Improving photochemical indicators for attributing ozone sensitivities in source apportionment analysis.

Comprehensive Air Quality Model with extensions (CAMx)-Decoupled Direct Method (DDM) simulations of first-order ozone (O) sensitivity to nitrogen oxides (NO) and volatile organic compounds (VOCs) emissions were performed and combined with modelled [Formula: see text] ratios to obtain a range of thresholds for determining O-sensitivity regimes for different areas of China. Utilising the new threshold ranges for photochemical indicators, the method for determining O formation in the Ozone Source Apportionment Technology (OSAT) module within CAMx was improved by a dynamically varied threshold of [Formula: see text] ratio. The O concentration contributions in the newly added transition regime were apportioned to NO and VOCs emissions in proportion to the relationship between the [Formula: see text] ratio and first-order O sensitivity. The source contributions of O concentrations from different emission sectors from June to September 2019 were compared using the original and improved CAMx-OSAT. The results showed that the O concentration contributions changed significantly in the NO-limited regime, with a maximum decrease of 21.89%, while the contributions increased by up to 7.57% in the VOC-limited regime, and were within 15 µg/m in the transition regime. The modified OSAT module enabled a more sophisticated attribution of O to precursor emissions and may have far-reaching implications for informing O pollution control policy.