Development of a portable cavity-enhanced absorption spectrometer for the measurement of ambient NO3 and N2O5: experimental setup, lab characterizations, and field applications in a polluted urban environment

摘要

A small and portable incoherent broadband cavity-enhanced absorptionspectrometer (IBBCEAS) for NO and NO measurement has beendeveloped. The instrument features a mechanically aligned non-adjustableoptical mounting system, and the novel design of the optical mounting systemenables a fast setup and stable operation in field applications. To removethe influence of the strong nonlinear absorption by water vapour, a dynamicreference spectrum through NO titration is used for the spectrum analysis.The wall loss effects of the sample system were extensively studied, and thetotal transmission efficiencies were determined to be 85 and 55 % forNO and NO, respectively, for our experimental setup. Thelimit of detection (LOD) was estimated to be 2.4 pptv (1) and2.7 pptv (1) at 1 s intervals for NO and NO,respectively. The associated uncertainty of the field measurement wasestimated to be 19 % for NO and 22–36 % for NOmeasurements from the uncertainties of transmission efficiency, absorptioncross section, effective cavity length, and mirror reflectivity. Theinstrument was successfully deployed in two comprehensive field campaignsconducted in the winter and summer of 2016 in Beijing. Up to 1.0 ppbNO+NO was observed with the presence of high aerosolloadings, which indicates an active night-time chemistry in Beijing.