0.7m s-1) from March to June (pre-monsoon). During this period, the prevailing wind direction

158

(WD) was from the southwest. At the end of June (onset of the monsoon), the pattern changed

159

with lower WS (0.8m s-1) and a shift to the southeast. The lowest mean WS (0.62±0.19 m s-1)

160

was observed during the winter when the WDs were mostly west and southwest. The annual

161

mean temperature was 24 ±4.5oCand varied from 3.0to 32.9oC. The seasonal mean temperatures

162

were monsoon (28.9oC), post-monsoon (24.4oC),pre-monsoon (23.9oC), and winter (18.4oC).

163

Annual mean RH was 82 ± 4.5% and varied from 63 to 92%. Seasonally, RH varied in order of

164

monsoon (85%), post-monsoon (84%), pre-monsoon (77%), and winter (82%).This region of

165

India receives heavy rainfall compared to other areas. The annual RF during the study period was

166

1970 mm with the highest deposition in the monsoon season (1164 mm) and lowest in winter (11

167

mm). Details on the variation of meteorological conditions over the region were presented in an

168

earlier report (Tiwari et al., 2016a; 2017).

169

Dispersion of atmospheric pollutants depends upon the meteorological conditions. The wind

170

speed and mixed layer height (MLH) defines the volume of air through which the pollutant

171

mixed (Collier et al., 2005). The ventilation coefficient (VC) is the product of MLH and mean

172

WS through the mixing layer. It reflects the transport rate in the mixing layer. The VC values

173

were estimated using the Equation (1) (Ashrafi et al., 2009):

174

VC=ZU

175

Where Z is atmospheric MLH (in meters) above the surface and U is the average WS (in m s-1)

176

within MLH. The MLH were estimated using the HYbrid Single-Particle Lagrangian Integrated

177

Trajectory model (HYSPLIT; Draxler, 1999), and details are presented elsewhere (Stein et al.,

178

2015; Dumka et al., 2015a, b). HYSPLIT was also used to calculate air parcel back trajectories

179

using the reanalysis meteorological data set. The values of U were measured at a height of 10 m

180

above the ground.

(1)

181 182

3.2. Ambient concentrations of O3, CO, CO2, NOx, CH4, and NMHC

183

The ambient concentrations of O3, CO, CO2, NOx, CH4, and NMHC were measured from 1st

184

January 2013 to 30th June 2014 are shown in Figure 2. The daily mean concentrations were 10.5

185

± 6.30ppb, 0.5 ± 0.2ppm, 411.7 ± 37.5 ppm, 19.6 ± 11.6ppb, 2045 ± 417ppb, and 173 ±93ppb,

186

respectively (Figure 2). The ozone concentrations in Guwahati are quite low and reflect the near-

187

source conditions at this site such that there is substantial titration of the ozone by freshly emitted

188

NO from the nearby highway. The O3concentrations were well below the World Health 6

189

Organization (WHO) 8-hour maximum standard (100 µg m-3or 50 ppb) that was established to

190

adequately protect public health (WHO, 2006). However, they have also reported that some

191

health effects may occur below this level.

192

For the period of July 1st, 2013, to December 31st, 2013, the NOx data also included NO and NO2

193

concentrations. Thus, the dynamics of the NOx species with ozone and solar radiation can be

194

examined.Figure 3 shows 3 days of data in early November 2013.During the evening hours,

195

ozone becomes depleted, and NO dominated the NOx. The variable NO concentrations showed

196

the influence of local NO sources whereas the NO2 declined monotonically from its maximum

197

value once the O3has been depleted. As the solar radiance increased after sunrise, ozone forms

198

and the NO is converted to NO2 but is then photolyzed to form more ozone reaching a photo-

199

stationary state (White, 1977).

200 201

3.3. Temporal Variations

202

The daily variations in ozone by hour of the week are shown in Figure 4, where the median

203

values for each hour of the day for each day of the week are plotted along with the 25th and 75th

204

percentile values as error bars. There is little day-to-day variability in the median values.

205

However, the ranges vary from day to day. The O3 concentrations were higher between 09:00 to

206

19:00 local time peaking in the early afternoon when solar irradiance was high. There is no clear

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“weekend effect” as observed in many developed countries (Cleveland et al., 1974; Lebron,

208

1975; Cleveland and McRae, 1978) where there is a substantial reduction in heavy-duty truck

209

traffic on weekends resulting in decreased NO emissions with a commensurate increase in ozone

210

concentrations.

211

The hour-of-the-week plots for the other measured species are shown in Supplemental

212

Information Figures S-1 to S-5. For CO, the dielfluctuations in its concentrations were smaller in

213

the pre-monsoon, monsoon and post-monsoonwhen compared to the winter period. Over

214

northern part of India, farmers burn their crop residues during the post-monsoon period and

215

produce ahuge amount of atmospheric pollution. Its impact could be observed in the CO

216

concentrations where there was no variation between daytime and nighttime CO concentrations

217

during that period. Figure S-6 shows a plot of the frequency of air parcel back trajectory

218

endpoints over the period of November 11 to December 15, 2013 for each 1º latitude by 1º

219

longitude grid cell. Effective transport from the area of the IGB, where agricultural burning is

220

widespread can be clearly seen. Goyal and Chalapati (2007) reported that during the pre7

221

monsoonal months, the relatively constant CO concentrations throughout the day were likely

222

related to the mixing of air masses within a deeper boundary layer. Thus, the transported

223

agricultural burning aerosol could be effectively admixed to ground level.

224

For CO2, there was little day-to-day variability observed. CO2 concentrations >420 ppmv were

225

observed in the overnight hours (21:00 to 07:00 LT) compared with daylight hours (08:00 to

226

20:00 LT) where concentrations dropped to ~390 ppmv. The lower concentrations of CO2 were

227

likely due primarily to the increased wind speeds during the day since the daytime source

228

emissions rates will be high with increased motor vehicle traffic and increased industrial activity.

229

Hourly O3concentrations were also analyzed by month (Figure 5) with substantial month-to-

230

month variation in median values and the extent of their dispersion. Mean values were lower

231

during the monsoon (7.5ppb) and post-monsoon (6.39ppb) seasons and higher in the pre-

232

monsoon (14.8ppb) and winter (10.0ppb) seasons (Table 2). However, there are significant

233

month-to-month variations that do not correlate with the variations in solar intensity (Figure S-7)

234

as would be anticipated if the measured ozone reflected regional concentrations.

235

Additionally, high winter O3 concentrations (second largest) may be affected by biomass burning

236

(BB) aerosol transported from the western IGB (Kaskaoutis et al., 2014; Tiwari et al., 2015) as

237

shown in Figure S-6 and shown by the impact of the transported aerosol on CO during this

238

period.BB produces significant amounts of reactive hydrocarbon species concentrations and

239

resulting high O3 (Thompson et al., 2001). The lower mean O3 concentration (7.5 ppb) observed

240

during the monsoon season was due to the intense rainfall that can wash out peroxy radicals

241

(HO2 and RO2) by wet deposition limiting the gas phase photochemistry of O3formation as well

242

as the reduction of solar radiation by clouds during rain events (Seinfeld and Pandis, 2016).The

243

post-monsoon season O3 concentrations are comparable to earlier studies at other urban locations

244

in India (Debaje and Kakade, 2009; Lal et al., 2012; Chelani, 2012; Tiwari et al.,

245

2016b).However, O3 concentrations are lower than the rural and high-altitude sites in India

246

where NO emissions are significantly lower (24-36 %) (Reddy et al., 2010). In Delhi, the

247

average concentration of O3 was reported much higher (23.6ppb) than the Guwahati (Tiwari et

248

al., 2015). The concentration of O3 over Guwahati was lower may be due to a significant amount

249

of untitrated NO which was produced due to local sources of NO in the surrounding of the

250

monitoring site such as traffic. The other gaseous species were also separated seasonally as

251

presented in Table 2.

8

252

The monthly patterns for the other 5 species are shown in Figures S-8 to S-12. Seasonally the

253

mean mixing ratios of CO was 0.73 ± 0.3 ppm (winter), 0.47 ± 0.3 ppm (pre-monsoon), 0.38 ±

254

0.2 ppm (monsoon) and 0.51 ± 0.2 ppm (post-monsoon). The mean CO concentration in

255

Guwahati was around three times lower than Delhi (Sharma et al., 2016) and equal to that in

256

Kanpur, an urban site in the central IGB region (Gaur et al., 2014).Generally, the CO

257

concentrations increase during the morning traffic hours.In the case of CO2, there is low

258

variability by season with concentrations always greater than 400 ppmv. Chhabra and Goel

259

(2017) have reported the mean CO2 concentration over India was ~ 399.73 (±1.89) ppm using

260

satellite-based

261

(http://www.globalcarbonatlas.org/en/CO2-emissions), an exponential growth of 5.47% per year

262

(Figure S-13) becoming the world's third-largest fossil-fuel CO2-emitting country.

263

NOXisemitted by high-temperature combustion and is likely dominated by emissions from the

264

nearby national highway. Figure S-14 presents a conditional bivariate probability function

265

(CBPF) plot for NOx over the 18 months of measurements. It can be seen that there is a strong

266

influence of the highway to the NW of the site under low wind speed conditions. As observed in

267

Figure 3, NO and NO2 concentrations are driven largely by photochemical nitrogen oxide cycle

268

and during daylight hours likely form a photo stationary state (White, 1977). In Guwahati, NOx

269

varied from 11.8 ppb (monsoon) to 32.5 ppb (winter).

270

Regional O3 is also produced during transport by chemical reactions with organic precursors

271

such as CH4 and non-methane volatile organic carbon, and UV radiation (Simpson et al., 2014).

272

Both CH4 and NMHC had a peak at the beginning of the sampling period and then had relatively

273

little variation throughout the rest of the monitoring period (Figure 2). The mean

274

CH4concentration(2041 ppb) in Guwahati was higher than the 2013 global mean concentration of

275

1813.32

276

relativelylow variability by season of 1945 ppb(winter) to 2111 ppb (monsoon).NMHCswill also

277

be involved in the regional chemistry through the formation of peroxy radicals that can oxidize

278

NO back to NO2 without consuming an O3 molecule. However, it does not appear to have a

279

substantial effect on the Guwahati values given the strong direct relationships with the NOx

280

species.However, in the downwind regions, it can be anticipated that there will be higher

281

concentrations of ozone with the potential for substantial crop damage.The NMHCs seasonal

282

behavior showed lowestconcentrations in the pre-monsoon (136 ppb) and highest in the post-

283

monsoon (210 ppb) seasons likely a result of biogenic emissions during the plant growth that

±

data.

0.62

ppm

Based

on

(Dlugokencky,

data

from

the

Global

Carbon

www.esrl.noaa.gov/gmd/ccgg/trends_ch4/)

9

Atlas

with

284

followed the rainfall. Nishant et al. (2014) suggested that the importance of NMHCs as O3

285

precursors depends largely on their reactivity and atmospheric concentrations.Other major

286

sources of NMHCs in the urban region are from chemical industries, solvent evaporation refinery

287

operations, and vehicular emissions (Liu et al., 2017). The nocturnal NMHC concentrations were

288

approximately double than the daylight hours (Figure S-5) reflecting the lower boundary layer

289

height, lower wind speeds, and possibly greater emissions.

290 291

3.4. Impact of ventilation coefficient and solar radiation on surface ozone

292

The MLH is a crucial parameter that determines the ability of atmospheric pollutants to

293

accumulate and disperse and about the structure of turbulence within the mixed layer (Seibert et

294

al., 2000).The concentrations of atmospheric pollutants are affected by the extent of dispersion as

295

reflected by the VC (Krishnan and Kunhikrishnan, 2004). Low VC values indicate poor

296

dispersion of pollutants whereas high VC conditions support dispersion of the pollutants. The

297

diel variation in VC by different season were calculated for Guwahati and presented in Figure6.

298

The hourly VC values show a similar trend for all seasons with the highest value (1935 m) in the

299

summer and lowest (209 m2 s) in the post-monsoon season. The VC starts increasing from 08:00

300

local time and reached sits maxima at 15:00 local time. The daytime (08:00 to 19:00 LT) and

301

nighttime (19:00 to 08:00 to local time) mean VC values are presented in Table 4.The VC values

302

were about one order of magnitude higher during the daytime (1360 m2 s) that at night. Although

303

higher VC conditions typically lead to lower pollution concentrations, for O3, the opposite

304

pattern was observed because of the higher photochemical activity during the day driven by the

305

solar radiation. For the other species (CO, NOx, CH4 and NMHC), lower concentrations were

306

measured during the daytime (Figures S-1 to S-5).

307

Eagleman (1991) proposed that the low pollution potential occurs when the VC is >6000 m2 s.

308

This criterion is met in pre-monsoon during 11:00–17:00 local time. However, the abovecriterion

309

is notfulfilled in the winter, monsoon, and post-monsoon seasons. Praveena and Kunhikrishnan

310

(2004) observed that the MLHwas lower during the winter over the Indian subcontinent. In the

311

monsoon season, the strong prevailing winds disperse the atmospheric pollutants quickly and

312

they are washed out by the precipitation. There is also less solar radiation reaching the ground

313

due to the cloud layers. Alternatively, both windspeed and MLH depth are lowerduring the

314

winter (Alappattu et al., 2009). A significant impact of MLH was observed in the case of CH4and

10

315

NMHC during thedaytime when the WS was highest favoring the dispersion of pollutants and

316

resulting in the lowest concentrations of CH4 and NMHCs.

317 318

3.5. Relationships among the measured species

319

The ratio between CO and NOx suggests the presence of local air masses at the receptor site (Ou

320

et al., 2015).Because of the presence of different emission sources in the IGB, the Bay of Bengal

321

and other regions surrounding the receptor site, the CO/NOx ratio can be used to categorize the

322

air mass origins.The CO/NOx ratios were estimated seasonally and were highest in pre-monsoon

323

(32.6) followed by the monsoon season (32.2) when the average wind direction was 1710 (south-

324

east) and 1830 (south) confirming the dominance of local air masses at this site given the local

325

anthropogenic activities in that direction. The lower CO/NOx ratios when the air masses arrive

326

from 2070 and 2230(southwest) were 27.5 and 25.1, respectively. Overall, this analysis revealed

327

that higher CO/NOx ratios indicated that the air was likely transported from distance source

328

instead of being dominated by local sources.

329

The seasonal mean O3, CO, NOx, CH4, and NMHC concentrations were separated by WD. The

330

concentrations of CO were highest when the air masses came from between 00 to 900 and 900 to

331

1800 during the winter and post-monsoon periods. NMHCs were significantly higher (>225 ppb)

332

when the air masses came from between 00 to 900 during the winter, monsoon, and post-monsoon

333

seasons but not pre-monsoon suggesting that more gasoline evaporation occurred. However,

334

during pre-monsoon, the higher level of photochemical activity may reduce the NMHC

335

concentrations. Jia et al. (2016) suggested that the O3 formation is more sensitive to NOx level at

336

high VOC/NOx ratios. The NHMCs/NOx ratio was 9.45 suggesting that NMHC can play an

337

important role in ozone formation in the region downwind of Guwahati.

338 339

4. Conclusions

340

The measurements of ozone (O3) and related gaseous species (carbon monoxide: CO, carbon di-

341

oxide: CO2, oxides of nitrogen: NOx, methane: CH4, and non-methane hydrocarbons: NMHC)

342

were conducted in Guwahati. Relatively low concentrations of O3 were measured in Guwahati

343

compared to other urban sites in India. Examination of the limited NO, NO2, and O3 data

344

suggested that this site is strongly affected by local NOx sources, one of which is an adjacent

345

major national highway. These results suggest that during much of the daylight hours, this site is

346

in or nearly in a photo-stationary state indicating a low impact of organic species on the ozone 11

347

concentrations. However, given the high NMHC to NOx ratios measured at this site, it is likely

348

that there is asubstantially higher ozone concentrations in the region downwind of Guwahati that

349

could result in substantial crop damage. Thus, non-urban monitoring for ozone is essential if all

350

of the effects of urban air pollution are to be effectively assessed. These results suggest that the

351

location of the monitoring sites is often too close to major NOx sources. Thus, to obtain a more

352

accurate assessment of ozone pollution, sites will need to establish in more suburban or rural

353

areas to avoid the effects of NO titration.

354 355

Acknowledgements

356

The authors would like to thank the Vice-Chancellor of Guwahati University for providing the

357

infrastructure and facilities at the university and the Director, Indian Institute of Tropical

358

Meteorology, Pune, for providing the financial support for installing the instruments at the

359

Department of Environmental Sciencesof Guwahati University. Authors conveyed the heartiest

360

thanks to Dr. Beig along with SAFAR groups who commissioned the station and continuously

361

take care of the instruments and manage the data generation. We would like to thank the NOAA

362

Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion

363

model. We would like to thanks the Prof. Mustafa Odabasi, Editor-in-Chief, Atmospheric

364

Pollution Research, and anonymous reviewers for their constructive comments and suggestions

365

to improve the scientific quality of manuscript.

366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381

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18

583

Table 1: Seasonal-mean of metrological parameters except for rainfall (total). The seasons are classified as pre-monsoon (March-

584

April-May), monsoon (June-July-August-September), post-monsoon (October-November), and winter (December-January-February),

585

respectively Seasons

Solar Radiation

Temperature 0

( C)

-2

Wind Speed

Wind 0

Direction ( )

-1

(Wm )

(ms )

Relative

Rainfall

Humidity

(mm)

(%)

Winter

109.5

18.4

0.6

223

81.6

11

Pre-monsoon

180.3

23.9

2.2

171

76.8

648

Monsoon

176.2

28.9

1.1

183

84.5

1164

Post-monsoon

148.3

24.4

0.6

207

83.7

147

Annual mean

153.5

23.9

1.1

196

81.6

1970

586 587

Table 2:Seasonal-mean concentrations of surface ozone and related species Seasons

O3

CO

CO2

NOx

CH4

NMHC

Winter

10.1

0.7

415.7

32.5

1945.0

187.7

Pre-monsoon

14.8

0.5

415.5

15.6

2035.9

135.8

Monsoon

7.5

0.4

402.2

11.8

2111.3

192.5

Post-monsoon

6.4

0.5

416.3

19.1

2071.4

209.8

Annual mean

9.7

0.5

412.4

19.8

2040.9

181.5

588 589

19

590

Table 3: Monthly mean concentrations (ppbv) of measured surface ozone in Guwahati, IGB, and India Sampling site

Concentrations of O3

References

Guwahati (BRV)

10.5

Present study

IGB

15.7

Present study

Ahmadabad

20.7

Beig et al., (2007)

Pune

30.1

Debaje and Kakade, (2009)

Delhi

41.5

Chelani (2012)

Patna

14.5

Tiwari et al., (2016)

Kanpur

27.9

Gaur et al., (2014)

Mt. Abu

39.9

Naja et al., (2003)

Udaipur

28.6

Yadav et al., (2016)

Anantapur

35.9

Ahammed et al., (2006)

Agra

21.0

Satsangi et al., (2004)

Kannur

18.4

Nishanth et al., (2012)

Varanasi

41.5

Tiwari et al., (2008)

Nainital

43.9

Kumar et al., (2010)

Trivandrum

11.5

David and Nair, (2011)

591 592

20

593

Table 4: Seasonal-mean ventilation coefficient (VC: m2s), surface ozone (O3: ppbv) and solar radiation (SR: W·m-2) and mixed layer

594

height (MLH: m) and Temperature (Tem.: 0C) Night (19:00-08:00 LT) Seasons

VC

O3

SR

BHL

TEM

WIN

49.8

3.36

3.8

114

15.8

PRM

367.4

7.79

11.9

228

23.2

MON

129.5

4.46

12.2

202

27.0

POM

40.5

2.48

8.2

125

21.8

Annual Mean

146.8

4.5

9.0

168

22.0

Day (08:00-19:00 LT) WIN

578.9

16.32

215.2

642

21.1

PRM

3502.6

18.21

348.6

1114

28.7

MON

980.7

10.45

340.1

634

30.7

POM

377.2

10.32

288.3

481

26.9

Annual Mean

1359.9

13.83

298.06

717

26.9

595

21

596 597 598

Figure 1: Geographical location of sampling site (i.e. Guwahati University) over the Indian subcontinent.

22

599 600 601 602

Figure 2: Daily and 20 days moving mean of O3 (ppb), CO (ppm), CO2 (ppm), NOx (ppb), CH4 (ppb) and NMHC (ppb) from January 1st, 2013 to 30th June 2014 in Guwahati, India. 23

700 15

40

30

10

20 5

10

600 500 400 300 200

Solar Radiation (w.m2)

50

100

0 11/06

603 604 605

800

20

NO NO2 O3 SR

O3 Concentration (ppb)

NO or NO2 Concentration (ppb)

60

11/07

0 11/09

11/08

0

Figure 3. Daily variation of NO, NO2, O3, and SR for 3 days in November 2013. 35

Ozone 30

Ozone (ppb)

25

20

15

10

5

0 Sunday

Monday

Tuesday

Wednesday Thursday

Friday

Saturday

Hour of the Week 606 607 608

Figure 4. Median ozone concentration plotted as a function of hour of the week. The error bars represent the 25th and 75th percentile values. 24

50

Solar Radiation (W m-2) Ozone (O3)

1000 900

Ozone (ppb)

800 700

30

600 20

500 400

Solar Radiation (W m-2)

40

10 300 0 Jan 609 610 611 612 613 614

Feb Mar Apr May Jun

Jul

Aug Sep

200 Oct Nov Dec

Figure 5. Median ozone concentrations in Guwahati by month (black points) with the error bars showing the 25th and 75th percentile values. Red star color gives the monthly median solar radiation with the error bars representing the 25th and 75th percentile values. The Ozone and solar radiation was taken from 09:00 to 15:00 local time.

25

615 616 617 618

Figure 6: Variations of hourly ventilation coefficient in different seasons at Guwahati during January 2013 to June 2014.

26

619

27

620

Evaluation of Urban Ozone in the Brahmaputra River Valley

621

U.C. Dumka1*¸ A.S. Gautam2, S. Tiwari3, D.S. Mahar2, S.D. Attri4, R. K. Chakrabarty,5, P.

622

Permita,5, Philip K. Hopke6, 7, Ritu Hooda8

623

1

Aryabhatta Research Institute of Observational Sciences, Nainital – 263 001, India

624

2

H.N.B. Garhwal University, Srinagar, Uttarakhand-246174, India

625

3

Indian Institute Meteorological Department, New Delhi Branch, 110060, India

626

4

India Meteorological Department, New Delhi, 110060, India

627

5

628

USA

629

6

630

5708, USA

631

7

632

Dentistry, Rochester, NY 14642 USA

633

8

Center for Aerosol Science and Engineering, Washington University in St. Louis, St. Louis, MO - 63130, Center for Air Resources Engineering and Science, Clarkson University, Potsdam, NY-13699Department of Public Health Sciences, University of Rochester School of Medicine and Department of Zoology, Maharshi Dayanand University, Rohtak, Haryana, India

634 635

Research Highlights

636

• Relatively low ozone concentrations were measured in Guwahati, India.

637

• Ozone concentrations in Guwahati were dominated by local NOx emissions.

638

• A high mean NHMCs/NOx ratio (9.45) suggests high downwind O3 values.

639 640

Abstract

641

Ozone (O3), carbon monoxide (CO), nitrogen oxides (NOx=NO+NO2), methane (CH4) and non-

642

methane hydrocarbons (NMHCs) were measured in Guwahati in the Brahmaputra River Valley

643

(BRV) of northeastern India from 1st January 2013 to 30th June 2014. The current analysis shows

644

that the annual mean concentrations of O3, CO, CO2, NOx, CH4, and NMHC in Guwahati were

645

10.5 (± 6.30 ppb), 0.5 (± 0.2 ppm), 411.7 (± 37.5 ppm), 19.6 (± 11.6 ppb), 2045 (± 417 ppb) and

646

173( ± 93 ppb), respectively. The pattern of O3 concentrations in Guwahati indicated it was

647

strongly influenced by local NOx sources with an adjacent national highway being the likely

*

Correspondence to: U. C. Dumka (E-mail: [email protected]; [email protected]); Suresh

Tiwari (E-mail: [email protected]); P. K. Hopke (E-mail: [email protected])

28

648

major source. Thus, the measured O3 values were reduced by local titration. The diel, weekly,

649

and seasonal patterns for each of these gaseous species showed the strong influence of local

650

sources. The mean ventilation coefficient was greater than >6000 m2 s during the day in the pre-

651

monsoon season indicating pollutant dispersion. The seasonally average solar radiation (SR)

652

fluxes decreased in the order: pre-monsoon (180 W m-2), monsoon (176 W m-2), post-monsoon

653

(148 W m-2) and winter (110 W m-2). The corresponding mean O3 concentrations were 14.8, 7.5,

654

6.4, and 10.1 ppb, respectively. In the pre-monsoon season, an impact of SR on the

655

photochemical formation of O3 was observed. High O3 winter concentrations were observed

656

likely driven by local biomass burning providing reactive VOCs that contributed to ozone

657

formation.

658 659

Keywords: Ozone, northeastern India, pollutant gases, NOx titration, Brahmaputra River Valley,

660

Nonmethane hydrocarbons (NMHCs)

661 662

5. Introduction

663

Atmospheric ozone is a secondary air pollutant produced by photochemical reactions with

664

primary pollutants. High concentrations of surface O3 affect crop productivity, damage

665

ecosystems and adversely affect human health (Anenberg et al. 2009; Travagli et al., 2010;

666

Zhang et al., 2014; Guo et al. 2015; Ghude et al., 2014; 2016a and b; Wang et al., 2017). Ozone

667

contributes to global

668

(https://www.epa.gov/ghgemissions/understanding-global-warming-potentials). During the past

669

three decades, increasing atmospheric pollution in developing countries like India and China

670

driven by rapid expansion of urban, industrial, vehicular emissions, biomass burning, etc. is a

671

major concern and the impacts extend from local to global scales (Yadav et al. 2016; Wang et al.,

672

2017).

673

Ozone is formed through complex chemical reactions involving oxides of nitrogen (NOx),

674

volatile organic compounds (VOCs), and sunlight (Seinfeld and Pandis, 2016). The

675

photochemical production rate of O3 depends on the ambient VOCs and NOx concentrations and

676

the solar radiation intensity (Seinfeld and Pandis, 2016). NOx lifetimes vary from an hour to

677

days depending on the photochemical environment (Seinfeld and Pandis, 2016). In addition to

678

biogenic emissions, VOCs sources include vehicular emissions, liquefied petroleum gas use, use

679

of solvents, gas leaks, chemical factory emissions, anaerobic wastewater treatment, etc. (Ling et

warming as one of the most important greenhouse gases

29

680

al., 2011). Ozone plays a crucial role in the oxidizing capacity of the atmosphere and contributes

681

to the formation of highly reactive hydroxyl radicals (Sharma et al., 2016). In the tropics

682

including the Indian subcontinent, photochemical O3 production is important because of the high

683

solar fluxes, atmospheric water content, and emitted nitrogen oxides (Andreae and Crutzen,

684

1997).

685

The lifetime of O3 near the surface varies from hours to weeks (Fleming et al., 2011). Although

686

O3 has a relatively short atmospheric lifetime, O3 precursors can be transported over long

687

distances (Brankov et al., 2003). The formation rates of O3 are proportional to temperature and

688

solar intensity (Nishanth et al., 2014). Naja and Lal (2002) reported that the tropospheric O3

689

concentrations are rising in the lower atmosphere in the northern hemisphere. Tropospheric O3 is

690

increasing by 0.5 to 2% yr−1 in the Northern Hemisphere due to increasing anthropogenic

691

activities (Vingarzan, 2004). O3 is also a greenhouse gas and thus, contributes to global radiative

692

forcing (+ 0.35 W/m2) in the troposphere (Akimoto, 2003; Solomon et al., 2007).

693

In India, surface O3 concentrations have been reported for various areas including urban (Delhi:

694

Jain et al., 2005; Pune: Beig et al., 2007; Kanpur: Gaur et al., 2014; Udaipur: Yadav et al., 2016),

695

rural (Anantapur: Ahammed et al., 2006), coastal (Thumba: Nair et al., 2011; and Kannur:

696

Nishanth et al., 2012), and in the Himalayan region (Ali et al., 2004; Kumar et al., 2010; Ojha et

697

al., 2012; Sarangi et al., 2014; Bhardwaj et al., 2018). O3 concentrations vary depending upon

698

climatic conditions and emission sources. Lelieveld et al. (2001) suggested that extreme solar

699

radiation (SR; W m-2), elevated atmospheric water content, and high NOx, enhance

700

photochemical production of O3 in the tropics especially on the Indian subcontinent. The major

701

sources of NOx in India are emissions from fossil fuel combustion in mobile and stationary

702

sources (David and Nair, 2011). In rural areas, a major source of CH4, CO, and volatile organic

703

compounds (VOCs) is the burning of biofuel/biomass (Khare, 2012) and these emissions lead to

704

increased surface ozone.

705

Further, non-methane hydrocarbons (NMHCs) are ozone-producing precursors through hydroxyl

706

radical initiated oxidation and subsequent reactions with NO to yield NO2 without the

707

consumption of O3 (Duan et al., 2008; Jia et al., 2016). Methane (CH4) is also an important

708

greenhouse gas (IPCC, 2013) and has a significant influence on the O3 concentration in the

709

background troposphere (West and Fiore, 2005; West et al., 2006). Rice fields are major methane

710

sources contributing ~15–20% of the total global anthropogenic CH4 emissions (Aulakh et al.,

711

2001). The world’s annual rice production must increase from 518 to 760 million tons in 1990 to 30

712

2020 to meet the needs of the growing population (Ma et al., 2008). Thus, the total atmospheric

713

CH4 emissions from paddy cultivation will likely increase in the future. Carbon monoxide (CO)

714

also is involved in controlling background troposphere O3 through its reaction with hydroxyl

715

radicals (Pochanart et al., 2003). Its major sources include motor vehicles and biomass burning.

716

In the present study, O3, CO (carbon monoxide), CO2 (carbon di-oxide), NOx (oxides of

717

nitrogen), CH4 (methane), and NMHCs (non-methane hydrocarbons) were measured from

718

January 2013 to June 2014 in Guwahati in Brahmaputra River Valley (BRV) region in eastern

719

India. This area is different from other parts of India because of the influence of Ganga Basin

720

and the Bay of Bengal. The BRV is a highly polluted, highly populated region with a unique

721

topography, where major sources of pollution include local traffic, natural emissions (dust

722

storms, lightning and sea-salt), seasonal open fires (agricultural and forest), open waste

723

burning, residential emissions from cooking, heating, emissions from small, medium and

724

heavy industries including power generation, emissions from diesel generator sets, brick

725

kiln emissions and long-range transport from other polluted regions. The study presents the

726

variability of measured pollutants including O3 and its precursors including CO, CO2, NOx, CH4,

727

and NMHCs. The main objectives of the present study were to determine the seasonal, day of

728

week, and diel characteristics of O3 concentrations, to identify the major source emissions of

729

ozone precursors especially CH4 and NMHCs, and to study the relationships between the

730

meteorological variables, ozone, and its precursors in a tropical setting.

731 732

6. Measurement Location and Methods

733

Hourly concentrations of O3 and related gases were measured on the premises of Guwahati

734

(GUW) University (26° 9'7.88"N, 91°39'24.89"E; 55 meters above sea level) in the State of

735

Assam, located in the Brahmaputra Valley Region (BVR) (Figure 1). The sampling site lies

736

between the BVR and Shillong Plateau region with an area of ~216 km2 surrounded by the

737

Nilachal, Narakasur, and Chitrachal Hills to the west, south, and north, respectively. Complete

738

details regarding the observational site were provided by Tiwari et al. (2016a; 2017). The area

739

around the observational site has a high population density of 4,393 persons per km2

740

(https://www.citypopulation.de/php/india-assam.php?cityid=1840607000). Green tea plants and

741

tropical rainforests surround the city. The area has a mild sub-tropical climate with warm (pre-

742

monsoon: March to May), rainfall (monsoon: June to September), post-monsoon (October to

743

November) and winter (December to February) periods. Sharma et al. (2009) reported a high 31

744

annual average rainfall (~1800 mm) with ~90% of rain occurring during the monsoon period

745

with the highest deposition in July/August. Near the sampling site, a national highway (NH) runs

746

toward Shillong, Meghalaya and contributes pollutant species. The Assam Science Technology

747

and Environment Council (ASTEC) reported ~400,000 registered (both personal and

748

commercial) vehicles without any emission clearance certificates enter the city every day such

749

that they emit huge quantities of pollutants (ASTEC, 2011).

750

The gaseous species, O3, CO, CO2, NOx, CH4, and NMHC were monitored on the roof of the

751

Department of Environmental Science building of Guwahati University at a height of ~10 meters

752

above ground level from 1st January 2013 to 30th June 2014. The measurements were made with

753

standard regulatory compliance monitors. The O3 was measured with an Ecotech model EC 9810

754

with auto ranging from 0-0.05 ppm to 0-20 ppm and a detection limit of 0.5 ppb. CO was

755

measured with an Ecotech model Serinus 30 with auto ranging from 0 to 200 ppm CO with a

756

detection limit of 40 ppb. The zero and span calibration of CO were performed regularly using a

757

standard calibration mixture (Yadav et al., 2014). NOx was measured using an Ecotech model

758

EC 9841 chemiluminescence analyzer with auto ranging from 0 to 1000 ppm with a resolution of

759

0.01 ppm. The measured NOx may have included other oxidized reactive nitrogen (N) that can

760

be converted by the molybdenum catalyst (Ran et al., 2011). CH4 and NMHC were monitored

761

with a Synspec model Alpha 115. Unlike other systems that determine NMHC by difference,

762

this unit uses gas chromatography to separate the methane from longer chain hydrocarbons and

763

thus, provides separate analyses of these two species.

764

The local surface meteorological parameters (wind speed: WS in m s-1, wind direction: WD in 0,

765

temperature: Temp in 0C, relative humidity: RH in % and rainfall: RF in mm) were measured in

766

Guwahati with an automatic weather monitoring system (Envirotech Instruments, WS10:

767

Anemometer; WD10: Potentiometric; Temp: RT10-Thermistor; RH: RT10- Capacitance; LI-

768

200SA: Photo Cell for solar radiation (SR in W m-2). The data were measured at a 10 m above

769

the ground level at 5-minute intervals and averaged to hourly values.

770 771

7. Results and Discussion

772

3.1. General Meteorological Conditions

773

The mean values of the meteorological parameters for each of the four major seasons (Pathak et

774

al., 2014) measured in Guwahati are presented in Table 1. Large seasonal differences were

775

observed reflecting the changing boundary layer conditions. The annual surface mean wind 32

776

speed (WS) was 1.12 ± 0.73 m s-1 and varied from 0.1 to 6.1 m s-1 with the highest mean values

777

(2.2 ± 0.7 m s-1) from March to June (pre-monsoon). During this period, the prevailing wind

778

direction (WD) was from the southwest. At the end of June (onset of the monsoon), the pattern

779

changed with lower WS (0.8 m s-1) and a shift to the southeast. The lowest mean WS (0.62 ±

780

0.19 m s-1) was observed during the winter when the WDs were mostly west and southwest. The

781

annual mean temperature was 24 ± 4.5oC and varied from 3.0 to 32.9oC. The seasonal mean

782

temperatures were monsoon (28.9oC), post-monsoon (24.4oC), pre-monsoon (23.9oC), and winter

783

(18.4oC). Annual mean RH was 82 ± 4.5% and varied from 63 to 92%. Seasonally, RH varied in

784

order of monsoon (85%), post-monsoon (84%), pre-monsoon (77%), and winter (82%). This

785

region of India receives heavy rainfall compared to other areas. The annual RF during the study

786

period was 1970 mm with the highest deposition in the monsoon season (1164 mm) and lowest

787

in winter (11 mm). Details on the variation of meteorological conditions over the region were

788

presented in an earlier report (Tiwari et al., 2016a; 2017).

789

Dispersion of atmospheric pollutants depend upon the meteorological conditions. The wind

790

speed and mixed layer height (MLH) defines the volume of air through which the pollutant

791

mixed (Collier et al., 2005). The ventilation coefficient (VC) is the product of MLH and mean

792

WS through the mixing layer. It reflects the transport rate in the mixing layer. The VC values

793

were estimated using the Equation (1) (Ashrafi et al., 2009):

794

VC=ZU

795

Where Z is atmospheric MLH (in meters) above the surface, and U is the average WS (in m s-1)

796

within MLH. The MLH were estimated using the HYbrid Single-Particle Lagrangian Integrated

797

Trajectory model (HYSPLIT; Draxler, 1999), and details are presented elsewhere (Stein et al.,

798

2015; Dumka et al., 2015a, b). HYSPLIT was also used to calculate air parcel back trajectories

799

using the reanalysis meteorological data set. The values of U were measured at a height of 10 m

800

above the ground.

(1)

801 802

3.2. Ambient concentrations of O3, CO, CO2, NOx, CH4, and NMHC

803

The ambient concentrations of O3, CO, CO2, NOx, CH4, and NMHC were measured from 1st

804

January 2013 to 30th June 2014 are shown in Figure 2. The daily mean concentrations were 10.5

805

± 6.30 ppb, 0.5 ± 0.2 ppm, 411.7 ± 37.5 ppm, 19.6 ± 11.6 ppb, 2045 ± 417 ppb, and 173 ± 93

806

ppb, respectively (Figure 2). The ozone concentrations in Guwahati are quite low and reflect the

807

near-source conditions at this site such that there is substantial titration of the ozone by freshly 33

808

emitted NO from the nearby highway. The O3 concentrations were well below the World Health

809

Organization (WHO) 8-hour maximum standard (100  µg m-3 or 50 ppb) that was established to

810

adequately protect public health (WHO, 2006). However, they have also reported that some

811

health effects may occur below this level.

812

For the period of July 1st, 2013, to December 31st, 2013, the NOx data also included NO and NO2

813

concentrations. Thus, the dynamics of the NOx species with ozone and solar radiation can be

814

examined. Figure 3 shows 3 days of data in early November 2013. During the evening hours,

815

ozone becomes depleted, and NO dominated the NOx. The variable NO concentrations showed

816

the influence of local NO sources whereas the NO2 declined monotonically from its maximum

817

value once the O3 has been depleted. As the solar radiance increased after sunrise, ozone forms

818

and the NO is converted to NO2 but is then photolyzed to form more ozone reaching a photo-

819

stationary state (White, 1977).

820 821

3.3. Temporal Variations

822

The daily variations in ozone by hour of the week are shown in Figure 4, where the median

823

values for each hour of the day for each day of the week are plotted along with the 25th and 75th

824

percentile values as error bars. There is little day-to-day variability in the median values.

825

However, the ranges vary from day to day. The O3 concentrations were higher between 09:00 to

826

19:00 local time peaking in the early afternoon when solar irradiance was high. There is no clear

827

“weekend effect” as observed in many developed countries (Cleveland et al., 1974; Lebron,

828

1975; Cleveland and McRae, 1978) where there is a substantial reduction in heavy-duty truck

829

traffic on weekends resulting in decreased NO emissions with a commensurate increase in ozone

830

concentrations.

831

The hour-of-the-week plots for the other measured species are shown in Supplemental

832

Information Figures S-1 to S-5. For CO, the diel fluctuations in its concentrations were smaller

833

in the pre-monsoon, monsoon and post-monsoon when compared to the winter period. Over

834

northern part of India, farmers burn their crop residues during the post-monsoon period and

835

produce a huge amount of atmospheric pollution. Its impact could be observed in the CO

836

concentrations where there was no variation between daytime and nighttime CO concentrations

837

during that period. Figure S-6 shows a plot of the frequency of air parcel back trajectory

838

endpoints over the period of November 11 to December 15, 2013 for each 1º latitude by 1º

839

longitude grid cell. Effective transport from the area of the IGB, where agricultural burning is 34

840

widespread can be clearly seen. Goyal and Chalapati (2007) reported that during the pre-

841

monsoonal months, the relatively constant CO concentrations throughout the day were likely

842

related to the mixing of air masses within a deeper boundary layer. Thus, the transported

843

agricultural burning aerosol could be effectively admixed to ground level.

844

For CO2, there was little day-to-day variability observed. CO2 concentrations >420 ppmv were

845

observed in the overnight hours (21:00 to 07:00 LT) compared with daylight hours (08:00 to

846

20:00 LT) where concentrations dropped to ~390 ppmv. The lower concentrations of CO2 were

847

likely due primarily to the increased wind speeds during the day since the daytime source

848

emissions rates will be high with increased motor vehicle traffic and increased industrial activity.

849

Hourly O3 concentrations were also analyzed by month (Figure 5) with substantial month-to-

850

month variation in median values and the extent of their dispersion. Mean values were lower

851

during the monsoon (7.5 ppb) and post-monsoon (6.39 ppb) seasons and higher in the pre-

852

monsoon (14.8 ppb) and winter (10.0 ppb) seasons (Table 2). However, there are significant

853

month-to-month variations that do not correlate with the variations in solar intensity (Figure S-7)

854

as would be anticipated if the measured ozone reflected regional concentrations.

855

Additionally, high winter O3 concentrations (second largest) may be affected by biomass burning

856

(BB) aerosol transported from the western IGB (Kaskaoutis et al., 2014; Tiwari et al., 2015) as

857

shown in Figure S-6 and shown by the impact of the transported aerosol on CO during this

858

period. BB produces significant amounts of reactive hydrocarbon species concentrations and

859

resulting high O3 (Thompson et al., 2001). The lower mean O3 concentration (7.5 ppb) observed

860

during the monsoon season was due to the intense rainfall that can wash out proxy radicals (HO2

861

and RO2) by wet deposition limiting the gas phase photochemistry of O3 formation as well as the

862

reduction of solar radiation by clouds during rain events (Seinfeld and Pandis, 2016). The post-

863

monsoon season O3 concentrations are comparable to earlier studies at other urban locations in

864

India (Debaje and Kakade, 2009; Lal et al., 2012; Chelani, 2012; Tiwari et al., 2016b). However,

865

O3 concentrations are lower than the rural and high-altitude sites in India where NO emissions

866

are significantly lower (24-36 %) (Reddy et al., 2010). In Delhi, the average concentration of O3

867

was reported much higher (23.6 ppb) than the Guwahati (Tiwari et al., 2015). The concentration

868

of O3 over Guwahati was lower may be due to a significant amount of untitrated NO which was

869

produced due to local sources of NO in the surrounding of the monitoring site such as traffic.

870

The other gaseous species were also separated seasonally as presented in Table 2.

35

871

The monthly patterns for the other 5 species are shown in Figures S-8 to S-12. Seasonally the

872

mean mixing ratios of CO was 0.73 ± 0.3 ppm (winter), 0.47 ± 0.3 ppm (pre-monsoon), 0.38 ±

873

0.2 ppm (monsoon) and 0.51 ± 0.2 ppm (post-monsoon). The mean CO concentration in

874

Guwahati was around three times lower than Delhi (Sharma et al., 2016) and equal to that in

875

Kanpur, an urban site in the central IGB region (Gaur et al., 2014). Generally, the CO

876

concentrations increase during the morning traffic hours. In the case of CO2, there is low

877

variability by season with concentrations always greater than 400 ppmv. Chhabra and Goel

878

(2017) have reported the mean CO2 concentration over India was ~ 399.73 (±1.89) ppm using

879

satellite-based

880

(http://www.globalcarbonatlas.org/en/CO2-emissions), an exponential growth of 5.47% per year

881

(Figure S-13) becoming the world's third-largest fossil-fuel CO2-emitting country.

882

NOX is emitted by high-temperature combustion and is likely dominated by emissions from the

883

nearby national highway. Figure S-14 presents a conditional bivariate probability function

884

(CBPF) plot for NOx over the 18 months of measurements. It can be seen that there is a strong

885

influence of the highway to the NW of the site under low wind speed conditions. As observed in

886

Figure 3, NO and NO2 concentrations are driven largely by photochemical nitrogen oxide cycle

887

and during daylight hours likely form a photo stationary state (White, 1977). In Guwahati, NOx

888

varied from 11.8 ppb (monsoon) to 32.5 ppb (winter).

889

Regional O3 is also produced during transport by chemical reactions with organic precursors

890

such as CH4 and non-methane volatile organic carbon, and UV radiation (Simpson et al., 2014).

891

Both CH4 and NMHC had a peak at the beginning of the sampling period and then had relatively

892

little variation throughout the rest of the monitoring period (Figure 2). The mean CH4

893

concentration (2041 ppb) in Guwahati was higher than the 2013 global mean concentration of

894

1813.32 ± 0.62 ppm (Dlugokencky, www.esrl.noaa.gov/gmd/ccgg/trends_ch4/) with relatively

895

low variability by season of 1945 ppb (winter) to 2111 ppb (monsoon). NMHCs will also be

896

involved in the regional chemistry through the formation of proxy radicals that can oxidize NO

897

back to NO2 without consuming an O3 molecule. However, it does not appear to have a

898

substantial effect on the Guwahati values given the strong direct relationships with the NOx

899

species. However, in the downwind regions, it can be anticipated that there will be higher

900

concentrations of ozone with the potential for substantial crop damage. The NMHCs seasonal

901

behavior showed lowest concentrations in the pre-monsoon (136 ppb) and highest in the post-

902

monsoon (210 ppb) seasons likely a result of biogenic emissions during the plant growth that

data.

Based

on

data

36

from

the

Global

Carbon

Atlas

903

followed the rainfall. Nishant et al. (2014) suggested that the importance of NMHCs as O3

904

precursors depends largely on their reactivity and atmospheric concentrations. Other major

905

sources of NMHCs in the urban region are from chemical industries, solvent evaporation refinery

906

operations, and vehicular emissions (Liu et al., 2017). The nocturnal NMHC concentrations were

907

approximately double than the daylight hours (Figure S-5) reflecting the lower boundary layer

908

height, lower wind speeds, and possibly greater emissions.

909 910

3.4. Impact of ventilation coefficient and solar radiation on surface ozone

911

The MLH is a crucial parameter that determines the ability of atmospheric pollutants to

912

accumulate and disperse and about the structure of turbulence within the mixed layer (Seibert et

913

al., 2000). The concentrations of atmospheric pollutants are affected by the extent of dispersion

914

as reflected by the VC (Krishnan and Kunhikrishnan, 2004). Low VC values indicate poor

915

dispersion of pollutants whereas high VC conditions support dispersion of the pollutants. The

916

diel variation in VC by different season were calculated for Guwahati and presented in Figure 6.

917

The hourly VC values show a similar trend for all seasons with the highest value (1935 m) in the

918

summer and lowest (209 m2 s) in the post-monsoon season. The VC starts increasing from 08:00

919

local time and reaches its maxima at 15:00 local time. The daytime (08:00 to 19:00 LT) and

920

nighttime (19:00 to 08:00 to local time) mean VC values are presented in Table 4. The VC

921

values were about one order of magnitude higher during the daytime (1360 m2 s) that at night.

922

Although higher VC conditions typically lead to lower pollution concentrations, for O3, the

923

opposite pattern was observed because of the higher photochemical activity during the day

924

driven by the solar radiation. For the other species (CO, NOx, CH4 and NMHC), lower

925

concentrations were measured during the daytime (Figures S-1 to S-5).

926

Eagleman (1991) proposed that the low pollution potential occurs when the VC is >6000 m2 s.

927

This criterion is met in pre-monsoon during 11:00–17:00 local time. However, the above

928

criterion is not fulfilled in the winter, monsoon, and post-monsoon seasons. Praveena and

929

Kunhikrishnan (2004) observed that the MLH was lower during the winter over the Indian

930

subcontinent. In the monsoon season, the strong prevailing winds disperse the atmospheric

931

pollutants quickly and they are washed out by the precipitation. There is also less solar radiation

932

reaching the ground due to the cloud layers. Alternatively, both wind speed and MLH depth are

933

lower during the winter (Alappattu et al., 2009). A significant impact of MLH was observed in

37

934

the case of CH4 and NMHC during the daytime when the WS was highest favoring the

935

dispersion of pollutants and resulting in the lowest concentrations of CH4 and NMHCs.

936 937

3.5. Relationships among the measured species

938

The ratio between CO and NOx suggests the presence of local air masses at the receptor site (Ou

939

et al., 2015). Because of the presence of different emission sources in the IGB, the Bay of Bengal

940

and other regions surrounding the receptor site, the CO/NOx ratio can be used to categorize the

941

air mass origins. The CO/NOx ratios were estimated seasonally and were highest in pre-monsoon

942

(32.6) followed by the monsoon season (32.2) when the average wind direction was 1710 (south-

943

east) and 1830 (south) confirming the dominance of local air masses at this site given the local

944

anthropogenic activities in that direction. The lower CO/NOx ratios when the air masses arrive

945

from 2070 and 2230 (southwest) were 27.5 and 25.1, respectively. Overall, this analysis revealed

946

that higher CO/NOx ratios indicated that the air was likely transported from distance source

947

instead of being dominated by local sources.

948

The seasonal mean O3, CO, NOx, CH4, and NMHC concentrations were separated by WD. The

949

concentrations of CO were highest when the air masses came from between 00 to 900 and 900 to

950

1800 during the winter and post-monsoon periods. NMHCs were significantly higher (>225 ppb)

951

when the air masses came from between 00 to 900 during the winter, monsoon, and post-monsoon

952

seasons but not pre-monsoon suggesting that more gasoline evaporation occurred. However,

953

during pre-monsoon, the higher level of photochemical activity may reduce the NMHC

954

concentrations. Jia et al. (2016) suggested that the O3 formation is more sensitive to NOx level at

955

high VOC/NOx ratios. The NHMCs/NOx ratio was 9.45 suggesting that NMHC can play an

956

important role in ozone formation in the region downwind of Guwahati.

957 958

8. Conclusions

959

The measurements of ozone (O3) and related gaseous species (carbon monoxide: CO, carbon di-

960

oxide: CO2, oxides of nitrogen: NOx, methane: CH4, and non-methane hydrocarbons: NMHC)

961

were conducted in Guwahati. Relatively low concentrations of O3 were measured in Guwahati

962

compared to other urban sites in India. Examination of the limited NO, NO2, and O3 data

963

suggested that this site is strongly affected by local NOx sources, one of which is an adjacent

964

major national highway. These results suggest that during much of the daylight hours, this site is

965

in or nearly in a photo-stationary state indicating a low impact of organic species on the ozone 38

966

concentrations. However, given the high NMHC to NOx ratios measured at this site, it is likely

967

that there is a substantially higher ozone concentrations in the region downwind of Guwahati that

968

could result in substantial crop damage. Thus, non-urban monitoring for ozone is essential if all

969

of the effects of urban air pollution are to be effectively assessed. These results suggest that the

970

location of the monitoring sites is often too close to major NOx sources. Thus, to obtain a more

971

accurate assessment of ozone pollution, sites will need to establish in more suburban or rural

972

areas to avoid the effects of NO titration.

973 974

Acknowledgements

975

The authors would like to thank the Vice-Chancellor of Guwahati University for providing the

976

infrastructure and facilities at the university and the Director, Indian Institute of Tropical

977

Meteorology, Pune, for providing the financial support for installing the instruments at the

978

Department of Environmental Sciences of Guwahati University. Authors conveyed the heartiest

979

thanks to Dr. Beig along with SAFAR groups who commissioned the station and continuously

980

take care of the instruments and manage the data generation. We would like to thank the NOAA

981

Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion

982

model. We would like to thanks the Prof. Mustafa Odabasi, Editor-in-Chief, Atmospheric

983

Pollution Research, and anonymous reviewers for their constructive comments and suggestions

984

to improve the scientific quality of manuscript.

985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000

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44

1196 1197 1198 1199 1200 1201

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45

1202

Table 1: Seasonal-mean of metrological parameters except for rainfall (total). The seasons are classified as pre-monsoon (March-

1203

April-May), monsoon (June-July-August-September), post-monsoon (October-November), and winter (December-January-February),

1204

respectively Seasons

Solar Radiation

Temperature 0

( C)

-2

Wind Speed

Wind 0

Direction ( )

-1

(Wm )

(m s )

Relative

Rainfall

Humidity

(mm)

(%)

Winter

109.5

18.4

0.6

223

81.6

11

Pre-monsoon

180.3

23.9

2.2

171

76.8

648

Monsoon

176.2

28.9

1.1

183

84.5

1164

Post-monsoon

148.3

24.4

0.6

207

83.7

147

Annual mean

153.5

23.9

1.1

196

81.6

1970

1205 1206

Table 2: Seasonal-mean concentrations of surface ozone and related species Seasons

O3

CO

CO2

NOx

CH4

NMHC

Winter

10.1

0.7

415.7

32.5

1945.0

187.7

Pre-monsoon

14.8

0.5

415.5

15.6

2035.9

135.8

Monsoon

7.5

0.4

402.2

11.8

2111.3

192.5

Post-monsoon

6.4

0.5

416.3

19.1

2071.4

209.8

Annual mean

9.7

0.5

412.4

19.8

2040.9

181.5

1207 1208

46

1209

Table 3: Monthly mean concentrations (ppbv) of measured surface ozone in Guwahati, IGB, and India Sampling site

Concentrations of O3

References

Guwahati (BRV)

10.5

Present study

IGB

15.7

Present study

Ahmadabad

20.7

Beig et al., (2007)

Pune

30.1

Debaje and Kakade, (2009)

Delhi

41.5

Chelani (2012)

Patna

14.5

Tiwari et al., (2016)

Kanpur

27.9

Gaur et al., (2014)

Mt. Abu

39.9

Naja et al., (2003)

Udaipur

28.6

Yadav et al., (2016)

Anantapur

35.9

Ahammed et al., (2006)

Agra

21.0

Satsangi et al., (2004)

Kannur

18.4

Nishanth et al., (2012)

Varanasi

41.5

Tiwari et al., (2008)

Nainital

43.9

Kumar et al., (2010)

Trivandrum

11.5

David and Nair, (2011)

1210 1211

47

1212

Table 4: Seasonal-mean ventilation coefficient (VC: m2s), surface ozone (O3: ppbv) and solar radiation (SR: W·m-2) and mixed layer

1213

height (MLH: m) and Temperature (Tem.: 0C) Night (19:00-08:00 LT) Seasons

VC

O3

SR

BHL

TEM

WIN

49.8

3.36

3.8

114

15.8

PRM

367.4

7.79

11.9

228

23.2

MON

129.5

4.46

12.2

202

27.0

POM

40.5

2.48

8.2

125

21.8

Annual Mean

146.8

4.5

9.0

168

22.0

Day (08:00-19:00 LT) WIN

578.9

16.32

215.2

642

21.1

PRM

3502.6

18.21

348.6

1114

28.7

MON

980.7

10.45

340.1

634

30.7

POM

377.2

10.32

288.3

481

26.9

Annual Mean

1359.9

13.83

298.06

717

26.9

1214

48

1215 1216 1217

Figure 1: Geographical location of sampling site (i.e. Guwahati University) over the Indian subcontinent.

49

1218 1219 1220 1221

Figure 2: Daily and 20 days moving mean of O3 (ppb), CO (ppm), CO2 (ppm), NOx (ppb), CH4 (ppb) and NMHC (ppb) from January 1st, 2013 to 30th June 2014 in Guwahati, India. 50

700 15

40

30

10

20 5

10

600 500 400 300 200

Solar Radiation (w.m2)

50

100

0 11/06

1222 1223 1224

800

20

NO NO2 O3 SR

O3 Concentration (ppb)

NO or NO2 Concentration (ppb)

60

11/07

0 11/09

11/08

0

Figure 3. Daily variation of NO, NO2, O3, and SR for 3 days in November 2013. 35

Ozone 30

Ozone (ppb)

25

20

15

10

5

0 Sunday

Monday

Tuesday

Wednesday Thursday

Friday

Saturday

Hour of the Week 1225 1226 1227

Figure 4. Median ozone concentration plotted as a function of hour of the week. The error bars represent the 25th and 75th percentile values. 51

50

Solar Radiation (W m-2) Ozone (O3)

1000 900

Ozone (ppb)

800 700

30

600 20

500 400

Solar Radiation (W m-2)

40

10 300 0 Jan 1228 1229 1230 1231 1232

Feb Mar Apr May Jun

Jul

Aug Sep

200 Oct Nov Dec

Figure 5. Median ozone concentrations in Guwahati by month (black points) with the error bars showing the 25th and 75th percentile values. Red star color gives the monthly median solar radiation with the error bars representing the 25th and 75th percentile values. The Ozone and solar radiation was taken from 09:00 to 15:00 local time.

52

1233 1234 1235

Figure 6: Variations of hourly ventilation coefficient in different seasons at Guwahati during January 2013 to June 2014.

53

1

Evaluation of Urban Ozone in the Brahmaputra River Valley

2

U.C. Dumka,1* A.S. Gautam2, S. Tiwari3, D.S. Mahar2, S.D. Attri4, R. K. Chakrabarty5, P.

3

Permita5, Philip K. Hopke6, 7

4

1

Aryabhatta Research Institute of Observational Sciences, Nainital – 263 001, India

5

2

Indian Institute Meteorological Department, New Delhi Branch, 110060, India

6

3

H.N.B. Garhwal University, Srinagar, Uttarakhand-246174, India

7

4

India Meteorological Department, New Delhi, 110060, India

8

5

9

USA

10

Center for Aerosol Science and Engineering, Washington University in St. Louis, St. Louis, MO - 63130,

6

Center for Air Resources Engineering and Science, Clarkson University, Potsdam, NY-13699-

11

5708, USA

12

7

13

Dentistry, Rochester, NY 14642 USA

Department of Public Health Sciences, University of Rochester School of Medicine and

14 15

Research Highlights

16

• Relatively low ozone concentrations were measured in Guwahati, India.

17

• Ozone concentrations in Guwahati were dominated by local NOx emissions.

18

• A high mean NHMCs/NOx ratio (9.45) suggests high downwind O3 values.

*

Correspondence to: U. C. Dumka (E-mail: [email protected]; [email protected]); Suresh

Tiwari (E-mail: [email protected]); P. K. Hopke (E-mail: [email protected])

1

o The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. o The authors declare the following financial interests/personal relationships which may be considered as potential competing interest. On behalf of all authors U C Dumka Corresponding author