INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. (2015) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/joc.4479 Temporal variations in the frequency and concentration of dust events over Iran based on surface observations O. Alizadeh-Choobari, a * P. Ghafarian b and E. Owlad b a Institute of Geophysics, University of Tehran, Iran b Iranian National Institute for Oceanography and Atmospheric Science, Tehran, Iran ABSTRACT: Using surface meteorological records of a 20-year period from 1991 to 2010, temporal variations in the frequency and concentration of transported dust events over Iran are investigated. Five regions of frequent dust events are identifed. In the order of importance, these areas are the Khuzestan Plain, the coastal plain of the Persian Gulf, west of Iran, Tabas and Sistan. The frst three areas create a belt of high frequency of dust events along the western foothills of the Zagros Mountains. The Khuzestan Plain is the area with the highest frequency of dust events, over which dust laden air is almost permanently present in summer, while the coastal plain of the Persian Gulf is the second most affected area. These two areas, along with west of Iran, are mostly infuenced by transported dust from sources outside of Iran, while Tabas and Sistan are mostly infuenced by arid lands in the interior of Iran. In contrast, the southern coastal strip of the Caspian Sea is the area with the least frequent dust episodes. Throughout Iran, the frequency of dust events strengthens in spring, peaks in summer and signifcantly weakens in autumn and winter, with the least observed frequency in winter. Signifcant monthly variations of the frequency of dust events were also identifed, with the most and least frequencies in July and December, respectively. In terms of long-term frequency of dust events, our observational analyses show an overall rising trend of the frequency of Iran’s dust events in recent years, predominantly attributed to increasingly frequent dust outbreaks in Iraq due to human intervention. KEY WORDS frequency of dust events; arid lands; surface observations; visibility; Iran; Khuzestan Plain; Tabas; Sistan Received 25 January 2015; Revised 7 April 2015; Accepted 15 July 2015 1. Introduction Mineral dust is believed to be mostly originated from natural sources (Tegen et al., 2004), predominantly from the so-called ‘dust belt’ region, extending from the west coast of North Africa, over the Middle East, Central and South-Southwest Asia, to China (Prospero et al., 2002). Iran in Southwest Asia is therefore located within the Earth’s ‘dust belt’ region, suggesting that it is often sig- nifcantly affected by strong dust episodes. A majority of Iran lies in the subtropical high-pressure belt, experiencing either dry or semi-dry climate charac- terized by less than 260 mm annual mean precipitation (Masoodian, 1998; Hasanean, 2004) and high evapora- tion rates that far exceed the annual rates of precipita- tion. Indeed, arid and semi-arid areas occupy as much as 60% of Iran, covering about 1 million km 2 , although in the foothills of the Alborz and Zagros mountains and the coastal strip of the Caspian Sea, a Mediterranean cli- mate is dominated. In spite of potential sources in the interior of Iran associated with arid or semi-arid climate, it is now well established that signifcant quantities of dust originate from external sources (Shahsavani et al., 2012). * Correspondence to: O. Alizadeh-Choobari, Institute of Geophysics, University of Tehran, P.O. Box 14155-6466, Tehran, Iran. E-mail: omid.alizadeh@ut.ac.ir Iran is located downwind of some major sources of dust, including arid lands of Aljazera, the Alluvial Plain of the Tigris–Euphrates basin and the Zubair Desert in Iraq, the Syrian Desert which occupies much of western Iraq, and three remarkable desert areas of An Nafud in the northwest, Ad-Dahna in the northeast and Rub al Khali in the southeast of the Arabian Peninsula. As desert lands of Iraq and the Syrian Desert are mostly composed of fne silt and very fne clay particles (COMET, 2003) and infu- enced by the summer Shamal (an Arabic word meaning north) winds (Abdi Vishkaee et al., 2012), they are easily susceptible to wind erosion, causing western Iran to be often infuenced by transported dust from these sources. A large proportion of the Arabian Peninsula is occupied by desert plains (mostly composed of sand and silt), acting as the second main global source of dust after North Africa. The important arid and semi-arid areas in the interior and outside of Iran are depicted in Figure 1. For detailed discussion about the internal sources of dust in Iran, the reader is referred to the study by Alizadeh-Choobari et al. (2014). Vast arid and semi-arid areas in the interior of Iran and the profound impact of transported dust from external sources imply that mineral dust has an important contribution in degrading the air quality of Iran and acts as a signifcant regional climate forcing. A preliminary study of the frequency and seasonality of dust events over Iran was made by Middelton (1986) using ground station observations. He postulated that the highest © 2015 Royal Meteorological Society