Temporal variations, regional contribution, and cluster analyses of ozone and NO in a middle eastern megacity during summertime over 2017-2019.

Particulate matter is usually regarded as the dominant pollutant in Tehran megacity in Iran. However, the number of ozone exceedance days significantly increased in recent years. This study analyzes simultaneous measurements of O and NO (NO+NO) concentrations to improve our understanding of ozone evolution during the summers of 2017 to 2019. The k-means clustering technique was used to select five representative air quality monitoring sites in Tehran to capture O and NO concentrations' variability. The findings show that all of the investigated sites failed to meet the ozone non-attainment criterion. The ozone weekend effect is seen in the study of weekday/weekend differences in 2017 and 2018, but not in 2019, which can be due to the shift in the ozone production regime. The summer mean variation analysis can also be used to deduce this regime change. In 2017, the O and NO summer mean variations suggest a holdback in the NO upward trend and a reversal in the O downward trend that had been in place since 2012. Air mass back trajectory clustering reveals that east and north-east air mass clusters have the most significant impact on Tehran's O pollution and the highest regional contribution to O. The study of O against NO shows that the regional contribution to O increased from 2017 to 2018 and then decreased in 2019; however, the local contribution is the opposite. The diurnal analysis of the regional and local contributions to O indicated that O in Tehran might be primarily affected by pollutants from a short distance. The findings reveal critical changes in the behavior of O in recent years, indicating that decision-makers in Tehran should reconsider air pollution control measures.