1 Introduction

The coronavirus disease (COVID-19) has been a recent global health crisis that impacted worldwide. The disaster has resulted in a tragically large number of fatalities and severe damage to people’s livelihoods and economic activities. Although the first COVID-19 case was reported in Wuhan (China), the disease affected practically every country in the world (Singh and Chauhan 2020; Tabarej and Minz 2022). Since the rapid transmission was facilitated by travel and mass gatherings, many countries adopted rigorous travel bans, physical distancing, and lockdowns forcing citizens to stay at home to slow down the rate of transmission. Since the lockdown was effective in controlling the transmission of the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) in India during the first wave (in 2020), the Government of India exerted another lockdown during the second surge of COVID-19 cases in 2021 through restrictions for travel as well as limiting industrial and commercial activities (Saharan et al. 2022; Kumar 2021). Similar to the effects of the lockdown in 2020, the restrictions imposed during the lockdown of 2021 also made noticeable impacts on the physical environment and air quality across India (Mahato and Pal 2022; Shukla et al. 2022; Ali et al. 2022).

Air pollution has been one of the most challenging problems in urbanized and industrialized areas worldwide and particularly in India (Khilnani and Tiwari 2018). Some of the urban areas in India are highly polluted (Lokhandwala and Gautam 2020) and about 90% of the population in India lives in areas where the air quality does not meet the WHO standards (Rodríguez 2020). Long-term exposure to air pollutants may result in a variety of health issues, mostly related to respiratory and cardiac systems, and even lead to premature death regions. Outdoor air pollution (in terms of particulate matter) in Kerala (a less-urbanized and less-industrialized State in India) has grown during recent years and the ambient air quality worsened at an alarming rate, specifically in the major urban centers, such as Thiruvananthapuram, Kochi and Kozhikode (Government of Kerala 2020). The number of automobiles in Kerala has increased more than 20 times since 1975, and Ernakulam, Thiruvananthapuram, and Kozhikode districts register the highest levels of automobile emissions (i.e., 40%). The major air pollutants in the region are particulate matter, sulphur dioxide (SO2), nitrogen oxides (NOx), carbon monoxide (CO), and ozone (O3) (Jyothi et al. 2019). Numerous researchers addressed the ambient air quality issues of various parts of Kerala over the past few decades (e.g., Aggarwal 1999; Bency et al. 2003; Sarasamma and Narayanan 2014; Jyothi et al. 2019). Further, a few studies (Tobollik et al. 2015; Sumesh et al. 2017) highlighted the significance of the particulate matter in the ambient air quality and life expectancy of the regional population.

Although the lockdown measures during the COVID-19 pandemic wrecked the economy at global as well as regional scales, the lockdown period of 2020, as well as 2021, is characterized by improved air quality in urbanized and industrialized areas across India (Navinya et al. 2020; Mahato and Ghosh 2020; Das et al. 2021; Singh and Tyagi 2021). The reduction of air pollutant emissions was significant even in the less urbanized and less industrialized regions in India (e.g., Kerala) (e.g., Thomas et al. 2020, 2021; Resmi et al. 2020). Even if anthropogenic activities were curtailed during the COVID-19 pandemic, it is reported that background air pollution (i.e., emissions from natural processes) predominated across South India, especially in the coastal regions (Madineni et al. 2021). The different studies carried out in Kerala to understand the effects of COVID-19 lockdowns in 2020 and 2021 observed unusually low emissions of air pollutants in the urbanized areas of the region. However, the impacts of the lockdowns on the ambient air quality of the coastal regions of Kerala, particularly in 2021, are less explored and discussed. Indeed, the lockdown period aids in understanding the natural sources of air pollution in such regions. Hence, the present study sought to address the impact of the COVID-19 lockdown (in 2021) on the ambient air quality in four coastal urban regions along the southwest coast of Kerala (India) during the lockdown associated with the second wave of COVID-19 in 2021.

2 Materials and Methods

2.1 Study Area

The present study focuses on the variations in the ambient air quality of four coastal urban centers in Kerala (India), namely, Thiruvananthapuram, Kollam, Kozhikode, and Kannur (Fig. 1). Physiographically, the region is classified into three major zones: highland (> 75.0 m above the mean sea level), midland (7.6–75.0 m above the mean sea level) and lowland and coastal plains (< 7.5 m above the mean sea level). The Western Ghats influences the physiographic variations and the orography determines the climate of Kerala. The windward slopes of the Western Ghats including the coastal plains enjoy a tropical humid climate (Am) with a mean annual rainfall of about 2800 mm (Thomas and Prasannakumar 2016) and a mean annual temperature of about 25 °C.

Fig. 1
figure 1

Ambient air quality monitoring stations (used in this study) of Kerala (India)

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Major anthropogenic sources of air pollution in Kerala are industries and traffic. Although Kerala does not have large-scale industrial complexes, such as in many other states of India, the region supports several industrial clusters and most of them are located near the coastal urban regions. Based on the census 2011 report, Kerala has a population of 33,387,677 (i.e., 859 persons km−2), which is nearly 3% of the population of India (https://censusindia.gov.in). Further, the region witnessed rapid urbanization in the past decades, where the urban population of Kerala increased from 25.9% in 2001 to 47.7% in 2011 (Lal and Nair 2017). The region is well connected via road, railway, air, and water transport with a road density of 3.9  km−2 and recorded an increase in road traffic (12–14%) annually. Being located at the southwestern fringe of the Indian peninsula, marine traffic along the coastal routes of the Arabian Sea also serves as a critical source of air pollutants (Thangalakshmi and Sivasami 2022).

2.2 Ambient Air Quality Monitoring Stations

2.2.1 Thiruvananthapuram (TM)

Thiruvananthapuram (TM) is the capital of Kerala and the headquarters of the Thiruvananthapuram district with a metropolitan population of about 1.7 million (Census of India 2011). The ambient air quality monitoring station is located at Plammoodu (Fig. 1; 76° 56′ 36" E and 8° 30′ 53" N), which is roughly 10 km from the shoreline. The region is accessible through road, railway, air, and water transportation modes. The city is a major hub of commercial and retail activities in Kerala and the urban area supports multiple industrial and chemical manufacturing units. Being the administrative capital of Kerala, the region hosts numerous government administrative offices. The region is also one of the major information technology hubs and academic nuclei in India. The mean annual rainfall of the region is about 1900 mm and the temperature ranges from 20 to 39 °C (Ansar et al. 2012; Nair et al. 2017).

2.2.2 Kollam (KL)

Kollam (KL) is another major urban area in the southern part of Kerala and is mainly developed along the coastal stretch of the Kollam district (Fig. 1). With a metropolitan population of 1.3 million (Census of India 2011), Kollam is one of the most densely populated areas in Kerala. The ambient air quality monitoring station is located at Kadapakkada (76° 36′ 26" E and 8° 52′ 43." N). A large number of industries (primarily chemical and food processing) exist in the region as the coastal belt is blessed with placer mineral deposits and due to the occurrence of one of the major fishing harbours in Kerala (Sarasamma and Narayanan 2014). The region receives an annual rainfall of 2400 mm with temperatures varying between 23 and 36 °C (Nair et al. 2017).

2.2.3 Kozhikode (KZ)

Kozhikode district is one of the developing urban centers with significant socioeconomic significance in Kerala. The ambient air quality monitoring station is located at Palayam (Fig. 1; 75° 47′ 3" E and 11° 14′ 56" N). It has a metropolitan population of over 2 million, making it one of the largest metropolitan areas in Kerala. The region is a major trade hub and commercial center in northern Kerala and supports a wide range of industries ranging from sawmills to chemical manufacturing. The annual average rainfall of the city is 3500 mm (Surendran et al. 2019) and the mean annual temperature is 26 °C.

2.2.4 Kannur (KN)

With 1.7 million population in the metropolitan region, Kannur is the sixth-most urbanized area in Kerala (Census of India 2011). The ambient air quality monitoring station at Kannur is geographically situated at Thavakkara (Fig. 1; 75° 21′ 19" E and 11° 52′ 8" N). The ambient air temperature ranges from 20 to 33 °C and the mean annual rainfall is 3400 mm (Whitaker and Whitaker 2006).

2.3 Data Collection

The air pollutant levels recorded in the four ambient air quality monitoring stations in Thiruvananthapuram, Kollam, Kozhikode, and Kannur districts (Fig. 1) were used for the present study. The daily mean concentrations of major pollutants, viz., particulate matter with a diameter of less than 2.5 µm and 10 µm (PM2.5 and PM10), carbon monoxide (CO), oxides of nitrogen (NO and NO2), sulphur dioxide (SO2), ammonia (NH3) and ozone (O3) from March 1 to July 31 in 2021 were used to calculate the air quality index (AQI) (CPCB 2014). Hourly data of eight air pollutants, including PM2.5, PM10, CO, NO, NO2, NH3, SO2, and O3 are available in the online air quality data portal of the Central Pollution Control Board, Government of India. (https://app.cpcbccr.com/ccr/#/caaqm-dashboard-all/caaqm-landing). The entire study period was classified into three: the pre-lockdown (PRLD) period from 1 March to 7 May, the lockdown (LD) period from 8 to 31 May, and the post-lockdown (PTLD) period between 1 June and 31 July. The AQI was calculated from the concentrations of these eight pollutants (up to a 24 h averaging period), with a minimum of three pollutants (at least one of which is PM2.5 or PM10) (Eq. 1). The daily average value was calculated for all the stations starting at 12:00 am for a period of 24 h.

$$\mathrm{AQIi }= \left[\left\{\frac{\left({IN}_{HI}-{IN}_{LO}\right)}{\left({B}_{HI}-{B}_{LO}\right)}\right\}\times \left({C}_{i}-{B}_{LO}\right)\right]+{IN}_{LO},$$
(1)

where BHI and BLO as breakpoint concentrations greater and smaller to Ci; Ci is the concentration of pollutant i; INHI and INLO are the corresponding AQI values. The AQI is classified as good (0–50), satisfactory (51–100), moderate (101–200), poor (201–300), very poor (301–400), and severe (401–500) (CPCB 2014).

3 Results and Discussion

3.1 AQI at Thiruvananthapuram (TM)

The AQI calculated at the air quality monitoring station of TM showed significant temporal variability between the PRLD, LD, and PTLD periods (Table 1). The AQI at TM shows significant improvement during the LD period (median = 44; 24–64) compared to the PRLD (52; 34–105) and PTLD (55; 32–100) periods (Fig. 2). Majority of the days during the PRLD (56%) and PTLD (60%) periods in 2021 recorded ‘satisfactory’ air quality (50 < AQI ≤ 100), whereas about 75% of the total days had ‘good’ air quality (AQI ≤ 50) during the LD period. Moreover, 46% of the days in the PRLD and 40% of the PTLD periods also recorded ‘good’ air quality at TM. However, a few days in the PRLD period appeared to have moderate air quality also (Fig. 2). The AQI statistics at TM indicate a clear declining trend in the AQI values during the lockdown restrictions. However, the air quality was reinstated to its original level (i.e., PRLD period) after the lifting of the lockdown measures. The prominent pollutant for each day was determined for the entire study period (Fig. 2). The relative dominance of different air pollutants during the different periods (i.e., PRLD, LD, and PTLD) indicates that PM10 was the dominant determining the AQI during the PRLD period. It may be noted that the AQI of about 68% of the total days during the PRLD period was influenced by PM10. However, CO was the prominent pollutant influencing the AQI during the LD (62%) and PTLD (70% days) periods, whereas PM10 has only secondary significance.

Table 1 Summary statistics of AQI of the ambient air quality stations in different periods
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Fig. 2
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Daily time series of AQI recorded at TM. Relative dominance of different air pollutants influencing the AQI during the PRLD, LD, and PTLD periods is also shown in the pie charts

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3.2 AQI at Kollam (KL)

A remarkable variation in air quality is noted at KL between the PRLD, LD, and PTLD periods (Table 1). Further, the temporal variability of the AQI at KL indicates an improvement in the air quality during the LD period (median = 49; 30–105) compared to the PRLD (89; 48–205) and PTLD (75; 47–124) periods (Fig. 3). About 58% of the days during the LD period recorded ‘good’ air quality, while 37% showed ‘satisfactory’ and 5% with moderate AQI. However, during the PRLD, 63% of the days had ‘satisfactory’ air quality, and 37% of the days were characterized by ‘moderate’ air quality. Although the AQI increased during the PTLD than the LD period, 83% of the days had satisfactory air quality while ‘moderate’ and ‘good’ air quality levels were noted during 12% and 5% of days, respectively. Similar to the temporal pattern of the AQI at TM, KL also showed an increasing trend in the AQI after lifting the lockdown restrictions. However, the AQI during the PTLD period did not rebound to the AQI levels of the PRLD period (Fig. 3). Among the different air pollutants recorded at KL, PM10 (62% days) and PM2.5 (38%days) dominated during the PRLD period (Fig. 3). Further, PM10 was the prominent pollutant during the PRLD and LD periods, but CO was the prominent pollutant during the PTLD period.

Fig. 3
figure 3

Daily time series of the AQI recorded at KL. Relative dominance of different air pollutants influencing the AQI during the PRLD, LD, and PTLD periods is also shown

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3.3 AQI at Kozhikode (KZ)

The effect of the lockdown measures on air pollutant concentrations at KZ was evident in the improvement of air quality. The AQI showed a marked improvement at KZ during the LD period (median = 37; 49–76) compared to the PRLD (76; 51–86) and PTLD (49; 27–85) periods. About 95% of the days during the LD period recorded ‘good’ air quality while another 5% of the days had ‘satisfactory’ AQI levels (Fig. 4). However, ‘satisfactory’ air quality levels prevailed during the PRLD period. Although the withdrawal of the lockdown measures increased the AQI, 56% of the days in the PTLD period remained ‘good’, while 44% of the days had a ‘satisfactory’ air quality level. Although PM10 was the prominent pollutant determining the AQI at KZ, CO was influential during the LD and PTLD periods (Fig. 4).

Fig. 4
figure 4

Daily time series of AQI recorded at KZ. Relative dominance of different air pollutants influencing the AQI during the PRLD, LD, and PTLD periods is also shown

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3.4 AQI at Kannur (KN)

The temporal variability of the AQI at KN also indicated an improvement in air quality during the LD period (Median = 86; 47–127) compared to the PRLD (102; 66–258) and PTLD (54; 32–135) periods. About 77% of the days in the LD period recorded ‘satisfactory’ air quality levels and 23% of the days showed ‘moderate’ AQI values (Fig. 5). However, during the PRLD period, 47% of the days had ‘satisfactory’ level and 53% of the days appeared to have ‘moderate’ air quality. Interestingly, 41% of the days during the PTLD period were characterized by ‘good’ air quality, while 44% of the days registered ‘satisfactory’ and 15% of the days had ‘moderate’ air quality levels. A declining trend of the AQI values at KN is noted due to the lockdown restrictions. However, the trend continued in the PTLD period also. PM10 was the dominant pollutant influencing the AQI at KN during all the periods. However, NO2 was also significant during the PTLD period (Fig. 5).

Fig. 5
figure 5

Daily time series of AQI recorded at KN. Relative dominance of different air pollutants influencing the AQI during the PRLD, LD, and PTLD periods is also shown

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The reduction in the AQI values in the LD period compared to the PRLD period at all the monitoring stations indicates an improvement in the ambient air quality due to the lockdown measures. However, the effect of the lockdown was spatially varying where the largest changes in the AQI were noted at KZ (51%) and KL (45%), whereas the changes at TM and KN were 13% and 16%, respectively. Among the different prominent pollutants influencing the AQI, PM10 was the determinant during the PRLD period at all the monitoring stations. Although PM10 was the prominent pollutant at KL and KN in the LD period, CO was the major pollutant at TM and KZ. Further, CO was the prominent pollutant during the PTLD period at all the stations except KN, where PM10 was the significant one controlling the AQI. In general, a gradual transition of the prominent air pollutant determining the AQI is noted from PRLD (PM10) to PTLD (CO).

3.5 Variability of Air Pollutant Concentrations at Different Monitoring Stations

Figure 6 shows the daily time series of the concentration of different air pollutants at all the monitoring stations. The manifestations of the AQI pattern at the different stations in Kerala are indicative of improvement in air quality due to a decrease in air pollutant levels during the LD period. However, the rate of reduction of the pollutant concentration during the LD period differs among the stations (Fig. 6). For instance, the concentration of all the air pollutants (except SO2) showed a significant reduction during the LD period at TM compared to the PRLD period. Significant reductions are noted in the particulate matter concentrations (i.e., 72 and 61% for PM2.5, and PM10, respectively), while NO (8%), NO2 (29%), NH3 (27%), CO (25%) and O3 (32%) levels also decreased during the LD period (Table 2). However, the SO2 levels in the LD period increased by about 7% compared to the PRLD period. The PTLD period at TM witnessed an increase in the concentration of most of the pollutants (i.e., PM2.5-11%, PM10-36%, NO2-24%, NH3-20%, SO2-6% and CO-17%) compared to the LD period. On the other hand, NO and O3 levels were reduced by 24% and 21%, respectively. The concentration of pollutants in the PTLD period was also lesser than in the PRLD period, i.e., such as PM2.5 (69%), PM10 (47%), NO (30%), NO2 (12%), NH3 (12%), CO (13%) and O3 (47%). But SO2 levels of the PTLD period were about 13% higher than the PRLD period (Table 2).

Fig. 6
figure 6

Daily time series of the concentration of different air pollutants recorded at TM, KL, KZ, KN: a PM2.5, b PM10, c CO, d NO, e NO2, f NH3, g SO2 and h O3

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Table 2 Mean concentration of the air pollutants at different monitoring stations during PRLD, LD, and PTLD periods
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A remarkable reduction in the concentration of all the pollutants except NH3 and CO was observed at KL during the LD period compared to the PRLD period (Fig. 6). The concentration of PM2.5, PM10, NO, NO2, SO2, and O3 was reduced by 57%, 47%,18%, 47%, 29%, and 44%, respectively (Table 2). However, the NH3 level was increased by 18% and CO concentration remained unchanged during the LD period. However, during the PTLD period, all the air pollutants except NH3 and O3 showed a notable increase than the LD period. Among the different pollutants, the concentration of CO doubled in the PTLD period compared to the LD period. A comparison of the PRLD and PTLD periods at KL indicates that PM2.5, PM10, NO2, NH3, SO2, and O3 concentrations were reduced during the PTLD period by 40%, 32%, 22%, 20%, 18%, and 49%, respectively. However, NO and CO levels of the PTLD period were significantly higher (15% and 133%, respectively) than that of the PRLD period (Table 2).

The concentration of all the air pollutants at KZ showed a decrease during the LD period than the PRLD period (Fig. 6). The concentration of PM2.5, PM10, NO, NO2, NH3, SO2, CO, and O3 were lowered by 42%, 57%, 61%, 59%, 34%, 9%, 25%, and 22%, respectively (Table 2). However, the PTLD period witnessed an increase in the concentration of PM2.5 (12%), NO (136%), NH3 (6%), SO2 (53%) CO (33%), and O3 (4%) compared to the LD period. While the rate of increase in the PM10 concentration in the PTLD period (compared to the PRLD period) was insignificant, NO2 concentration was reduced by about 5%. Further, PM2.5, PM10, NO, NO2, NH3 and O3 levels in the PTLD period were lower than the PRLD period by 35%, 56%, 9%, 61%, 30%, and 19% respectively (Table 2). However, the concentration of SO2 showed an increase (38%) during the PTLD compared to the PRLD period.

The concentration of PM2.5 (49%), PM10 (15%), CO (17%), and O3 (40%) at KN decreased during the LD period compared to their concentration during the PRLD period (Fig. 6; Table 2). While the concentration of NO, NO2, and NH3 remained unchanged during the LD period and SO2 increased by 21%. The particulate matter levels (PM2.5—19%, PM10—26%), SO2 (29%), and O3 (33%) showed a further decrease in the PTLD period than the LD period, whereas oxides of nitrogen increased (NO by 18% and NO2 by 70%). The concentration of most of the air pollutants at KN showed a significant reduction in the PTLD period than the PRLD period, i.e., PM2.5 (58%), PM10 (37%), SO2 (14%), CO (17%) and O3 (60%). Contrastingly, the concentration of NO and NO2 increased by 18% and 70% respectively (Table 2).

In general, all the ambient air quality stations in the coastal urban areas of Kerala witnessed a reduction in air pollutants during the LD period compared to the PRLD period. However, the concentration of various air pollutants returned to its original level (i.e., PRLD) after relaxing the lockdown measures during the PTLD period. Thomas et al. (2022) noted such behaviour of air pollutants in the Kochi urban area of Kerala. However, most of the pollutants recorded at KN show a continuation of the declining trend of the LD period in the PTLD period also.

4 Discussion

The results of this study indicate a reduction in the different air pollutants and an overall improvement in the ambient air quality in the coastal urban regions of Kerala in the LD period of 2021. Air pollution in Kerala is primarily contributed by vehicular traffic, industries, and construction works (Jyothi et al. 2019). Since the lockdown restricts most of such activities, the emission of pollutants from these sources would be lesser in the LD period compared to the PRLD period. The withdrawal of lockdown restrictions during the PTLD period resulted in an increase in the concentrations of prominent pollutants during PTLD period as compared to the LD period. Most of the pollutant species recorded in the monitoring stations follow such a trend, i.e., a drastic reduction in the LD period compared to the PRLD period and an increase in the PTLD period. Among the different pollutant species, particulate matter (PM2.5 and PM10) showed a significant reduction in the LD period. Since the particulate matter levels are mostly associated with construction activities and vehicular traffic and hence, cessation of such activities was reflected in the particulate matter levels. Since many of the gaseous pollutants, such as oxides of nitrogen (NO and NO2) and CO are also primarily contributed by vehicular traffic, travel restrictions during the LD period could have reduced their concentration in the LD period compared to the PRLD period. Similarly, many of the industries of Kerala are located near the ambient air quality monitoring stations, e.g., Travancore Titanium Products (TM), and Kerala Minerals and Metals Limited (KL). Hence, restrictions in industrial activities are reflected in the reduced concentration of air pollutants. Although many of the pollutants in the PTLD period bounced back to their concentration to the level of the PRLD period, some of the pollutants (e.g., PM2.5 and PM10) did not return to their previous level, which is attributed to the effect of seasonal (monsoon) rainfall purging the particulate matter in the atmosphere.

It is interesting to note that the concentration of some of the pollutants got amplified during the LD period at many stations, even with the cessation of industrial and construction activities and travel restrictions. For instance, SO2 recorded at TM and KN showed an increased concentration during the LD period (9.5 and 1.7 μg/m3, respectively) than the PRLD period (8.9 and 1.4 μg/m3) (Table 2). However, the concentration of O3 at most of the stations (TM, KL, and KN) showed a gradual reduction from the PRLD to PTLD period. Moreover, many of the pollutants (e.g., PM2.5, PM10, NH3, and CO) at KN also exhibited a gradual reduction throughout the period (Table 2). Further, the temporal pattern of different pollutants also differs among the monitoring stations, which is an indication of the differences in the regional activities resulting in the emissions. Except for KN, CO levels at all the monitoring stations during the PTLD period either returned to the PRLD period or increased the concentration than the PRLD period. This is evident in the dominance of CO as the major factor determining the AQI in the PTLD period in most of the stations. Even during the LD period, CO was significant in controlling the AQI at many of the stations (TM, KL, and KZ) (Figs. 2, 3, and 4). Although CO is primarily contributed by automobile emissions and industries, emissions from other sources such as household activities, biomass burning, and agriculture activities could also be significant in the regional context. Although noted only at KL, the concentration of NH3 increased by 18% during the LD compared to the PRLD period. Agricultural activities particularly before the onset of monsoon could be the main source of NH3 emissions apart from other minor sources (Behera et al. 2013).

In the southwest coastal region, the Arabian Sea could be a substantial source of pollutants in the coastal region. The increase in SO2 levels during the LD period particularly at TM could be attributed to the marine sources as Shanavas et al. (2020) observed that the emissions from marine traffic contribute to the concentration of SO2 along the coastal regions of south India. According to Madineni et al. (2021), the relative increase in the concentration of some trace gases in the southern part of India suggests a contribution from other sources including emissions from natural sources (e.g., forest fires, biomass burning) and long-range transport. The results of the back-trajectory analysis by Thomas et al. (2020) also indicate that pollutant transport from nearby regions might have affected the concentration of various air pollutants in the region during the LD period of 2020. It is indisputable that the lockdown measures help reduce air pollutants thereby improving the air quality of the coastal regions of Kerala in 2021. However, the characteristic behaviour of many gaseous pollutants deviating from the generalized pattern is indicative of other sources of air pollution which are unaffected by the lockdown measures.

5 Summary and Conclusion

The impacts of the lockdown (due to the surge in COVID-19 cases in Kerala in 2021) on the ambient air quality of the coastal urban areas of Kerala (India) were investigated in this study. Temporal variability of different air pollutants (PM2.5, PM10, CO, NO, NO2, NH3, SO2, and O3) and the AQI at different ambient air quality monitoring stations (TM, KL, KZ, and KN) was analyzed to understand the effect of lockdown measures on the pollutant concentration and air quality. We observed a notable reduction in the concentration of major pollutants and the enhancement of air quality at all the coastal air quality monitoring stations during the LD period compared to the PRLD period. However, the improvement in the ambient air quality was more significant at KZ (51%) and KL (45%), than at TM (13%) and KN (16%). Among the different pollutants, significant reductions in the LD period were noted in the particulate matter (PM2.5 and PM10) concentrations than the gaseous pollutants. The prominent pollutant determining the AQI at all the monitoring stations in the PRLD period was PM10 which gradually changes to CO during the PTLD period. Although the lockdown restrictions imposed on anthropogenic activities were reflected in the reduction of air pollutant concentration and enhancement of ambient air quality across the region, the increase in the concentration of the pollutants, viz., CO, SO2, and NH3 during the LD period attributes to the contribution from the sources other than vehicular traffic, industries, and construction activities. Such sources include household emissions, burning of biomass, agriculture activities, onshore as well as long-range transport. Further, the relative influence of such sources strongly varies across the region, and extended research to understand the regional factors controlling the ambient air quality of Kerala (India).