A determination that a source may be present. Typically requires on analysis of one or a series of measured anomalies. Detections can be defined in terms of magnitude and/or duration of elevated mixing ratio or emission rate. For example, a detection event could be defined as an anomaly that reaches an estimated mass emission rate of 5 standard deviations above a 24-hour baseline Unlike an anomoly, a detection is intended to lead to a further action, e.g., follow-up and/or root-cause analysis.