Abstract: System for, and method of, monitoring airborne particulate, including particulate of the PM.sub.2.5 class. The system for monitoring airborne particulate includes an optical sensor to measure size characteristics of sampled airborne particulate and a humidity sensor to measure relative humidity. An adjusted airborne concentration value is then produced in response to the measured size characteristics and to the humidity measurement. According to one embodiment the optical sensor is implemented as a multi-wavelength nephelometer, e.g., two-wavelength nephelometer. According to another embodiment the humidity sensor is placed relatively near the optical sensing region and makes a relative humidity measurement.