An air quality monitoring system that enables a wide scale deployment of instruments with enough accuracy for meaningful and actionable data is provided. In one aspect, an advanced technique is used to calibrate limited-capability gaseous chemical sensors to obtain accurate measurements by cross-calibrating those sensors with reference sensors to correct sensitivities to parameters that cause errors to measurements of targeted gases. In another aspect, air quality measurements are used to identify sources of chemicals in a localized level by accounting for local conditions using data such as ambient condition data and user-provided data about the local environment. In yet another aspect, a gaseous chemical sensor with an improved encasement having a cell for reflecting and lengthening light path is provided to reduce the limitations and enhance the accuracy of a conventional spectrophotometric gaseous chemical sensor.