Nitrogen dioxide (NO2) is a key species in studying photochemical smog and formation mechanisms of nitrate in fine particles.However,the conventional chemiluminescence (CL)-based method often has uncertainties in measuring NO2 because of interferences with other reactive nitrogen species.In this study,an Aerodyne Cavity Attenuated Phase Shift Spectroscopy (CAPS) NO2 monitor that essentially has no interferences with nitrogen-containing species was deployed in Beijing for the first time during August 2012.The CAPS NO2 monitor is highly sensitive with a detection limit (3σ) of 46.6 ppt for 1-minute integration.The NO2 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 NO2 measurements between CAPS and CL appear to be NOz dependent with larger differences at higher NOz concentrations (e.g.> 14 ppb).As a result,the ozone production efficiency of NOx (OPEx) derived from the correlations of Ox-NOz with the CL NO2 can be overestimated by 19-37% in Beijing.The daily OPEx calculated with the CAPS NO2 ranges from 1.0 to 6.8 ppb/ppb with an average (±1σ) of 2.6 (±1.3) for the entire study.The relatively low OPEx and the relationship between OPEx and NOx suggest that ozone production chemistry is VOC sensitive during summer in Beijing.Two case studies further show that high concentrations of NOx can significantly enhance the formation of nitrate in fine particles in the presence of high O3 and favorable meteorological conditions.