Trend analysis of measured surface ozone at the megacity of Tehran for the summertime 2007–2021.

High concentrations of surface ozone in urban and industrial regions worldwide have been a major air quality issue. Tehran, as the capital and the most populous city of Iran, has experienced intense ambient air pollution over the last three decades. This study aims to assess the long-term trends of ozone over this city in recent decades. Data series of maximum 8-h daily average (MDA8) ozone were obtained for the period between 2007 and 2021. To determine the MDA8 ozone trend attributed to the long-term changes in ozone precursor emissions, we isolated the effects of meteorology and short-term ozone variability. The results show a downward trend from 2007 to 2014 (P1) and an upward trend from 2014 onward (P2). The long-term precursor emissions changes contribute to a 29.1% negative trend (=−0.7 ppb year−1) over P1 and an 8.95% upward trend (=0.1 ppb year−1) over P2. To find the causes of the trend transition, we investigated the relationship between the adjusted MDA8 and several ozone precursors, namely NO x and CO. It is found that, over P1, the dominant chemistry regime is mixed, in which the decline of CO and NO x (attributed to industrial sources) is associated with the downward trend of ozone. On the other hand, the upward trend of ozone over P2 is associated with an increase in NO x mostly attributed to road transport (passenger cars) and residential sectors. • Surface ozone trend over the megacity of Tehran in the last decade is derived. • The influences of meteorology and short-term variability on the trend are minimized. • A transition point in the ozone trend attributable to road transport is found. [ABSTRACT FROM AUTHOR]