Comparison of Air Pollutants and Air Quality Index using Spatio-Temporal Variation in Chennai City, Tamil Nadu

. In recent years, air pollution in Chennai city in India causes some health effects. This study examines the spatial-temporal characteristics of ambient air quality in five stations Adyar, Anna Nagar, Kilpauk, Nungambakkam and Thiyagaraya Nagar from 2017 to 2022. The surface level aerosol pollutants like particulate matters (PM 2.5 & PM 10 ) and gaseous pollutants Sulfur dioxide (SO 2 ) & Nitrogen dioxide (NO 2 ) were obtained from Tamilnadu Pollution Control Board (TPCB) for five years which includes pre-COVID, during and Post-COVID - period. The results showed that fine particulate matter (PM 2.5 ) and coarse particulate matter (PM 10 ), decreased by 19.49% and 31.91% respectively and gaseous pollutant SO 2 and NO 2 slightly increased by 7.84% and 1.2 % respectively during 2021 as compared with 2017.The particulate matter (PM 2.5 & PM 10 ) level exceeded the National Ambient Air Quality Standards (NAAQS) as well as the WHO recommended Air Quality Guidelines during 2017-2019(Pre-COVID) and low during 2020-2021(During COVID and Post-COVID). The average Air Quality Index (AQI), calculated from the date decreased from 120(2018-2019) to 93(2020-2021) in Chennai city. The AQI and PM 2.5 /PM 10 showed the highest pollution level in winter and lower in summer. PM 10 was the primary pollutant, followed by NO 2 , PM 2.5 & SO 2 with spatial and temporal variations. The proportion of pollutants PM 2.5 and PM 10 decreased but increased for SO 2 and NO 2 . This study offers useful data and resources for further research on Chennai's air quality.


Introduction
Air Quality Index (AQI) about Chennai was analysed and studied with the use of Spatio-Temporal variations obtained by correlating the various pollutants such as PM2.5, PM10, SO2, NO2, CO and O3 [1]. The impact of the lockdown on ambient air quality of Delhi was assessed and stated that in initial days of lockdown O3 levels reduced due to almost constant meteorology, but increased later due to solar radiation and ambient temperature [2]. Utilizing data from 34 monitoring stations located throughout the megacity of Delhi and seven pollutant metrics (PM10, PM2.5, SO2, NO2, CO, O3, and NH3) and used the National Air Quality Index (NAQI) to display the spatial pattern of air quality before and during lockdown [3]. Using geospatial information systems, the pollution and the high-resolution population data were interpolated to study the extent of the pollutants of Chennai City, India [4]. The ambient air pollutants, meteorological factors, and their interactive effects on the number of COVID-19 confirmed cases to understand the relationship between transmission of the disease and environmental factors [5]. In two typical regions (Beijing-Tianjin-Hebei (BTH) and Yangtze River Delta (YRD)), the processes of atmospheric aerosol evolution before, during, and after the lockdown of COVID-19, as well as the variations of six criteria air pollutants (PM2.5, O3, NO2, SO2, CO, and PM10) from January 1 to April 11, 2020, were compared to the same period in 2019 [6]. Different aggregation function used in calculating AQI or API for various types of pollutants was reviewed by Gorai et.al. [7]. In this paper, comparison between the various pollutants such as PM10, PM2.5, SO2 & NO2 and its air quality index for the period of 2017-2021 in the city of Chennai using Spatial-temporal variation. The assessment of the ambient air quality values with the National Ambient Air Quality Standards (NAAQS) [8] and World Health Organisation (WHO) [9].

Study area and data sources
Chennai an area about 426 km 2 which lies between geocoordinate 13.04°N to 80.17°E on the southeast coast of India and in the northeast corner of Tamil Nadu. The average concentration of four air pollutants PM2.5, PM10, SO2, NO2 during 2017-2021 for the Chennai city from five different location like Adyar, Anna Nagar, Kilpauk, Nungambakkam and T-Nagar were used for analysis of the pollutants. Pollutants datas were downloaded from the Tamilnadu Pollution Control Board website (Home (tnpcb.gov.in). Fig.1 shows the location of study in Chennai City. In this study, the spatial distributions of the four pollutants PM2.5, PM10, SO2 and NO2 in five major location of Chennai during 2017 to 2021 were presented. Fig.2 shows the concentration of PM2.5, from the plot it was observed that high PM2.5 level was recorded during 2019 when compared to 2020 and 2021. In particular the location Thiyagaraya Nagar recorded high level of PM2.

Spatial and temporal variation of a pollutant SO2
The annual variation of concentration of SO2 was plotted in Fig. 4. From the plot it was observed that the maximum SO2 level was recorded in the station Nungambakkam during 2019 with the concentration of 12 µg/m 3 and reduced gradually to 9 µg/m 3 during covid pandemic. In Adyar location, the concentration of SO2 recorded was uniform with a level of 9 µg/m 3 during 2017 to 2021. The average SO2 concentration of Kilpauk and Thiyagaraya Nagar during the duration of study was 10 µg/m 3 and 11 µg/m 3 respectively.

Spatial and temporal variation of a pollutant NO2
The Fig. 5 shows the annual variation in the concentration of NO2 between 2017 to 2021. In Nungambakkam and Thiyagaraya Nagar the maximum level of NO2 concentration recorded was 21 µg/m 3 . From the plot it was observed that the increasing concentration of NO2 from 9 µg/m 3 to 11 µg/m 3 during 2017 to 2021. in general, the concentration of NO2 recorded in the five places of Chennai city was reduced during covid period (2019 and 2020) and increased during 2021. From the overall analysis, it was observed that of the particulate matter (PM2.5 & PM10) decreased by 19.49% and 31.91% respectively, but the gaseous pollutants SO2 and No2 increased by 1.2% and 7.84% respectively during the period from 2017 to 2021 which includes the Pre-COVID, during COVID and Post-COVID period. The percentage reduction during COVID period (2020) of the air pollutants was relatively high due to the implementation of lockdown in the city. During the study period (2017-2021), the average concentration of PM10, increased by 48.4% (2018) and then decreased by 28.1%, 50.2%, 44.8% during 2019,2020 and 2021 respectively (Fig. 6). During the period between 2019 -2021, the concentration of PM2.5 decreased by 11.7%, 28.2%, 34.4%, 33.1% and 31.9% in station Adyar, Anna Nagar, Kilpauk, Nungambakkam and Thiyagaraya Nagar respectively (Fig. 7). The green and red dotted lines in Fig. 6 and Fig.7 shows the average concentration of SO2 and NO2 recommended in NAAQS (National Ambient Air Quality Standards) and WHO (World Health Organization) respectively.  The average concentration of SO2 increased 22% in Adyar, 16.5% in Anna Nagar, 10.3% in Kilpauk, 7.17% in Nungambakkam and 6.9% in Thiyagaraya Nagar during the study period, whereas in 2020 it got reduced by 11.2% (Fig. 8). The average concentration of NO2 during the period 2017-2022 increased by 12.9%, 17.5%, 20.6% and 26.3% in station Adyar, Anna Nagar, Kilpauk, Nungambakkam and Thiyagaraya Nagar respectively (Fig. 9).

Air Quality Index (AQI) for different pollutants
An indicator of overall air quality, health consequences, vulnerable populations, and the need for preventative actions is the air quality index (AQI), a color-coded scale that condenses many pollution concentrations into a single numerical number. Moderate (Air Quality acceptable-moderate health concern for very small population)

101-200
Poor (Members of sensitive groups may experience health effects)

201-300
Very poor (Triggers health alert, everyone may experience health effects)

301-400
Severe (Affects healthy people and seriously impacts those with existing diseases) 401-500  Figure 10. From the bar chart in Figure 10, it was observed that there were decrease in the trend of AQI from Winter to Autumn during 2018-2021. In 2018, the % decrease in the AQI from Winter to spring, summer and Autumn are 7.1%, 42.4% and 38% respectively. The average variation of the AQI during summer was reduced drastically 60% when compared to winter in the study period 2020. The percentage reduction in AQI during summer was relatively very low as compared to other seasons of the study period 2018-2021. Figure 11 shows the monthly average concentration of PM2.5( Fig.10(a)), PM10 (Fig,10(b)), SO2 (Fig.10(c)), NO2 (Fig.10(d)), AQI (Fig.10(e)) and ratio between PM2.5/PM10 (Fig.10(f)). A greater PM2.5/PM10 ratio indicates an increased proportion of PM2.5, and a lower PM2.5/PM10 ratio suggests a higher concentration of PM10 in the environment. The PM2.5/PM10 ratio typically represents the composition and quality of the air. In Figure  11(a & b), the U-shaped curve indicates that the emission of pollutants during summer was relatively low value of PM2.5 (15 µg/m 3 ) and PM10 (35 µg/m 3 ) as referred to NAAQS and WHO standards.

Conclusion
In this study, the ambient monitoring data of four air pollutants including PM2.5, PM10, SO2, NO2 during the period 2017 to 2021 was analysed and investigated using spatial and temporal variation in Chennai city. In comparison to 2017, the data revealed that fine particulate matter (PM2.5) and coarse particulate matter (PM10) decreased by 19.49% and 31.91%, respectively, but gaseous pollutants SO2 and NO2 slightly increased by 7.84% and 1.2%, respectively, during 2021. The particulate matter (PM2.5 & PM10) level exceeded the National Ambient Air Quality Standards (NAAQS) as well as the WHO recommended Air Quality Guidelines during 2017-2019(Pre-COVID) and low during 2020-2021(During COVID and Post-COVID). The percentage reduction in AQI during summer was relatively very low as compared to other seasons of the study period between 2018 to 2021. The AQI and PM2.5/PM10 showed the highest pollution level in winter and lower in summer. PM10 was the primary pollutant, followed by NO2, PM2.5 & SO2 with spatial and temporal variations. The proportion of pollutants PM2.5 and PM10 decreased but increased for SO2 and NO2. This study provides pertinent information and sources for more study on Chennai's air quality.