Aerosols in the Pre-industrial Atmosphere

Abstract

Purpose of Review We assess the current understanding of the state and behaviour of aerosols under pre-industrial conditions and the importance for climate. Recent Findings Studies show that the magnitude of anthropogenic aerosol radiative forcing over the industrial period calculated by climate models is strongly affected by the abundance and properties of aerosols in the preindustrial atmosphere. The low concentration of aerosol particles under relatively pristine conditions means that global mean cloud albedo may have been twice as sensitive to changes in natural aerosol emissions under preindustrial conditions compared to present-day conditions. Consequently, the discovery of new aerosol formation processes and revisions to aerosol emissions have large effects on simulated historical aerosol radiative forcing.Summary We review what is known about the microphysical, chemical, and radiative properties of aerosols in the pre-industrial atmosphere and the processes that control them. Aerosol properties were controlled by a combination of natural emissions, modification of the natural emissions by human activities such as land-use change, and anthropogenic emissions from biofuel combustion and early industrial processes. Although aerosol concentrations were lower in the pre-industrial atmosphere than today, model simulations show that relatively high aerosol concentrations could have been maintained over continental regions due to biogenically controlled new particle formation and wildfires. Despite the importance of preindustrial aerosols for historical climate change, the relevant processes and emissions are given relatively little consideration in climate models, and there have been very few attempts to evaluate them. Consequently, we have very low confidence in the ability of models to simulate the aerosol conditions that form the baseline for historical climate simulations. Nevertheless, it is clear that the 1850s should be regarded as an early industrial reference period, and the aerosol forcing calculated from this period is smaller than the forcing since 1750. Improvements in historical reconstructions of natural and early anthropogenic emissions, exploitation of new Earth system models, and a deeper understanding and evaluation of the controlling processes are key aspects to reducing uncertainties in future.

Aerosol characteristics and particle production in the upper troposphere over the Amazon Basin

“…This had also been concluded based on a large statistical sampling of UT air in the Northern Hemisphere by the CARIBIC aircraft measurement program (Weigelt et al, 2009). The importance of NPF in the UT for the budget of CN and CCN had been proposed previously on the basis of modeling studies (Yu et al, 2008;Merikanto et al, 2009;Carslaw et al, 2017), and is evident in the global enhancement of CN in the UT, especially in tropical regions, seen in compilations of data from numerous aircraft campaigns (Yu et al, 2008;Reddington et al, 2017). In this way, aerosol production by BVOC oxidation in the UT can provide the "missing source" of FT organic aerosol, which had been deduced from a mismatch between models and observations (Heald et al, 2005).…”

Forests, atmospheric water and an uncertain future: the new biology of the global water cycle

“…Much remains unknown about the behaviour and impact of ice nucleating particles in the atmosphere (Carslaw et al 2017;Coluzza et al 2017) and we know almost nothing about these particles in the tropics (Yakobi-Hancock et al 2014). Methods now permit a much more detailed assessment of bacterial communities and their activities (Hill et al 2014;Failor et al 2017).…”

“…Though specific information is limited it is generally assumed that atmospheric aerosol concentrations were lower in the pre-industrial era (reviewed in Hamilton 2015; Carslaw et al 2017). In such conditions the climate system is thought to have been more sensitive to changes in these concentrations (Spracklen and Rap 2013;Carslaw et al 2017).…”

“…In such conditions the climate system is thought to have been more sensitive to changes in these concentrations (Spracklen and Rap 2013;Carslaw et al 2017). Over the past century the products of industry, fires and other activities have added considerably to atmospheric particles though many details of these emissions and their dynamics remain unclear (Fuzzi et al 2015;Sánchez Gácita et al 2017).…”

“…For example, observations in France indicate that atmospheric acidification reduces their influence (Pouzet et al 2017). Nonetheless evidence that human activities have impacted ice nucleating particles or their properties remains too limited to generalise (Carslaw et al 2017;Coluzza et al 2017). …”