A Photoacoustic Extinctiometer (PAX) is a sensitive, high-resolution, and fast-response instrument designed for measuring the optical properties of aerosols, specifically light absorption and scattering coefficients. It is a crucial tool for climate change research and carbon particle sensing, providing direct, in-situ measurements without the artifacts associated with filter-based methods. The PAX operates by using a modulated diode laser (e.g., 870 nm as standard, with optional 405 nm or 532 nm wavelengths) to illuminate aerosol particles. When particles absorb light, they heat the surrounding air, generating amplified pressure waves that are detected acoustically. This photoacoustic approach directly measures light absorption. Simultaneously, a wide-angle integrating reciprocal nephelometer measures the light scattering coefficient. From these direct measurements, the PAX can derive other critical parameters, including the extinction coefficient, single scattering albedo, and black carbon (soot) mass concentrations. Measurement ranges for absorption and scattering can extend from less than 1 Mm⁻¹ to 100,000 Mm⁻¹. The instrument offers rapid response times (e.g., less than 10 seconds) and one-second resolution, providing real-time data. PAX instruments are indispensable for climate modeling, helping scientists understand the radiative forcing effects of black carbon and brown carbon aerosols. They are widely used in atmospheric research, air quality monitoring, and studies of combustion emissions and biomass burning. Their suitability for deployment on mobile platforms, at fixed sites, and in laboratory settings makes them versatile for diverse environmental studies. The Photoacoustic Extinctiometer is designed for continuous and autonomous operation, eliminating the need for filter collection and thus avoiding associated measurement artifacts. It features low power consumption, straightforward calibration procedures, and integrated data storage. The instrument can be controlled via an intuitive touchscreen interface on the front panel or remotely through a web browser on a networked computer, allowing for real-time display of measured and calculated data, instrument status, and configuration of operating parameters. Data is typically stored in easy-to-read ASCII files for convenient download, archiving, and analysis.