Nephelometers have played a central role in air quality research and regulatory monitoring since the 1960s. They are also among the core instruments used within the ACTRIS (Aerosol, Clouds, and Trace Gases Research Infrastructure) network to provide high-quality measurements of atmospheric aerosols. Despite their long-standing use, interpreting nephelometer data remains a challenge—even for experienced researchers. In this webinar, we will review the fundamentals of how nephelometers measure particulate matter, clarify what insights this data can and cannot provide, and discuss key factors that influence measurement capabilities and data quality. We will use the AirPhoton IN101 nephelometer—approved for use in ACTRIS, widely deployed globally by the SPARTAN network, and used by many research institutions in India—as a reference to explain the basic measurement principles. We’ll then introduce the IN102 model, highlighting how its hardware upgrades and integration with GRASP software enable enhanced characterization of particles, including estimates of size distribution and dry particulate mass.