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(NO_(x))and volatile organic compounds(VOCs)emissions were performed and combined with modelled P_(H_(2)O_(2))/P_(HNO_(3))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_(H_(2)O_(2))/P_(HNO_(3))ratio.The O_(3)concentration contributions in the newly added transition regime were apportioned to NO_(x)and VOCs emissions in proportion to the relationship between the P_(H_(2)O_(2))/P_(HNO_(3))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 NO_(x)-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 15μg/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.

Applying model simulation and photochemical indicators to evaluate ozone sensitivity in southern Taiwan

Ozone sensitivity was investigated using CAMx simulations and photochemical indicator ratios at three sites （Pingtung City, Chao-Chou Town, and Kenting Town） in Pingtung County in southern Taiwan during 2003 and 2004. The CAMx simulations compared fairly well with the hourly concentrations of ozone. Simulation results also showed that Pingtung City was mainly a volatile organic compounds （VOC）-sensitive regime, while Chao-Chou Town was either a VOC-sensitive or a NOx-sensitive regime, depending on the seasons. Measurements of three photochemical indicators （H202, HNO3, and NOy） were conducted, and simulated three transition ranges of H202/HNO3 （0.5-0.8）, O3/HNO3 （10.3-16.2） and O3/NOy （5.7-10.8） were adopted to assess the ozone sensitive regime at the three sites. The results indicated that the three transition ranges yield consistent results with CAMx simulations at most times at Pingtung City. However, both VOC-sensitive and NOx-sensitive regimes were important at the rural site Chao-Chou Town. Kenting Town, a touring site at the southern end of Taiwan, was predominated by a NOx-sensitive regime in four seasons.

Photochemical indicators of ozone sensitivity： application in the Pearl River Delta, China

Surface 03 production has a highly nonlinear relationship with its precursors. The spatial and temporal heterogeneity of O3-NOx-VOC-sensitivity regimes complicates the control- decision making. In this paper, the indicator method was used to establish the relationship between 03 sensitivity and assessment indicators. Six popular ratios indicating ozone-precursor sensitivity, HCHO/NOy, H2O2/ HNO3, O3/NOy, O3/NOz, O3/HNO3, and H2O2/NOz, were evaluated based on the distribution of NOx- 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 VOCsensitive regime, while its north-east is NO y-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 H202/HNO3 being the best indicator. The threshold sensitivity levels for HCHO/NOy, H2O2/HNO3, O3/NOy, O3/NOz, O3/HNO3, and H2O2/NOz 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/ NOy, O3/NOz, O3/HNO3, and H202/NOz were able to identify more than 95% of VOC- and NOx-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/NOz 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.

Modeling of spatial and temporal variations of ozone-NO_(x)-VOC sensitivity based on photochemical indicators in China

Comprehensive air quality model with extensions(CAMx)-decoupled direct method(DDM)was used to simulate ozone-NO_(x)-VOCs sensitivity of for May-November in 2016-2018 in China.Based on the relationship between the simulated ozone(O_(3))sensitivity values and the ratio of formaldehyde(HCHO)to NO_(2)(FNR)and the ratio of production rate of hydrogen peroxide(H_(2)O_(2))to production rate of nitric acid(HNO_(3))(P_(H_(2)O_(2))/P_(HNO_(3))),the localized range of FNR and P_(H_(2)O_(2))/P_(HNO_(3))thresholds in different regions in China were obtained.The overall simulated FNR values are about 1.640-2.520,and P_(H_(2)O_(2))/P_(HNO_(3))values are about 0.540-0.830 for the transition regime.Model simulated O_(3)sensitivities or region specific FNR or P_(H_(2)O_(2))/P_(HNO_(3))thresholds should be applied to ensure the accurate local O3 sensitivity regimes.Using the tropospheric column FNR values from ozone monitoring instrument(OMI)satellite data as an indicator with the simulated threshold values,the spatial distributions of O_(3)formation regimes in China are determined.The O_(3)sensitivity regimes from eastern to central China are gradually from VOC-limited,transition to NO_(x)-limited spatially,and moving toward to transition or NO_(x)-limited regime from 2005 to 2019 temporally.