Assessment of the Temporal Variation of Ambient Air Quality in a Metropolitan City

Key words: Air pollutants, Carbon Monoxide, Nitrogen Dioxide, Respirable Suspended Particulate Matter, Sulphur Dioxide, Suspended Particulate Matter

1. Introduction

Urban areas exhibit both the highest level of pollution and largest target of impacts on human health (Goyal and Sidhartha, 2003). In this developed and modern age, cities are fastly turning into gas chambers with the advent of more and more technological innovations and Delhi is a good example of this phenomenon (Jes, 1999; CPCB, 2000). Delhi was ranked fourth among the most polluted 41 cities in the world, as there were over 2.2 million vehicles emitting 1280 tones of pollutants daily. Diesel exhaust is considered to contain those substances, which are hazardous to human health (Aggarwal, 1999; Halankar, 1999; Gurjar et al., 2004).  This paper discussed the criteria pollutants (SO₂, NO₂, CO, RSPM, SPM) scenario of traffic zone in Delhi, India.

The gaseous air pollutants like SO₂ enter the atmosphere mostly from combustion of coal and oil, rubber plants and chemical industries. Once in the atmosphere, sulfur dioxide can be oxidized to SO₃ (sulfur trioxide) and with water vapor, SO₃ is converted to sulfuric acid mist. Other basic oxides combine with SO₃ to form sulfate aerosols (Derek et al., 2002; Fabio, 2004). These compounds can be transported over long distances and fall back to earth as acid rain. SO₂ damages plant leaves and in moist air if forms acids that harms various  structures and buildings. The respiratory tract can also be affected (Geargil, 1978). NO₂ is formed from the burning of coal, oil, gas, and gasoline, when combustion temperatures are extremely high. NO₂ has enormous detrimental effect on human health. Carbon Monoxide is produced by incomplete combustion of carbon compounds, principally in internal combustion engines. Concentration of CO in the vicinity of busy highways are higher than ambient concentrations upto even more than 100 meters. Carbon monoxide is not an irritant and has little or no effect on plants or materials; however, it reacts with heamoglobin in the bloodstream and deprives the heart and brain from oxygen. Moderate concentrations of it may significantly reduce brain function and high concentrations can be even lethal (NAAQMS series, 1998-2004).

Suspended particulate matter in ambient air is a complex, multi-phase system consisting of particle sizes ranging from < 0.01 µm to >100 µm (Wan-Kuen et al., 2005 & 2006).  When breathed in, these particles can reach the deepest regions of the lungs and lead to many significant health problems ranging from aggravated asthma to premature death (Winchester, 1993;  Freiman et al., 2006; Benjamin et al., 2006).

Concentration of various air pollutants differs depending upon the location and time. Thus, monitoring of air pollution is a complex task as pollutant concentration at each measurement point changes with the local pollution sources, the peripheral natural environment (hills, ponds, trees and marshes) and existence of manmade structures such as buildings, roads and height of measurement point.

In the present study various criteria air pollutants (sulphur dioxide, nitrogen dioxide, carbon monoxide, suspended particulate matter and respirable particulate matter) were monitored during 1999- 2003 in the traffic zone of Delhi, India. Temporal analysis of these pollutants was also done for the percent variation in their concentration during years 2000-2003 keeping 1999 data as base year.

2. Materials and Methods

Figure 1 · Map of the study site for monitoring of ambient air quality in Delhi, India

The site selected for the present investigations was India Gate in Delhi city (a traffic zone), India as shown in Figure 1. The monitoring of pollutants was carried out with a frequency of twice a week, thus giving 106 observation days in a year. Average value had been calculated for all the pollutants for each year. The air monitoring was carried out using high volume air samplers (Envirotech Model APM 415) equipped with a gaseous sampling attachment. The sampling inlet was located 25 ft above the ground level to collect the samples for SO_2, NO_2, CO and particulate matter. Samples were taken for particulate matter at an interval of 8 hr and sampling of SO₂, NO₂ and CO was done at an interval of 4 hr in a day as per standard methods described in APHA handbook, (Kartz, M.,) 1989.

SO₂ concentration was analyzed by West-Gaeke colourimetric procedure (standard/reference method) at wavelength of 560 nm_. Samples from the ambient air were collected through a solution of sodium tetrachloromercurate in an impinger at the airflow rate of 1 liter/min. For measurement of NO₂ concentration, sodium arsenate- modification of Jacobs- Hochheiser method was used. Nitrogen dioxide was absorbed in alkaline solution (NaOH- sodium arsenate absorbent) in an impinger at the rate of 1 lit/min. Carbon monoxide was measured continuously by infrared absorption photometry. Air was drawn through a sample cell where infrared light passes through it.  When CO molecules in the air absorbed part of the infrared light, intensity of the light reaching on the sensor reduced. This portion of the infrared light absorbed by CO is converted into an electrical signal reflecting the CO concentration, and it was recorded.

For analysis of particulate matter samples were collected on a 25 cm² Glass fiber filter (GFF) dried at 105° for an hour. The Weight of the GFFs was measured before and after sampling by using a Mettler AT2001 analytical balance and the difference in weight gave the concentration of particulate matter.

3. Results and Discussion

The Central Pollution Control Board (CPCB) standard for the permissible limits of criteria air pollutants like Sulphur dioxide, nitrogen dioxide, carbon monoxide, suspended particulate matter and respirable particulate matter were followed for traffic zone as shown in Table 1:

The monitoring of the criteria air pollutants was done from India Gate in Delhi city (a traffic zone) during 1999 - 2003 as shown in Table 2:

All the results for 2000, 2001, 2002 and 2003 were compared by keeping 1999 data as a base year to get the trend of different criteria pollutants during this period as depicted by Table 3:

The data for 1999-2003 were also compared for these criteria pollutants with CPCB standards and the deviation of data from the CPCB standards for these pollutants were calculated in term of increase or decrease during 1999-2003 as shown in Table 4:

The important interesting aspects of these criteria air pollutants were as below:

The average sulphur dioxide concentration during 1999-2003 was 19.6 mg/m³, 17.7 mg/m³, 15.3 mg/m³, 13.8 mg/m³, and 12.2 mg/m³ respectively and these were below the national ambient air quality standards (60 mg/m³ for residential area) as shown in Table 2. There was 10.7 % decrease in SO₂ concentration in 2000 as compared to previous year and maximum decrease was 37.7 % in 2003. There is a constant trend of around 10-8 % further decrease than the previous years during this whole study period of five years. The deviation of SO₂ concentration from standard limits was found well within permissible limits during the whole study period from this study. The values of difference of Sulphur dioxide concentration from CPCB norms during 1999-2000 were  - 40.4 mg/m³, -42.5 mg/m³, - 44.7 mg/m³, - 46.2 mg/m³ and - 47.8 mg/m³ respectively as depicted by Table 4.

The average concentration of nitrogen dioxide during 1999, 2000, 2001, 2002 and 2003 were 39 mg/m³, 32 mg/m³, 30 mg/m³, 29 mg/m³ and 28 mg/m³ respectively as represented by Table 2. All the NO₂ concentration were within permissible limits during the complete study period as prescribed by CPCB norms (60 mg/m³). The average variation of nitrogen dioxide concentration during 2000-2003 as compared to 1999 were 17.9 % decrease, 23.07  % decrease, 25.64  % decrease and 28.2 % decrease respectively as shown in Table 3. It was clear from Table 4 that the concentration of nitrogen dioxide was under permissible limits as laid down by CPCB norms and the difference in concentration of nitrogen dioxide during 1999-2003 from CPCB norms was  - 21 mg/m³, -28 mg/m³, -30 mg/m³, -31 mg/m³ and - 32 mg/m³ respectively.

The SPM concentration during 1999, 2000, 2001, 2002 and 2003 were 470 mg/m³, 440 mg/m³, 298 mg/m³, 295 mg/m³ and 275 mg/m³ respectively as shown by Table 2. The average variation of SPM concentrations for 2000, 2001, 2002 and 2003 when compared to 1999 and these were 6.38 % decrease, 36.69 % decrease, 37.23 % decrease and 41.48 % decrease respectively as shown in Table 3. The deviation of SPM concentration from CPCB standards (140 mg/m³) were + 330 mg/m³, + 300 mg/m³, + 158 mg/m³, + 155 mg/m³ and + 80 mg/m³ respectively for 1999, 2000, 2001, 2002 and 2003 as given in Table 4. These were much more than the permissible limits prescribed by CPCB for SPM. The maximum concentration of SPM was 470 mg/m³ in 1999; while in subsequent years there was a trend of decrease in concentration but not with definite pattern.

The RSPM concentration during 1999-2003 was 390 mg/m³, 360 mg/m³, 330 mg/m³, 275 mg/m³ and 220 mg/m³ respectively as shown in Table 2. The % average variation of RSPM concentrations for 2000, 2001, 2002 and 2003 when compared with 1999 as a base year were 7.69 % decrease, 15.38 % decrease, 29.48 % decrease and 43.58 % decrease for years 2000, 2001, 2002 and 2003 respectively as shown in Table 3. The deviation of RSPM from CPCB standards (60 mg/m³) were + 330 mg/m³, + 300 mg/m³, + 270 mg/m³, + 215 mg/m³ and + 160 mg/m³ during 1999, 2000, 2001, 2002 and 2003 respectively as shown by Table 4. It was much above the permissible limits during the complete study period of five years.

For carbon monoxide, the average concentration during 1999- 2003 were 4200 mg/m³, 3730 mg/m³, 3350 mg/m³, 2870 mg/m³ and 2490 mg/m³ respectively as represented by Table 2. The % average variation of carbon monoxide concentration during 2000-2003 by keeping 1999 as a base year were 11.2 % decrease, 20.23  % decrease, 31.66 % decrease and 40.71 % decrease respectively as shown in Table 3. The maximum concentration of CO was in 2000 and after that there was a regular decrease during the subsequent years. It was clear from Table 4 that the concentration of CO was much more than the prescribed limits of CPCB (2000 mg/m³) and it was almost more than double during 2000. The deviation of CO concentration from CPCB norms during the period 1999-2003 were + 2200 mg/m³, + 1730 mg/m³, + 1350 mg/m³, + 870 mg/m³ and + 490 mg/m³ respectively.

Figure 2 · Comparison of the temporal trend of various criteria air pollutants measured from India Gate, Delhi during 1999-2003

Figure 2 represented the Comparative temporal trend of NO₂, SO₂, SPM and RSPM concentration measured from India Gate, Delhi during 1999-2003. It was found from the Figure that there was a continuous decrease in the concentration of these pollutants.

Figure 3 · Ttemporal trend of CO measured from India Gate, Delhi during 1999-2003

Figure 3 represented the temporal trend for CO concentration and it showed that CO also had a continuous trend of decrease in concentration for the period 1999-2003 measured at India Gate, Delhi.

The temporal decrease in concentration of NO₂, SO₂, SPM, RSPM and CO during the study period could be due to introduction of CNG fuel for commercial vehicles in 2000 and phasing out all those vehicles, which were more than 8 years old. Closer of hazardous industries during this period was also an important reason for reduction of these criteria pollutants in air. Introduction of low sulphur (0.05) diesel in national capital region (NCR) from April 2000 may be other important reasons of lowering down of concentration of sulphur.  Better road infrastructure, Road widening and construction of large number of flyovers for diverting traffic routes, Construction of bypass roads for heavy vehicles were also other factors responsible for reducing the concentration of these pollutants in air. From meteorological point of view, there were no chronological changes in the key parameters during this study period (Goyal et al., 2003). The State Pollution Control Boards/Central Pollution Control Board had also taken legal action against the defaulting industrial units and that also helped in reducing air pollution.

4. Conclusions

Present study discussed the 5-year (1999-2003) scenario of ambient air pollutants in traffic zone of Delhi, India. It was found that there was a gradual decreasing trend of ambient air pollutants in Delhi and that is possible due to integrated plan of Delhi Government and Central Pollution Control Board to control air pollution in Delhi. It was found that sulphur dioxide and Nitrogen Dioxide were well under permissible limits and possibly the concentration of SPM, RSPM and CO will come under control after some years if these efforts are continued in future also. As Delhi has achieved the highest rank in number of CNG-run vehicles and now it has become a city of flyovers in the world. It may be conclude from this paper that the air pollutants particularly from vehicular exhaust and industrial emissions can be controlled in any city or at any location by following an integrated approach involving meticulate use of transport policies, industrial emission norms and stringent pollution control regulations. The existing air quality monitoring system requires institutional strengthening, capacity building in various pollution control boards and human resource development including international cooperation.

References

***

Copyright © 2007, ECO Services International