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-3) sensitivity to nitrogen oxides (NOx) and volatile organic compounds (VOCs) emissions were performed and combined with modelled P-H2O2/PHNO3 ratios to obtain a range of thresholds for determining O-3-sensitivity regimes for different areas of China. Utilising the new threshold ranges for photochemical indicators, the method for determining O-3 formation in the Ozone Source Apportionment Technology (OSAT) module within CAMx was improved by a dynamically varied threshold of P-H2O2/PHNO3 ratio. The O-3 concentration contributions in the newly added transition regime were apportioned to NOx and VOCs emissions in proportion to the relationship between the P-H2O2/PHNO3 ratio and first-order O-3 sensitivity. The source contributions of O-3 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-3 concentration contributions changed significantly in the NOx-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 mu g/m(3) in the transition regime. The modified OSAT module enabled a more sophisticated attribution of O-3 to precursor emissions and may have far-reaching implications for informing O-3 pollution control policy. . (c) 2024 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.