Mesoscale controls on particulate matter pollution for a mega-city in a semi-arid mountainous environment: Tehran, Iran

Tehran, the mega-city capital of Iran, suffers from high concentrations of PM10 (particles less than 10 µm in aerodynamic diameter) throughout the year. Emissions from transport combined with mesoscale atmospheric features related to the mountainous terrain lead to a distinctive diurnal pattern in concentrations. Air quality data from monitoring stations show that the highest concentration of PM10 are in the morning and evening periods, associated with peak traffic volumes and transition in local meteorology from a stable nocturnal down-slope flow to a daytime upslope regime. There is a clear north south gradient in PM10 associated with the transport by down-slope winds.

The observations indicate that a clear diurnally reversing orographic wind system is dominant in the Tehran basin and controls much of the local meteorology on a day-to-day basis. Especially in summer, the circulation is clearly pronounced. The combined effect of reduced insolation to drive thermally generated flows and the regular passage of the eastward propagating depressions that pass over Tehran make the bi-modal behaviour of wind more diffuse in spring and winter.

This paper presents an analysis of the dominant mesoscale controls – particularly on a diurnal scale – on PM10 concentrations for Tehran through examining its spatial and temporal patterns as measured by 5 permanent monitoring stations. Three of the monitoring stations measure pollutants and meteorology and are roughly aligned on a north-south axis, the other two stations only measure meteorological variables and are situated in the eastern and western parts of the city.

The northernmost station shows a distinct diurnal pattern with clear morning and evening peaks, whereas for the other stations, concentrations are more evenly spread throughout the day. For the southernmost station, slightly elevated levels of pollutants are measured during the night which suggests a southward PM10 transport with the nocturnal drainage winds from the foothills.

As the presented diurnal mesoscale wind system coincides with peak traffic in Tehran, it potentially amplifies concentrations at the time of peak emission release. This overlap however, prohibits a clear separation between the traffic related and the meteorology related signals in the concentration measurements. An attempt to quantify the traffic and meteorology related contributions has been undertaken through a year long modelling study of the local low level wind field using The Air Pollution Model (TAPM), but this will be presented in a separate paper.