1. Introduction
The first positive case of SARS-CoV-2 (hereinafter COVID-19) was detected in Turkey on 11 March 2020. After the confirmation of the first case, some measures such as a stay-at-home order and restrictions on airway transportation were implemented by the Turkish government. However, the number of cases reached 4500 on 10 April. Then, more strict measures were applied for 30 crowded cities including Samsun. These measures were lockdown on the weekends, cancellation of intercity travel, etc.
Several studies declared an improvement in air quality due to the partial and total lockdowns related to COVID-19 [
1,
2,
3,
4,
5,
6,
7]. The work of [
4] showed that the improvement in air quality in Barcelona was related to the quarantine measures. The study of [
6] investigated the improvement in ozone levels in Rio de Janeiro, Brazil. The work of [
7] showed an increase in air quality in terms of the reduction in the concentrations of PM
2.5, PM
10, SO
2, CO and NO
2 in China. Moreover, Ref. [
6] discussed the impact of the partial lockdown on the ozone levels in the city of Rio de Janeiro, Brazil. Additionally, the study of [
4] showed that the quarantine measures led to an improvement in air quality in Barcelona. Studies in several European cities such as Barcelona, Istanbul, Madrid, Milan and Munich also reported an improvement in air quality. The work of [
5] reported reductions in PM
10 (32–43%), PM
2.5 (19–47%), NO
2 (29–44%), CO (40–58%) and SO
2 (34–69%) in Istanbul, a megacity in Turkey.
Meanwhile, a few studies included mobility to understand the effect of traffic on air quality during the COVID-19 measures [
8,
9]. The work of [
8] found a significant correlation between mobility and trends of PM
2.5 and NO
2 for Singapore. The study of [
9] declared that mobility and airborne particulate matter have an impact on the risk of COVID-19 transmission.
Samsun, located in the central part of the Black Sea Region of Turkey, is the most crowded and developed city of the region. In this study, the change in air pollutant concentrations during the lockdown in Turkey was investigated. The effect of mobility was considered as well.
2. Materials and Methods
Samsun, located in the central part of the Black Sea Region of Turkey, is the most crowded and developed city of the region (
Figure 1). Its population was 1,356,000 by the end of 2020 [
10], and it is the 16th crowded city of Turkey. It is surrounded by the Black Sea in the north, Ordu city in the east, Tokat and Amasya in the south and Sinop and Çorum cities in the west. The city has an area of about 9725 km
2, and 45% of this area is mountains, 37% plateaus and 18% plains.
There are 6 air quality monitoring stations in the city operated by the Central Black Sea Clean Air Center. However, only Tekkekoy station has data completeness greater than 95% during the period of the COVID-19 lockdown measures. PM
10, CO, NO
x and NO
2 are monitored at Tekkekoy station. The information about Tekkekoy station is presented in
Table 1. The change in the concentrations of air pollutants during the period of the COVID-19 lockdown measures and the number of exceedances of the limit values according to the World Health Organization [
11] were calculated. According to the WHO guidelines, the limit for PM
10 for 24 h is 50 µg/m
3, while the NO
2 limit for 1 h is 200 µg/m
3. Therefore, only the exceedance numbers for PM
10 and NO
2 were calculated.
Some periods were defined according to the COVID-19 measures in order to conduct the comparison of air pollutant concentrations. Some periods were chosen to perform the comparisons of the pollutant concentrations. The period from 1 January to 17 March 2020 was defined as pre-lockdown, and the period from 18 March to 1 June 2020 was defined as lockdown. The changes in the concentrations of pollutants were analyzed for three different steps. Firstly, the change in concentrations and the exceedance numbers during the lockdown with respect to the pre-lockdown were investigated.
Secondly, the change in the concentration levels of air pollutants during the lockdown was investigated by dividing the lockdown period into two parts: the first one covers the dates where full lockdown measures were applied for metropolitan cities, and the second covers the remaining lockdown days. Thirdly, the concentrations during the lockdown were compared to the same period in the previous year.
In the last part of the study, mobility data of [
12] were used to analyze the effect of mobility on the air quality. These data are represented as the percent change from a baseline in six categories. The baseline is the median value of mobility data covering the period from 3 January to 6 February 2020. The mobility data are explained in six different categories as introduced by [
12], but only workplace mobility data were used to represent the traffic change.
3. Results
3.1. Comparison of Lockdown and Pre-Lockdown Periods
The daily concentrations of CO, NO
2, NO
x and PM
10 for the period from 1 January to 1 June 2020 are shown in
Figure 2. The decline in the concentrations of CO, NO
2 and NO
x are clear, while the change in the concentration of PM10 is not clear. The mean concentrations before the lockdown and during the lockdown are shown in
Table 2. As shown in
Table 2, the mean concentrations of CO, NO
2 and NO
x declined by 72, 37 and 35%, while the mean concentration of PM
10 increased by 3% during the lockdown according to the period before the lockdown.
The numbers of exceedances for NO
2 and PM
10 are shown in
Table 3. NO
2 did not exceed the WHO limits for both the pre-lockdown and lockdown periods. PM
10 concentrations exceeded the WHO limits 19 times before the lockdown and 16 times during the lockdown, although the mean concentrations of PM
10 increased during the lockdown according to the pre-lockdown period. The days that exceeded the WHO limits decreased by 16% during the lockdown with respect to the pre-lockdown period.
3.2. Comparison of Sub-Periods in Lockdown Period
The mean concentrations of air pollutants during the period of the full lockdown measures and the remaining days and the percent change during the full lockdown days with respect to the remaining days are shown in
Table 4. As shown in
Table 4, the concentration of all the pollutants decreased. The CO concentration decreased just 1% during the period of the full lockdown measures with respect to the remaining days of the lockdown. NO
2, NO
x and PM
10 decreased by 25, 20 and 18%, respectively.
3.3. Comparison of the Lockdown Concentrations with the Same Period of 2019
The mean concentrations of the air pollutants during the lockdown period and the same period in 2019 are shown in
Table 5. NO
2 concentrations did not change during the lockdown with respect to 2019. CO and NO
x increased by 4 and 2.5%, respectively, whilst PM
10 decreased by 21%.
3.4. Relationship between Mobility and Air Pollution
The time series of the mobility data and daily average NO
x, NO
2 and PM
10 concentrations during the lockdown period is shown in
Figure 3. The CO concentrations are not plotted due to the greater values of CO. As shown in
Figure 3, the overall average of work mobility change in Samsun was 49%. The peak mobility values in the figure represent the full lockdown measures applied for metropolitan cities. During these peaks, declines in the mean concentration of the air pollutants are obvious.
The correlation between mobility change and pollutant concentrations was calculated and tested via Pearson’s test, and the results are shown in
Table 6. The correlation coefficients between mobility and CO, NO
x and PM
10 were 0.15, 0.27 and 0.08, respectively, and were not statistically significant. The correlation coefficient between mobility and NO
2 was 0.30, and it was statistically significant at the 99% confidence level.
4. Conclusions
The daily routines of people in all countries have been changed by COVID-19. In Turkey, after the confirmation of the first case on 11 March 2020, authorities implemented some rules such as shutting down airway transportation and moving to online education. However, by 10 April, the number of cases reached 4500, and more restrictive measures were imposed on 31 major cities including Samsun. The changes in the concentrations of CO, NOx, NO2 and PM10 in Samsun during the period of these measures and their relationship with the decrease in mobility were investigated in this study. The main findings are as follows:
The mean concentrations of CO, NO2 and NOx declined by 72, 37 and 35%, while the mean concentration of PM10 increased by 3% during the lockdown according to the pre-lockdown period. The PM10 concentrations exceeded the WHO limits 19 times before the lockdown and 16 times during the lockdown, although the mean concentrations of PM10 increased during the lockdown according to the pre-lockdown period.
The mean concentration of all the pollutants decreased during the period of the full lockdown measures compared to the remaining lockdown days at different levels. The CO concentration decreased just 1% during the period of the full lockdown measures with respect to the remaining days of the lockdown. NO2, NOx and PM10 decreased by 25, 20 and 18%, respectively.
NO2 concentrations did not change during the lockdown with respect to 2019. CO and NOx increased by 4 and 2.5%, respectively, whilst PM10 decreased by 21%.
The correlation between mobility change and pollutant concentrations was calculated and tested via Pearson’s test. Only the correlation coefficient between mobility and NO2 was statistically significant at the 99% confidence level, with a value of 0.30.