NOX level determination adopting reducing-agent mass sensor

The invention relates to NOX level determination adopting a reducing-agent mass sensor. An after-treatment system comprises an SCR system, a reducing-agent storage tank, a reducing-agent introducing assembly and the reducing-agent mass sensor, wherein the reducing-agent introducing assembly is coupled to the reducing-agent storage tank and the SCR system fluidly, and used for introducing a reducing agent into the SCR system. A controller is coupled to the reducing-agent mass sensor in a communication mode, and configured to explain an output signal coming from the reducing-agent mass sensor, and the output signal indicates the mass of the reducing agent. The controller determines an outlet NOx quantity in downstream exhaust gas of the SCR system by using the mass of the reducing agent, and further determines whether the outlet NOx quantity exceeds a predefined threshold value or not. The controller indicates to a user that the outlet NOx quantity exceeds the predefined threshold value as a response to that th