Located in the dust belt, the Arabian Peninsula is a major source of atmospheric dust. Frequent dust outbreaks and some 15 to 20 dust storms per year have profound effects on all aspects of human activity and natural processes in this region. To quantify the effect of severe dust events on radiation fluxes and regional climate characteristics, we simulated the storm that occurred from 18 to 20 March 2012 using a regional weather research forecast model fully coupled with the chemistry/aerosol module (WRF–Chem). This storm swept over a remarkably large area affecting the entire Middle East, northeastern Africa, Afghanistan, and Pakistan. It was caused by a southward propagating cold front, and the associated winds activated the dust production in river valleys of the lower Tigris and Euphrates in Iraq; the coastal areas in Kuwait, Iran, and the United Arab Emirates; the Rub al Khali, An Nafud, and Ad Dahna deserts; and along the Red Sea coast on the west side of the Arabian Peninsula. Our simulation results compare well with available ground-based and satellite observations. We estimate the total amount of dust generated by the storm to have reached 94 Mt. Approximately 78% of this dust was deposited within the calculation domain. The Arabian Sea and Persian Gulf received 5.3 Mt and the Red Sea 1.2 Mt of dust. Dust particles bring nutrients to marine ecosystems, which is especially important for the oligotrophic Northern Red Sea. However, their contribution to the nutrient balance in the Red Sea remains largely unknown. By scaling the effect of one storm to the number of dust storms observed annually over the Red Sea, we estimate the annual dust deposition to the Red Sea, associated with major dust storms, to be 6 Mt.

The spectral dependence of light absorption by atmospheric particulate matter has major implications for air quality and climate forcing, but remains uncertain especially in tropical areas with extensive biomass burning. In the September-October 2007 biomass-burning season in Santa Cruz, Bolivia, we studied light absorbing (chromophoric) organic or “brown” carbon (BrC) with surface and space-based remote sensing. We found that BrC has negligible absorption at visible wavelengths, but significant absorption and strong spectral dependence at UV wavelengths. Using the ground-based inversion of column effective imaginary refractive index in the range 305–368 nm, we quantified a strong spectral dependence of absorption by BrC in the UV and diminished ultraviolet B (UV-B) radiation reaching the surface. Reduced UV-B means less erythema, plant damage, and slower photolysis rates. We use a photochemical box model to show that relative to black carbon (BC) alone, the combined optical properties of BrC and BC slow the net rate of production of ozone by up to 18% and lead to reduced concentrations of radicals OH, HO₂, and RO₂ by up to 17%, 15%, and 14%, respectively. The optical properties of BrC aerosol change in subtle ways the generally adverse effects of smoke from biomass burning.

The near-surface temperature in Mediterranean climate-type regions has increased overall similarly or more rapidly than the global mean rates. Although these regions have comparable climate characteristics and are located at similar latitudes, recent warming acceleration is most pronounced in the Mediterranean Basin. Here, we investigate the contributions of several climate drivers to regional warming anomalies. We consider greenhouse gases, aerosols, solar irradiance, land–atmosphere interactions, and natural climate variability modes. Our results highlight the dominant role of anthropogenic greenhouse gas radiative forcing in all Mediterranean climate-type regions, particularly those in the northern hemisphere. In the Mediterranean Basin, the recent warming acceleration is largely due to the combined effect of declining aerosols and a negative trend in near-surface soil moisture. While land-atmosphere feedbacks are also important in other locations (e.g., California and Southern Africa), this synergy is unique in the Mediterranean Basin. These two regional climate drivers have natural and anthropogenic components of equivalent importance. Such feedbacks are not fully resolved in the current regional climate projections.

In the Middle East, desert dust is assumed to dominate air pollution, being in permanent violation of public health guidelines. Here we present ship-borne measurements from around the Arabian Peninsula and modeling results to show that hazardous fine particulate matter is to a large extent of anthropogenic origin (>90%), and distinct from the less harmful, coarse desert dust particles. Conventionally, it was understood that desert dust dominates both the fine and coarse aerosol size fractions, which obscures the anthropogenic signal. We find that the annual excess mortality from the exposure to air pollution is 745 (514-1097) per 100,000 per year, similar to that of other leading health risk factors, like high cholesterol and tobacco smoking. Furthermore, anthropogenic pollution particles account for a major part (~53%) of the visible aerosol optical depth. Therefore, in the Middle East anthropogenic air pollution is a leading health risk and an important climatic factor.

Volcanic explosions are the most critical replenishing mechanism of the stratospheric aerosol Junge layer. A fresh volcanic cloud comprises mostly sulfur‐bearing gases, volcanic ash, and water vapor. It is commonly assumed that only sulfate aerosols remain in an aged volcanic cloud. Accurate simulation of the initial evolution of multicomponent fresh volcanic clouds is largely missing due to insufficient spatial resolution and a lack of relevant physics in global climate models. However, this initial stage is essential, as the vertical structure, composition, and altitude of a freshly developed volcanic cloud affect its long‐term evolution. To fill this gap, we modified a regional WRF‐Chem model to study the dispersion of a Pinatubo‐size volcanic cloud in the equatorial belt with a 25 km grid spacing explicitly accounting for the SO₂, ash, sulfate, water vapor, and hydrometeors radiative effects. The model best reproduces the observed evolution of the Pinatubo optical depth when eruptive products are injected above the cold tropical tropopause at 17 km. During the first week, the volcanic cloud in our simulations rises 1 km/day. Ash is primarily responsible for the heating and lofting of the volcanic products. Radiative heating of SO₂ is weaker than that of ash and sulfate but is sufficient to position the core of the SO₂ layer 1–2 km above the sulfate layer. Utilizing a more realistic description of the volcanic cloud's initial stage potentially improves overall volcanic cloud predictability. It might also be essential to designing geoengineering technologies based on the injection of aerosol precursors in the lower stratosphere.