Interrupted time series ARMA modeling of air pollution (NO2, SO2 and PM10) during the COVID-19 pandemic in Stara Zagora, Bulgaria

After the outbreak of COVID-19 pandemic in Bulgaria, the government has declared “state of emergency” (from March 13th, 2020) and took instant and decisive measures, including suspension of all educational activities, ban on access to all public places, installation of checkpoints on entrance-exit roads of the regional centers, to prevent the spread of the infection. The aim of this article is to evaluate the effectiveness of the “state of emergency” on the air quality improvement. In particular, an interrupted time series modelling approach is employed to test if a significant change in the level and the trend of the pollutant time series has occurred after the “state of emergency” measure. The case study regards the city of Stara Zagora, Bulgaria and focuses on the comparison of the period before (January 1st – March 12th, 2020) and after (March 13th – December 15th, 2020) the state of emergency. After adjusting by meteorology and Sunday effect, the results confirm for negative and significant effect in the level for NO2 and SO2 immediately after the state of emergency and positive and significant trend only for NO2. For the remaining PM10 series of data, it is not possible to conclude that the intervention was helpful in improving air quality. These results are reasonable due to the stay-at-home order has certainly lowered road traffic on the one hand, thus reducing the main emission source connected to NO2 concentrations and on the other hand increase PM10 concentrations because in Stara Zagora city, solid fuel heating is the main source for PM.