[1] Desert dust deposition to the ocean may be a significant source of biogeochemically important elements, specifically iron. The bioavailability of iron in the oceans requires it to be in a soluble form, and because atmospheric iron in desert dust is typically insoluble, understanding the atmospheric processes that convert insoluble iron to more soluble forms is essential. Understanding these relationships is especially important in remote ocean regions where iron may be the limiting nutrient. Observations of soluble iron from 2001 cruise-based aerosol measurements over the Atlantic and Pacific Oceans ranged from 0 to 45% (mean of 4 ± 9%) in the fine mode (<2.5 mm in diameter) and 0 to 87% (mean of 2 ± 10%) in the coarse mode. We test two simple hypotheses of soluble iron enhancement in the atmosphere using a global model of mineral aerosols. The first method assumes that iron solubility increases as iron is exposed to solar radiation, approximating photoreduction reactions that are important pathways for enhancement of soluble iron in the presence of acidic solutions. The second process imitates cloud processing of iron by increasing the amount of soluble iron when the mineral aerosol comes into contact with a cloud. Both methods resulted in similar average magnitudes of percent soluble iron compared to observations but did not capture specific events or have sufficient variability, perhaps because the model does not include aerosol interactions between species other than mineral dust or other processes that may be important.

Understanding the spatial and temporal variability in fine mineral dust (FD, mineral aerosols with diameters less than 2.5 µm) and coarse aerosol mass (CM, mass of aerosols with diameters between 2.5 and 10 µm) is important for accurately characterizing and perhaps mitigating their environmental and climate impacts. The spatial and seasonal variability of ambient FD and CM was characterized at rural and remote sites across the United States for 2011–2014 using concentration and elemental chemistry data from the Interagency Monitoring of Protected Visual Environments (IMPROVE) aerosol monitoring network. FD concentrations were highest (and had ≥50% contributions to PM2.5 mass) in the southwestern United States in spring and across the central and southeastern United States in summer (20–30% of PM2.5 mass). CM was highest across the Southwest and southern Great Plains in spring and central United States in spring, summer, and fall (≥70% contributions to PM10 mass). Similar FD and CM seasonal variability was observed near source regions in the Southwest, but a seasonal decoupling was observed in most other regions, suggesting the contribution of nonlocal sources of FD or perhaps non‐dust‐related CM. The seasonal and spatial variability in FD elemental ratios (calcium, iron, and aluminum) was fairly uniform across the West; however, in the eastern United States a shift in summer elemental composition indicated contributions from nonlocal source regions (e.g., North Africa). Finally, long‐term trend analyses (2000–2014) indicated increased FD concentrations during spring at sites across the Southwest and during summer and fall in the southeastern and central United States.

No abstract

Recent modeling and field studies have highlighted a relationship between sulfate concentrations and secondarily formed organic aerosols related to isoprene and other volatile biogenic gaseous emissions. The relationship between these biogenic emissions and sulfate is thought to be primarily associated with the effect of sulfate on aerosol acidity, increased aerosol water at high relative humidities, and aerosol volume. The Interagency Monitoring of Protected Visual Environments (IMPROVE) program provides aerosol concentration levels of sulfate (SO4) and organic carbon (OC) at 136 monitoring sites in rural and remote areas of the United States over time periods of between 15 and 28 years. This data set allows for an examination of relationships between these variables over time and space. The relative decreases in SO4 and OC were similar over most of the eastern United States, even though concentrations varied dramatically from one region to another. The analysis implied that for every unit decrease in SO4 there was about a 0.29 decrease in organic aerosol mass (OA = 1.8 × OC). This translated to a 2 μg/m3 decrease in biogenically derived secondary organic aerosol over 15 years in the southeastern United States. The analysis further implied that 35% and 27% in 2001 and 2015, respectively, of average total OA may be biogenically derived secondary organic aerosols and that there was a small but significant decrease in OA not linked to changes in SO4 concentrations. The analysis yields a constraint on ambient SO4–OC relationships that should help to refine and improve regional‐scale chemical transport models.