Air Qual Atmos Health DOI 10.1007/s11869-014-0309-0
Spatial and temporal variability of surface ozone and nitrogen oxides in urban and rural ambient air of Delhi-NCR, India Amit Kumar & Deepak Singh & Bhupendra Pratap Singh & Manoj Singh & Kumar Anandam & Krishan Kumar & V. K. Jain
Received: 29 October 2014 / Accepted: 17 December 2014 # Springer Science+Business Media Dordrecht 2014
Abstract Ozone (O3), nitric oxide (NO), nitrogen dioxide (NO2), and nitrogen oxides (NOx) were measured continuously at three sites viz. urban background (JN), urban/traffic (CP), and rural (DP) in Delhi-NCR during the years 2013–2014. Meteorological parameters (temperature and relative humidity) were also measured in order to evaluate the relationship with targeted pollutants. The study shows that highest concentration of O3 was in summer while the lowest ones were recorded in winter and autumn for all the three sites. However, the level of NOx was observed maximum in CP (22.6 ppb) during winter and minimum in DP (5.3 ppb) during autumn. The diurnal variation of O3 was characterized by day-time maxima/(night-time minima) having concentrations 50.2/ (17.2), 46.1/(15.7), and 56.7/(23.6)ppb at JN, CP, and DP, respectively. Distinct differences in concentrations were observed for O3 and its precursors during weekends and weekdays for all the three sites. The analysis revealed that higher/ (lower) levels of O3 were observed during weekend/(weekdays). The moderate weekend effect was noticed for all the three sites but highest at rural site; DP.O3 was negatively correlated to RH and NOx, while it was positively correlated to temperature. The observed mean concentrations of O3 and NO2 were found to be below the recommended guideline values established by WHO and the European Union.
Keywords Ozone . Rural . Urban . Diurnal . Meteorological parameters . Weekend effect
A. Kumar (*) : D. Singh : B. P. Singh : M. Singh : K. Anandam : K. Kumar : V. K. Jain School of Environmental Sciences, Jawaharlal Nehru University, New Delhi, India e-mail:
[email protected]
Introduction Ozone (O3) is one of the important chemical constituents of the atmosphere that play a key role in tropospheric chemistry and global climate change (Reddy et al. 2012). O3 is a precursor of hydroxyl radical (OH) which controls the oxidizing power of the atmosphere (Beygi et al. 2011). It is a secondary pollutant formed through a series of photochemical reactions between oxides of nitrogen (NOx =NO and NO2), carbon monoxide (CO), and volatile organic compounds (VOCs) under intense solar radiation (Pudasainee et al. 2006; Tu et al. 2007; Tsakiri and Zurbenko 2011; Toro et al. 2014). Increase in O3 levels in the troposphere is directly proportional to solar intensity and temperature of the atmosphere (Nishanth et al. 2012). The diurnal, seasonal, and yearly variations of O3 levels in the troposphere are affected by chemical conditions and meteorological and topographic factors (Hassan et al. 2013; Im et al. 2013). O3 precursors may be transported over long distances under the effect of meteorological processes, leading to O3 formation far from sources (Brankov et al. 2003). A phenomenon in which the concentrations of O3 is higher on weekend in comparison to weekdays is termed as “weekend effect”, despite relatively low concentrations of O3 precursors (NOx and VOCs) at weekends (Pudasainee et al. 2006). The weekend effects of ozone formation are still not well understood; however, few hypotheses have been proposed to explain the weekend effect of O3 (Blanchard and Fairley 2001; Debaje and Kakade 2006; Murphy et al. 2007). According to several studies, primary cause of higher O3 levels in weekends is due to reduction in NOx emissions in a VOC-limited chemical regime (Heuss et al. 2003; Alghamdi et al. 2014). Less absorption of sunlight due to lower fineparticle concentrations at weekends resulting in enhanced O3 formation could be the cause of weekend effect (Marr and Harley 2002).
Air Qual Atmos Health
The elevated O3 and its precursors at the ground level have a particular concern due to their harmful effects on public health (chronic and acute), various natural materials, manufactured goods, vegetation, and forest (WHO 2000; European Union 2008; Mavroidis and Ilia 2012; Fann and Risley 2013; Szyszkowicz et al. 2012; CPCB 2012–13; Kheirbek et al. 2013; Kumar et al. 2013, 2014; Singh et al. 2014). Taking into account the adverse health effects, the World Health Organization (WHO), European Union (EU), and Central Pollution Control Boards (CPCB), India have established the air quality guidelines for both O3 and NO2. The permissible limit of O3 (100 μg/m3 8-h average) and NO2 (40 μg/m3 annual average) is provided by WHO and CPCB, India. In addition to this, EU recommended the threshold value for O3 (120 μg/m3—maximum daily 8-h mean value, not to be exceeded more than 25 days per calendar year, averaged over 3 years) and NO2 (40 μg/m3 annual average and 200 μg/m3 hourly, as a value not to be exceeded more than 18 times/year). Due to their aforementioned significance, a number of studies have been done on atmospheric oxidants extensively in many rural and urban areas across the world (Duenas et al. 2004; Pudasainee et al. 2006; Tu et al. 2007; Schipa et al. 2009; Han et al. 2011; Im et al. 2013; Wang et al. 2013; Dominguez-Lopez et al. 2014). In India, several researchers have characterized the levels of O3 and NOx in different cities (Debaje et al. 2003; Londhe et al. 2008; Reddy et al. 2012; Singla et al. 2011; Swamy et al. 2012). So far, no study has been done regarding continuous monitoring of O3 and NOx in
and around the Delhi-National Capital Region (NCR) which encompasses the rural and urban area. The unique topography, type of settlement, meteorological conditions, and large number of vehicles prompted us to carry out this study. Therefore, the main objectives of the present study are to (1) assess the ground level O3 and NOx concentrations in urban and rural areas in Delhi-NCR, (2) study the diurnal and seasonal variations in O3 concentration and its relation with temperature and relative humidity, and (3) examine the difference in O3 concentrations in weekdays and weekends.
Materials and methods Measurement sites The continuous monitoring of ozone and oxides of nitrogen (NO and NO2) has been carried out at three sites in DelhiNCR, India. The geographical locations of the monitoring sites are depicted in Fig. 1. The three targeted sites are categorized as urban and rural areas. Jawaharlal Nehru University (JN) and Connaught Place (CP) are considered as background and high traffic areas, respectively, in urban locations. However, the third monitoring site is Dariyapur village (DP), a rural site. All the three locations are at the same height (216 m) above sea level. The details regarding the monitoring sites and their characteristics are listed in Table 1. The JN campus is situated southwest of Delhi having a diverse group of natural vegetation covering 1,000 acres of
Fig. 1 Map of Delhi-NCR showing the monitoring sites marked with red circles
Air Qual Atmos Health Table 1 Details of monitoring sites and its characteristics
Monitoring sites
Lat/long
Site type
Abbreviation
Jawaharlal Nehru University Connaught Place Dariyapur
28.54, 77.16 28.63, 77.21 28.81, 77.00
Urban background Urban traffic Rural
JN CP DP
area which is occupied by buildings and green areas, enriched with natural flora and fauna. CP is located in central Delhi and is a highly urbanized area having the largest commercial, financial, and business centers of Delhi. It is surrounded by huge road circles having much heavy traffic density. The rural site DP is located west of Delhi, situated 45 and 35 km of JN and CP, respectively. Although the area has recently experienced a demographic increase, it has always been characterized as an agricultural area mainly dedicated to crop production. In addition to agriculture field, a vast area of the village is occupied by subtropical trees.
(e.g., high buildings and trees) in the surrounding areas. To prevent particles from entering the instruments, particulate filter was used and replaced once every 2 weeks. Moreover, an inverted Teflon funnel was fitted at the tube entrance to avoid dust and rainwater from entering the tube and measuring systems. Instrument maintenance (monthly and annually) was carried out as per manufacturer guidelines and calibration is done every 6 months. The instrument meets the technical specifications for US EPA (Environmental Protection Agency). The O3, NO, and NO2 concentrations were recorded every 1 min. Hourly averaged data were derived from the original 1-min average interval data.
Measurement techniques and instruments Statistical analyses Sampling was conducted for the pollutants (O3, NO, NO2, and NOx) for the three seasons, i.e., summer, autumn, and winter, at three sites during the years 2013–2014. A 1-week sampling program was done in each season from Monday to Friday as weekday, and Saturday through Sunday as weekend at all three sites. Along with this, meteorological parameters (temperature and relative humidity) were also continuously measured during the sampling campaign. Surface ozone was monitored with an automatic ozone analyzer (Model EC 9810 series O3), operated on the principle of photometric detection of the specific absorption of UV light by ozone. It is a microprocessor-controlled analyzer that uses a system based on the Beer–Lambert law for measuring the low ranges of ozone in ambient air. Other pollutants (NO, NO2, and NOx) were monitored continuously using an ambient analyzer (Model Ecotech Sernious 40). The analyzer works on the principle that nitric oxide (NO) and ozone (O3) react to produce a characteristic luminescence with intensity linearly proportional to the NO concentration. Detailed descriptions of the two instruments are listed in Table 2. The instruments were placed at 7–10 m from the ground level, and ambient air samples are drawn through a 4-m Teflon tube. The sampling sites were free from the influence of nearby emissions sources and any other kinds of obstacles Table 2
Time series plotting technique was used to investigate the diurnal and seasonal variation of different air pollutants. Statistical analyses were performed by using SPSS (version 16.0.; SPSS Inc., Chicago, IL, USA) and MATLAB (R2011b; MathWorks, Natick, MA, USA) software. To check the normality of the air pollutant’s data and meteorological parameters, Kolmogorov–Smirnov test was used. It is found that most of the data were not normally distributed. Therefore, Kruskal– Wallis and Mann-Whitney non-parametric tests were applied to investigate the seasonal and weekday–weekend differences, respectively, for the air pollutants.
Results and discussions Seasonal and diurnal variability of ozone Figure 2a–c represents the seasonal and diurnal variability of O3 during different seasons at the three monitored sites using hourly concentrations averaged over the sampling duration. The differences in O3 levels are mainly influenced by the
Technical specifications and sampling methods of the instruments
Air pollutant
Instrument
Method
Measurement range
Detection limit
O3 NOx
Model EC 9810 series O3 analyzer Model Ecotech Sernious 40
UV photometric method Chemiluminescence
0–20 ppm 0–20 ppm
15 ppbv, (b) moderate weekend effect if O3 difference is 5– 15 ppbv, and (c) no weekend effect if O3 difference is
