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Why Tehran AOD and visibility move opposite seasons

Winter mountain trapping cuts visibility while column AOD is lowest; summer dust raises AOD even as surface visibility recovers.

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What seasonal AOD–visibility relationship is the major finding?

They vary inversely: winter AOD is low while city visibility is worst.

Why do mountains worsen winter Tehran pollution?

Peaks over 5000 m inhibit ventilation and trap pollutants.

What are Mehrabad winter visibility minima?

6.3 km in December and 6.8 km in January.

When do satellite AOD values increase?

Late spring and summer from local/regional dust (and perhaps biomass burning).

What weekday visibility anomaly points to local emissions?

A positive visibility anomaly on Friday (Iran’s weekend).

What happens to AOD and visibility after rain?

AOD decreases and Mehrabad visibility rises for two days after versus before rain.

What fraction of Mehrabad/Semnan rain falls November–April?

80%.

Which model supplied speciated AOD?

GOCART (dust, sulfate, organic and black carbon, sea salt).

What trajectory tool mapped Tehran source regions?

HYSPLIT six-hourly trajectories at 500, 1000, and 3000 m.

Which source region dominates low-level arrivals most of the year?

Deserts west and southwest of Tehran (Region A).

Why can winter AOD be low even when visibility is poor?

Aerosols are trapped in the lowest layers, so the column optical depth stays small.