Introduction

An aerosol is defined as a suspension of particles in air. The sizes of aerosol particles range from a few nanometers to several tens of micrometers, and the chemical compositions of aerosol particles show large variability depending on the emission sources and formation processes. Fig. 1 shows example aerosol size distributions in the atmosphere. The majority of the particle number is found in the diameters smaller than 100 nm, whereas that of the particle mass is found in the diameters larger than 100 nm. Aerosols significantly affect air quality in urban air as they have adverse effects on human health. Aerosols can also affect the regional and global climate by altering the radiative balance of the Earth’s atmosphere (Fig. 2). The physical and chemical characterization of aerosol particles is important for improving our understanding of the environmental impacts of aerosols. ...

Atmospheric aerosol particles comprise sulfates, nitrates, organic matter, black carbon (soot), mineral dust, sea salt, and their mixtures. This article discusses the climate impacts of particulate matter (Fig. 1) from a physical science perspective, focusing on topics relevant to research themes in our laboratory. Atmospheric aerosols influence Earth’s radiation budget through aerosol-radiation interactions, involving scattering and absorption of solar radiation1. Sulfate aerosols predominantly scatter solar radiation back to space with minimal absorption, causing cooling of the atmosphere-ocean system. In contrast, black carbon strongly absorbs solar radiation, heating the surrounding atmosphere. While tropospheric black carbon warms the atmosphere-ocean system, stratospheric black carbon may cause net cooling of the climate system by heating the stratosphere while reducing incoming solar radiation to lower layers2. ...

Reducing greenhouse gas emissions is essential for mitigating global warming. However, practical difficulties of reducing global emission in a required timescale increase risks of severe impacts on human society and global ecosystems. Consequently, the climate science community has recently intensified researches on climate intervention as a potential measure to buy time until sufficient emission reductions and warming mitigation are achieved. Among geoengineering approaches for controlling the climate1, climate intervention refers to methods involving atmospheric physical and chemical processes. Current most focused methods include Marine Cloud Brightening (MCB)2 and Stratospheric Aerosol Injection (SAI)3 (Fig. 1). SAI is currently considered a relatively robust and cost-effective approach to counteracting greenhouse gas radiative forcing4,5. SAI artificially replicates the temporary cooling effects of volcanic aerosols on the climate; the 1991 Mount Pinatubo eruption demonstrated this mechanism, where stratospheric sulfate aerosols increased solar reflection, reducing global mean temperature by approximately 0.5°C6. Key scientific and technical challenges in SAI research include: ...