Re-examining Dust Chemical Aging and Its Impacts on Earth's Climate.

Abstract

The impact of atmospheric particulate matter (i.e., aerosols) on Earth's radiative balance has been and continues to be the leading source of uncertainty with respect to predictions of future temperature increases due to climate change. Mineral dust particles transported from deserts and semiarid regions across the globe are a dominant contributor to the aerosol burden. Dust has many and diverse effects on Earth's climate: it directly scatters and/or absorbs incoming sunlight; it reacts with trace gases leading to impacts on the oxidizing capacity of the atmosphere that affect both the lifetime of the greenhouse gas methane in addition to concentrations of tropospheric ozone-a greenhouse gas and criteria air pollutant; it influences the production as well as the lifetime and radiative properties of clouds; and it deposits nutrients to aquatic and terrestrial ecosystems that can stimulate primary production and facilitate the sequestration of atmospheric carbon dioxide (CO2). This Account will focus on the last three effects. The ability of dust to affect clouds and biogeochemical cycles hinges upon the chemical nature of dust particles-in particular, whether the compounds found in dust particles are water-soluble. The solubility of nutrients found in dust is particularly critical for determining the impact of atmospheric deposition on ocean productivity. The traditional viewpoint is that dust is inherently insoluble but reactive toward trace acidic gases, a process herein referred to as chemical aging. These reactions are thought to affect the oxidizing capacity of the atmosphere while effectively transforming the chemical composition of dust by increasing its solubility. Consequently, chemical aging is hypothesized to substantially increase the impact of dust on cloud droplet formation and marine biogeochemical cycles.This Account presents recent advances in our understanding of the mechanisms that determine how efficiently dust undergoes chemical aging and what the consequences of these processes are for the different effects of dust on Earth's climate. This Account will re-examine the traditional viewpoint that dust chemical aging strongly impacts marine biogeochemical cycles as well as the ability of dust to nucleate cloud droplets. Laboratory studies on environmental samples are combined with chemical analysis of field samples collected at dust transport receptor sites to better understand chemical aging mechanisms and determine the impact of dust on tropospheric oxidants, clouds, and biogeochemical cycles. Our results highlight the important role that dust mineralogy plays in both the nucleation of clouds as well as the kinetics responsible for the chemical aging of dust. This Account will present cases where dust contains inherently soluble minerals and does not require chemical aging in order to efficiently nucleate clouds in the atmosphere. Lastly, this Account illustrates the critical role that nondust aerosols, namely, wildfire and combustion emissions, play as a supplier of soluble nutrients important for biogeochemical cycles, particularly in marine environments. This Account will discuss these findings and highlight future research directions and recommendations to better understand dust-climate interactions and the emerging role of biomass burning aerosol in marine biogeochemical cycles.