Atmospheric multiphase chemistry influencing climate and health in the Anthropocene: from sulfate production to secondary organic aerosol formation and related effects

The Anthropocene as the current period of Earth history is characterized by a globally pervasive influence of human activities on the planet - from the equator to the poles and from the land surface, atmosphere and biosphere to the oceans and deep sea. The intensive use of land and water as well as large emissions of air pollutants, aerosols, and greenhouse gases lead to climate change and adverse effects on ecosystems, biodiversity, and human health. Since industrialization in the 18^th century and the great acceleration in the mid-20^th century, the atmospheric concentration levels and the global biogeochemical cycles of carbon, reactive nitrogen, and sulfur in the Earth system have been substantially altered by human interference. Among the first studies to quantify regional and global impacts of sulfate aerosols on atmospheric radiation, clouds, and climate were seminal papers published in the journal Tellus. Assessing the climate impacts of atmospheric aerosols requires a quantitative and predictive understanding of their sources, including the formation of sulfate and organic aerosols by oxidation and gas-to-particle conversion of gaseous precursors in the atmosphere. These multiphase processes include chemical reactions, mass transport, and phase transitions of gaseous, liquid, and solid substances. For sulfate aerosols, a number of formation pathways have been identified and quantitatively described in atmospheric chemistry and transport models. These pathways comprise reactions of sulfur dioxide and dimethyl sulfide with hydroxyl radicals in the gas phase, or with ozone, hydrogen peroxide, and transition metal ions in aerosol or cloud water. More recently, the reaction of sulfur dioxide with nitrogen dioxide has been discovered as another pathway of high relevance for haze formation under polluted environmental conditions. The reaction rates and relative importance of different sulfate formation pathways are strongly dependent on aerosol acidity (pH), which in turn depends on aerosol water content and is widely buffered by anthropogenic ammonia. Different reaction pathways, phase changes, and gas-particle partitioning are also relevant for the formation, growth and effects of secondary organic aerosols in the atmosphere. Historic and recent developments will be outlined and discussed.