The ES-405 is a near reference air quality sensor designed to simultaneously provide accurate real-time measurements of PM10, PM4, PM2.5, and PM1 particulate concentrations. It reports these four key particulate concentrations in both indoor and outdoor environments. The rugged weatherproof enclosure allows use in the harshest environments for reliable continuous outdoor operation, and the inlet heater reduces moisture-related measurement errors. The sensor incorporates sheath air to prevent particles from contaminating the internal optics of the sensor. The ES-405 can be wall-mounted or installed on a vertical mast.