Significance of volatile organic compounds and oxides of nitrogen on surface ozone formation at semi-arid tropical urban site, Hyderabad, India R. Venkanna, G. N. Nikhil, P. R. Sinha, T. Siva Rao & Y. V. Swamy
Air Quality, Atmosphere & Health An International Journal ISSN 1873-9318 Air Qual Atmos Health DOI 10.1007/s11869-015-0347-2
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Author's personal copy Air Qual Atmos Health DOI 10.1007/s11869-015-0347-2
Significance of volatile organic compounds and oxides of nitrogen on surface ozone formation at semi-arid tropical urban site, Hyderabad, India R. Venkanna 1 & G. N. Nikhil 1 & P. R. Sinha 2 & T. Siva Rao 3 & Y. V. Swamy 1
Received: 8 January 2015 / Accepted: 22 April 2015 # Springer Science+Business Media Dordrecht 2015
Abstract The chemistry and variation in light molecular weight (C2–C5) volatile organic compounds (VOCs) and nitrogen oxides (NOx =NO+NO2) over the formation of tropospheric ozone (O3) was studied for a time period of 1 year (2013) at a tropical urban site located in Deccan plateau region of Hyderabad, India, with semi-arid climate. Diel pattern of hydrocarbons showed maxima in the morning and night and minima in the afternoon. Ethylene and propylene showed relatively larger diurnal amplitude than other hydrocarbons. Among the analyzed hydrocarbons, acetylene was the most abundant with an annual mean of 5.5±1.3 ppbv. All the VOCs exhibited a seasonal variation with monsoon and summer minimum and winter maximum. Ozone formation potentials (OFP) and propylene-equivalents (propy-equiv.) were calculated to account the contribution of individual hydrocarbons towards formation of O3. Propylene had the highest contribution of propy-equiv. (34 %) and OFP (28.4 %) among the VOCs observed. The concentrations of VOCs and their reactivity with hydroxyl radicals played a significant role on the levels of propy-equiv. and OFP. Strong correlations 0.65 and 0.77 were observed between O3 vs. propy-equiv. and O3 vs. OFP, respectively. The crossover point relationship between NOx, VOCs, and O3 showed enhancement of O3 at lower levels and decreased at higher levels of NOx in the range of
* Y. V. Swamy
[email protected] 1
Bioengineering and Environmental Sciences, Indian Institute of Chemical Technology, Hyderabad 500 007, Andhra Pradesh, India
2
National Balloon Facility, Tata Institute of Fundamental Research, Hyderabad 500 062, Andhra Pradesh, India
3
Department of Inorganic and Analytical Chemistry, College of Science and Technology, Andhra University, Visakhapatnam, India
VOCs concentrations studied. Among hydrocarbons, propylene (10) and ethane (6.5) showed the highest and lowest crossover points, respectively. Keywords Correlation analysis . Hydrocarbons . Nitrogen oxides . Peroxy radicals . Surface ozone
Introduction Volatile organic compounds (VOCs) are the major precursors of ozone (O3, secondary pollutant) and play an important role in tropospheric chemistry. Emissions of VOCs have natural (vegetation and seawater) as well as anthropogenic sources in the troposphere. The global emission rate is estimated at ~1× 1014 gC yr−1 for anthropogenically derived compounds (stationary and mobile sources) (Piccot et al. 1992) and ~8× 1014 gC yr−1 for biogenic compounds (Guenther et al. 1995; Zimmerman et al. 1978). In Hyderabad city, the air pollution load is also mainly contributed by anthropogenic emissions due to rapid urbanization and increased economic activity. The present experimental work was carried out at Tata Institute of Fundamental Research-National Balloon Facility (TIFR-NBF), Hyderabad, to analyze the relative variability in VOCs concentrations. Ambient air samples were collected intermittently at TIFR and from different surrounding sites viz., ECIL X Roads (2 km), Tarnaka (10 km), and Habsiguda (12 km) which are away from the experimental site. The emission sources at these sites are majorly contributed by automobile traffic (both light and heavy motors) and liquefied petroleum gas (LPG) leakage from the nearby gas filling stations. The reports by Census India 2011 (http://www.census2011.co. in/census/district/122-hyderabad.html) recorded the population density of 18,172 people per square kilometer for Hyderabad. This city comprises of many industrial
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development areas and the main experimental site is surrounded by many industries such as Electronics Corporation of India Limited (ECIL), Hindustan Cables Limited (HCL), Nuclear Fuel Complex (NFC), petroleum storage containers, bottling units of Hindustan Petroleum Corporation Limited (HPCL), and Bharat Petroleum Corporation Limited (BPCL). The records of central pollution control board (CPCB) showed that there were over 1.1 million vehicles on road in this city in the year 2006 (http://cpcb.nic.in/). Guttikunda and Kopakka (2014) reported the sector-specific emissions for 2010–2011 for this city, accounted for 113,400 t of VOCs emissions. The predominant anthropogenic sources of VOCs are vehicular and industrial emissions such as fossil fuel combustion, biomass burning, LPG and natural gas (NG) leakages, fuel evaporation, petroleum distillation, oil refineries, and solvent usage (Barletta et al. 2005; Chen et al. 2001; Cheng et al. 1997; Duan et al. 2008; Na et al. 2001; Seila et al. 2001; Tang et al. 2007; Watson et al. 2001). In polluted urban areas, contribution of alkanes (45 %) in ambient air is higher than that of alkenes (10 %), and aromatic hydrocarbons (20 %) towards VOCs (Solomon 1994). Biogenic VOCs like isoprene also significantly contribute in the urban and regional environments. Anthropogenic and biogenic VOCs are important precursors for secondary organic aerosol (SOA) formation, where organic component of particulate matter transfers to the aerosol phase from the gas phase as products of gas-phase oxidation of parent organic species in polluted regions (Kanakidou et al. 2005). A large number of complex photochemical reaction take place by wide range of VOCs with different reaction mechanisms in the troposphere at urban and rural areas (Carter 1994). The photochemical oxidation of VOCs initiated by rapid reaction with hydroxyl radicals (OH˙) involves various chemical chain reactions and several gaseous intermediates such as alkyl peroxy radicals (RO2˙), peroxy acetyl nitrate (PAN), carbonyls and organic nitrate compounds (RONO2). Photochemical processes involving VOCs in the presence of sufficient amount of nitrogen oxides (NOx) leads to O3 formation (Caselli et al. 2010; Duan et al. 2008; Seinfeld 1989; Sillman 1999; Steinfeld 1998; Han et al. 2011; Swamy et al. 2012; Yerramsetti et al. 2013b). Further, the reaction of OH˙ with intermediate oxidation products (acetone, aldehydes, hydroperoxides etc.,) enhances the lifetime of greenhouse gases (GHGs) (Offenberg et al. 2011; Poisson et al. 2000). The reactivity of each VOC determines its respective impact over regional atmospheric chemistry. Peroxy radicals (HO2˙) can be formed as a result of photolysis processes. In brief, O3 is formed through oxidation of hydrocarbons by forming peroxy radicals according to the following chain reactions. CH3 −CH3 þ OH˙ →CH3 −CH2 ˙þ H2 O one example f or alkyl radical R˙
ð1Þ
CH3 −CH2 ˙ þ O2 →CH3 −CH2 O2 ˙
ð2Þ
Alkyl peroxy radicals and hydro peroxy radical can oxidize NO to NO2 to form hydroxyl radical. CH3 −CH2 O2 ˙ þ NO→CH3 CH2 O˙ þ NO2
ð3Þ
CH3 CH2 O˙ þ O2 →CH3 CHO þ HO2 ˙
ð4Þ
˙
˙
HO2 þ NO→OH þ NO2
ð5Þ
In the troposphere, NO2 is formed in several processes provides the primary source of oxygen atoms in presence of sunlight which is required for O3 formation. ð6Þ NO2 þ hν ð< 420 nmÞ→NO þ O 3 P The oxygen atom combines with an oxygen molecule to produce O3 O 3 P þ O2 þ M→O3 þ M ð7Þ The O3 formed in troposphere is one among the GHGs that shows adverse effects on human health and plant growth when exceeded above the ambient air quality standards (Burnett et al. 1994; Choi and Hwang 2011; Lai et al. 2009; Mudliar et al. 2010; Shao et al. 2009). In India, the National Ambient Air Quality Standards (NAAQ) has proposed 50 ppbv of O3 in the surface level (http://cpcb.nic.in/National_Ambient_Air_ Quality_Standards.php). Hyderabad is on the most polluted city in south Indian region, which has many sources of hydrocarbons. However, there were no monitoring studies on concentration and speciation of VOCs in this tropical urban site. Therefore, an investigation was carried out to determine the factors influencing the temporal variations of VOCs and emission sources and empirically analyze the contribution of hydrocarbons towards O3 formation at this site. Estimation of the reactivity and contribution of individual hydrocarbons to photochemical O3 formation was performed by using reactivity scale methods such as the propylene-equivalent concentration (propy-equiv.) and the ozone formation potential (OFP). Maximum incremental reactivity (MIR), which is a good indicator for comparing OFP of individual hydrocarbons, was used for the study.
Experimental methodology Analysis of O3 and NOx Continuous monitoring of O3 and NOx was carried out using online gas analyzers which are installed at Tata Institute of Fundamental Research-National Balloon Facility (TIFRNBF), Hyderabad, Telangana, India. The O3 analyzer (49i; Thermo Scientific, USA) operates on the UV light absorption principle at a wavelength of 254 nm and NOx analyzer (42i;
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Thermo Scientific, USA) works on chemiluminescence principle where light intensity is linearly proportional to the NO concentration. Accuracy of the analyzers was maintained by calibrating every fortnight with NIST (US National Institute of Standards and Technology) traceable standard gas by using multi-gas calibrator (Model 146i, Thermo Scientific, USA). Detailed methodology of trace gas measurements was previously reported (Swamy et al. 2012; Venkanna et al. 2014). Analysis of VOCs Ambient air samples were collected at the experimental TIFR site and sub-sites (one rural site and two traffic junctions, Habsiguda and Tarnaka, in Hyderabad) using a 500-mL glass air sampling unit. Measurements of ethane, propane, i-butane, n-butane, n-pentane, i-pentane, ethene, propene, and acetylene were carried out using the pre-concentration set-up connected to the gas chromatography (Shimadzu GC-17A, Japan). The collected air samples were transferred into an evacuated stainless steel canister of 250 ml. The canister was connected to an 8valve port (V1), U-type column (25 cm long with 6.3 mm outer diameter) filled with glass beads of 60/80 mesh was connected between one port of V1 with another 6-valve port (V2). The air from the canister was loaded into the U column and pre-concentrated for a pre-determined time under cryogenic conditions by using liquid nitrogen. After pre-concentration, the column was kept under isolated condition and heated with warm water; later, the desorbed volatile gases were injected into GC. Sample loading, isolation, and injection were carried out by operating V1 and V2 valves. VOCs were analyzed using GC equipped with flame ionization detector (FID) and PLOT capillary column (Supelco) of 30 M long, 0.53 mm i.d. with Al2O3/ KCl as stationary phase. High purity nitrogen (N2) was used as carrier gas. The column oven temperature was ramped from 40 to 180 °C. Initially, 40 °C was maintained for the first 5 min; afterwards, the temperature was raised at a rate of 5 °C/min until it reached 120 °C and kept at this temperature for 5 min. In the final step, the temperature was increased to 180 °C at a rate of 25 °C/min, and then maintained at 180 °C till the end of the analysis. Pure standard of the individual gases were injected to identify their retention times in chromatogram. Besides, for quantification of each peak, the calibration was done using two different calibration gas mixtures (NIST traceable) and found to be more reliable. GC calibration was done before analysis. The analytical performance was evaluated based on linearity and accuracy parameters. The minimum detection limits for all the nine VOCs were in the range of 50–150pptv. The overall uncertainties were in the range of 10–20 % of detection limits for all the gases. A calibration graph of area versus
concentration was drawn for the quantification of each air sample (Swamy et al. 2012).
Results and discussion Diurnal variations of hydrocarbons The annual mean diel variations of C2–C5 VOCs during different time intervals were evaluated for the TIFR site for the year 2013 and are graphically represented in Fig. 1. Vertical bars in the figure show the standard deviation (1s) from the mean. Diel patterns of hydrocarbons showed morning and evening peak concentrations and low concentrations during afternoon hours (1200 to 1600 h). The maxima at morning/ night time and minima at afternoon is a distinct feature of an urban polluted site. The VOCs concentrations during morning and evening are attributed to emissions from heavy vehicular traffic and weak vertical diffusion in the shallow boundary layer (Liu et al. 2015; Toro et al. 2014a, b). Low wind speed and high relative humidity during night time causes the weak diffusion of gases. The lower VOCs levels during noon time are due to the increased mixing boundary layer height and photochemical oxidation reaction with OH· radicals in presence of high solar radiation and temperature (Liakakou et al. 2009; Tang et al. 2007). Ethylene and propene showed larger amplitude compared to other hydrocarbons almost throughout the year. Afternoon concentrations of ethylene and propylene were 51 and 54 % lower than the evening peak hour levels. These variations might be due to high reactivity of alkenes (ethene and propene) with hydroxyl radicals (OH·) predominantly in presence of high temperature and solar radiation compared to other hydrocarbons (Atkinson 1997; Hagerman et al. 1997). With increasing VOC reactivity, the diel amplitude (percentage of the nighttime maximum to the daytime minimum concentration) also increases. However, diel amplitude was less in case of n-pentane (18 %) compared to other hydrocarbons due to low reactivity towards photochemical oxidation as well as contribution by the emission of pentanes from gasoline and solvent evaporation at high temperatures at noon time (Barletta et al. 2005; Tan et al. 2012). The average concentrations of nine hydrocarbons are placed in the order of propylene
