• Chemical actinometer (CA) was built for NO₂ photolysis frequency, j(NO₂), analysis.

• CA data (10% accuracy) were compared with spectroradiometer (SR) data.

• Long-term j(NO₂) measurement was validated by comparison of CA and SR.

• Dis-benefit of aerosol on j(NO₂) due toparticulate matter (PM_2.5) control on O₃.

The photolysis frequency of NO₂, j(NO₂), is an important analytical parameter in the study of tropospheric chemistry. A chemical actinometer (CA) was built to measure the ambient j(NO₂) based on a high precision NO_x instrument with 1 min time resolution. Parallel measurements of the ambient j(NO₂) by using the CA and a commercial spectroradiometer (SR) were conducted at a typical urban site (Peking University Urban Environmental Monitoring Station) in Beijing. In general, good agreement was achieved between the CA and SR data with a high linear correlation coefficient (R² = 0.977) and a regression slope of 1.12. The regression offset was negligible compared to the measured signal level. The j(NO₂) data were calculated using the tropospheric ultraviolet visible radiation (TUV) model, which was constrained to observe aerosol optical properties. The calculated j(NO₂) was intermediate between the results obtained with CA and SR, demonstrating the consistency of all the parameters observed at this site. The good agreement between the CA and SR data, and the consistency with the TUV model results, demonstrate the good performance of the installed SR instrument. Since a drift of the SR sensitivity is expected by the manufacturer, we propose a regular check of the data acquired via SR against those obtained by CA for long-term delivery of a high quality series of j(NO₂) data. Establishing such a time series will be invaluable for analyzing the long-term atmospheric oxidation capacity trends as well as O₃ pollution for urban Beijing.

The distribution of NO_x- and VOC-sensitive regimes in the PRD are identified.

The effectiveness of six popular chemical indicators for discriminating O₃ sensitivity regimes is evaluated.

Threshold levels for HCHO/NO_y, H₂O₂/HNO₃, O₃/NO_y, O₃/NO_z, O₃/HNO₃ were derived and verified.

The indicators H₂O₂/HNO₃ and H₂O₂/NO_z performed best and maintained relatively stable threshold levels.

Surface O₃ production has a highly nonlinear relationship with its precursors. The spatial and temporal heterogeneity of O₃-NO_x-VOC-sensitivity regimes complicates the control- decision making. In this paper, the indicator method was used to establish the relationship between O₃ sensitivity and assessment indicators. Six popular ratios indicating ozone-precursor sensitivity, HCHO/NO_y, H₂O₂/HNO₃, O₃/NO_y, O₃/NO_z, O₃/HNO₃, and H₂O₂/NO_z, were evaluated based on the distribution of NO_x- and VOC-sensitive regimes. WRF-Chem was used to study a serious ozone episode in fall over the Pearl River Delta (PRD). It was found that the south-west of the PRD is characterized by a VOC-sensitive regime, while its north-east is NO_x-sensitive, with a sharp transition area between the two regimes. All indicators produced good representations of the elevated ozone hours in the episode on 6 November 2009, with H₂O₂/HNO₃ being the best indicator. The threshold sensitivity levels for HCHO/NO_y, H₂O₂/HNO₃, O₃/NO_y, O₃/NO_z, O₃/HNO₃, and H₂O₂/NO_z were estimated to be 0.41, 0.55, 10.2, 14.0, 19.1, and 0.38, respectively. Threshold intervals for the indicators H₂O₂/HNO₃, O₃/NO_y, O₃/NO_z, O₃/HNO₃, and H₂O₂/NO_z were able to identify more than 95% of VOC- and NO_x-sensitive grids. The ozone episode on 16 November 16 2008 was used to independently verify the results, and it was found that only H₂O₂/HNO₃ and H₂O₂/NO_z were able to differentiate the ozone sensitivity regime well. Hence, these two ratios are suggested as the most appropriate indicators for identifying fall ozone sensitivity in the PRD. Since the species used for indicators have seasonal variation, the utility of those indicators for other seasons should be investigated in the future work.