According to Tehran Air Quality Control Company, mostly “moderate” status dominated the month, keeping Air Quality Index between 51 and 100.
The index categorizes conditions dictated by a measure of polluting matters into good (0-50), moderate (51-100), unhealthy for sensitive groups (101-150), unhealthy (151-200), very unhealthy (201-300) and hazardous (301-500).
Pollutants pushed the API to the “unhealthy” threshold for the sensitive group in the remaining six days. Children, pregnant women, cardiovascular and respiratory patients and the elderly form the group.
With the dilution of ground-level ozone during the month, the main culprits for the polluted days were PM2.5 and PM10, atmospheric particulate matters that have a diameter of less than 2.5 and 10 micrometers respectively.
O3 was responsible for five unhealthy days for the sensitive group in August, pushing up the AQI between 101 and 150.
Moderate air quality prevailed in the remaining 26 days of the month.
Comparative data analysis of ozone pollution in the year-ago September shows it adversely affected air quality in recent days.
In September 2020, moderate status was recorded for 28 days and high ozone density could only affect AQI on two days, keeping the air quality status in the range of unhealthy for the sensitive group.
Good air quality condition was not recorded even for one day during the period under review.
Scientific studies have shown that ground-level ozone is a secondary pollutant, formed when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants and other sources react chemically in the presence of sunlight.
Bad Ozone Pollution to Persist
Apart from the “good” ozone that occurs naturally in the stratosphere and forms a protective layer to shield the Earth from the sun's harmful ultraviolet rays, there is "bad" ozone in the lower atmosphere, near ground level, which is a harmful air pollutant.
The ground-level ozone results from chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC), created in high concentration in the presence of sunlight.
Experts say emissions from industrial facilities, motor vehicle exhausts, gasoline vapors and chemical solvents are some of the major sources of NOx and VOC.
Ozone is a colorless gas, 1.5 times denser than oxygen.
Breathing ozone can trigger a variety of health problems, particularly for the children, the elderly and people of all ages who have lung disorders such as asthma. Ground-level ozone can also have harmful impacts on sensitive vegetation and ecosystems.
Recommended remedial measures to alleviate this kind of pollution are similar to those regularly suggested for other types of pollution, such as detecting and removing mobile or stationary sources of nitrogen oxides and hydrocarbon emissions.
Vehicles burning diesel fuel and carburetor-equipped motorcycles are the most common producers of toxic emissions that lead to the emergence of ground-level ozone.
The capital's pollution levels often reach unhealthy levels in the cold season when the phenomenon of inversion occurs, but PM2.5 is chiefly responsible for poor air quality in this period.
In the warm season, which offers relief from smog, high levels of pollution are blamed on ozone and dust particles.
TAQCC charts indicated that ozone pollution hit a record in worsening Tehran's air conditions last year.
Main Culprits
Besides the hot weather, which is the main reason behind ozone formation in the already polluted metropolis, some officials also blame Tehran Municipality.
The Health Ministry has criticized the inaction of TM in identifying the sources of pollution and alleviating the problem on several occasions.
“Catalytic converters installed on vehicles are not in conformity with the required standards, if at all vehicle owners bother to equip their cars with the device," Abbas Shahsavani, the head of health group at the ministry, said.
A catalytic converter is an exhaust emission control device that reduces toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalyzing a redox reaction.
“Many commercial vehicles and public transport buses, which mostly add to air pollution in the city, do not use catalyst converters,” he added.
Shahsavani noted that it is TM's responsibility to monitor and crack down on smog-inducing two- and four-wheelers plying the streets of Tehran.
The capital’s urban managers have always censured passenger vehicles for their detrimental effects on air pollution and set several traffic regulations, schemes and fines to curb the use of private cars.
This is while a recent study by TAQCC on the nature and sources of pollutants shows that the highest level of emission is spewed by passenger buses, equal to 31%.
Heavy-duty vehicles are the next most polluting source with 23.7%, followed by motorcycles with 10%, airplanes with 5% and minibuses with 4.3%.
All these mobile sources account for a total of 76% of PM2.5 particles released into the air and the remaining 24% are emitted by industrial units.