Atmospheric Chemistry: A Century of Expanding Scientific Discovery and Societal Relevance (Core Science Keynote)

The impressive evolution of atmospheric chemistry from the early 20^th century to the present day has been driven by a mix of scientific curiosity about the air we live in and breath on Earth, the desire to understand and mitigate the detrimental impacts of air pollutants to humans and the environment, and in the last half century, a parallel quest to understand the atmospheres of other planets. In 1920, there was already an appreciation of the harmful health effects of pollutants from fossil fuel combustion, but the precise understanding of the chemistry involved was yet to come. Over the following 50 years, through expanding atmospheric measurements and laboratory kinetics studies, a rudimentary understanding of the chemistry of air pollution and the stratospheric ozone layer, together with simple computer models of these phenomena, and the earliest air pollution regulations, had evolved. Here I will focus on the last 50 years that I have personally witnessed.

This half-century has seen the careful observation of the rapidly growing size of human relative to natural influences on the atmosphere, and the more and more urgent need to mitigate the detrimental influences through regulation. This half-century began with the first studies of the effects of supersonic aircraft exhaust and of nitrous oxide and chlorofluorocarbon (CFC) emissions on the ozone layer, and the beginnings of research on the atmospheric chemistry of other planets. The discovery and explanation of the Antarctic Ozone Hole, and the passage of the Montreal Protocol to protect the ozone layer followed. Of course, the atmosphere knows no borders and more and more nations were getting involved in atmospheric chemistry studies. The research formally became international with the advent of the International Global Atmospheric Chemistry (IGAC) program in 1988, whose overall goal was to develop a fundamental understanding of the natural and anthropogenic processes that determine the composition of the atmosphere, and of the interactions between atmospheric composition and biospheric and climatic processes. Recurring assessments also became international with the first WMO Atmospheric Ozone report in 1985 (that evolved into the successive WMO Scientific Assessments of Ozone Depletion in 1994 onwards), and in 1990 the first WMO-UNEP report on Climate Change:The IPCC Scientific Assessment.

Clear metrics of the rise in human influences on atmospheric chemistry were and are the long-term chemical trends being observed around the globe: both in air pollutants like O₃, CO, NOx, SO₂, inorganic and organic aerosols and organic gases; and in long-lived chemically and radiatively active substances like CO₂, CH₄, N₂O and a growing number of purely synthetic trace gases (CFCs, HCFCs, HFCs, PFCs, NF₃, SF₆, etc.). The NASA-supported Advanced Global Atmospheric Gases Experiment and the NOAA Global Monitoring Division have now accumulated over 40 years of measurements of these long-lived gases, augmented by measurements of old air trapped in firn and ice cores. These trends, indicating that concentrations of almost all of these long-lived gases are now well beyond their ranges in the pre-industrial era, have led to the proposal that we have now left the Holocene epoch and have entered a new Anthropocene epoch. There are continuing important efforts to determine trends in the OH free radical, which is a key player in defining the oxidation and self-cleaning capacity of the atmosphere. There are also important lessons from all the measurements being learned along the way. CFCs were replaced by HFCs that are powerful greenhouse gases and are now accumulating rapidly in the atmosphere, and while the success of the Montreal Protocol is very evident in the overall trends, there is now evidence from the above networks that new CFC-11 emissions have recently emerged in apparent violation of the Protocol.

The human activities driving these changes form a long list: fossil fuel combustion, agriculture, refrigeration, air conditioning, industrial solvents, plastic foams, electricity distribution grids, and flat screen displays to name just a few. Solutions to lowering and halting the detrimental changes can be relatively straightforward (e.g. affordable alternatives are readily developed, as for CFCs) or very challenging (e.g. transforming the current fossil fuel-dominated energy generation system to low and zero greenhouse gas emitting technologies, while sustaining economic growth). But these latter challenges must be met: the longer the delay, the greater the cost of action, and the greater the damage from climate change.