Aerosol physical characterization: A review on the current state of aerosol documentary standards and calibration strategies

Aerosols have a wide-ranging impact on the climate, air quality, human health, and agriculture. Despite the ongoing advances in aerosol measurement science and technology, the uncertainties in quantifying aerosol physical properties remain significant in many applications. The accurate characterization of airborne particles - including number and mass concentration, size distribution and light absorption - is critical for understanding their behavior in the atmosphere and environmental fate. We delve into the physical characterization of aerosols, highlighting the measurement and documentary standards that underpin measurement traceability and enable comparison of data collected by instruments based on measurement principles at different times or locations. In particle metrology, recent advances have led to sophisticated primary measurement standards, with relative expanded measurement uncertainties down to 1.1 % (coverage factor k = 2; 95 % confidence interval). These standards enable time- and cost-effective instrument calibration to support research, industry, and legislation. We discuss documentary standards and regulations related to air quality and control of particle emissions from vehicles, aviation, shipping, and stationary sources, with the aim to increase awareness of these documents and underline differences in measurement protocols in different sub-fields of aerosol sciences. Importantly, we emphasize the need for further harmonization of measurement procedures, providing specific examples and making suggestions towards this goal. This review, with its comprehensive coverage of aerosol measurement and documentary standards across different sub-disciplines, can serve as a reliable guide for scientists and regulators interested in improving the accuracy of their measurements.