Nitrogen dioxide measurement by cavity attenuated phase shift spectroscopy (CAPS) and implications in ozone production efficiency and nitrate formation in Beijing, China

Abstract

Abstract Nitrogen dioxide (NO 2 ) is a key species in studying photochemical smog and formation mechanisms of nitrate in fine particles. However, the conventional commercially available chemiluminescence (CL)‐based method often has uncertainties in measuring NO 2 because of interferences with other reactive nitrogen species. In this study, an Aerodyne Cavity Attenuated Phase Shift Spectroscopy (CAPS) NO 2 monitor that essentially has no interferences with nitrogen containing species was deployed in Beijing for the first time during August 2012. The CAPS NO 2 monitor is highly sensitive with a detection limit (3 σ ) of 46.6 ppt for 1 min integration. The NO 2 measured by CAPS shows overall agreement with that from CL, yet large differences up to 20% were also observed in the afternoon. Further, the discrepancies of NO 2 measurements between CAPS and CL appear to be NO z dependent with larger differences at higher NO z concentrations (e.g., > 14 ppb). As a result, the ozone production efficiency of NO x (OPE x ) derived from the correlations of O x ‐NO z with the CL NO 2 can be overestimated by 19–37% in Beijing. The daily OPE x calculated with the CAPS NO 2 ranges from 1.0 to 6.8 ppb/ppb with an average (±1 σ ) of 2.6 (±1.3) for the entire study. The relatively low OPE x and the relationship between OPE x and NO x suggest that ozone production chemistry is VOC sensitive during summer in Beijing. Two case studies further show that high concentrations of NO x can significantly enhance the formation of nitrate in fine particles in the presence of high O 3 and favorable meteorological conditions.