Abstract: A system and method for using path-averaged concentration measurements for estimating the amount of substance being released from a particular location. One or more open-path sensing systems are set up in appropriate locations around possible substance release sites and tuned to detect a particular substance of interest. Path-averaged concentration measurements of the particular substance are collected over time and communicated to an open-path back calculation algorithm. The algorithm calculates time-averaged measurements for a plurality of time periods. Back calculation is then performed with the time-averaged measurements to estimate the release rate and the total release of the substance.