Abstract: An inexpensive, yet accurate particle matter (e.g., PM) detector is described that combines the features of two different optical particle sensors with a modeling tool to provide a comprehensive, effective air quality measurement instrument (e.g., sensing instrument). The modeling tool enables the use of a combination low cost optical particle sensors, such as a nephelometer and an optical particle counter to determine the amounts of unhealthy particles that are present in air. The sensing instrument determines the total mass loading at specific size breakpoints, such as PM1, PM2.5, and PM10. In some embodiments, the sensing instrument may include gas sensors so that the particular gaseous pollutants can be identified and measured. In some embodiments, the sensing instrument may further include one or more of a humidity sensor, a temperature sensor, and a pressure sensor.