Identification of close relationship between large-scale circulation patterns and ozone-precursor sensitivity in the Pearl River Delta, China.

To curb the continuous deterioration of ozone (O3) pollution in China, identifying the O3-precursor sensitivity (OPS) and its driving factors is a prerequisite for formulating effective O3 pollution control measures. Traditional OPS identification methods have limitations in terms of spatiotemporal representation and timeliness; therefore, they are not appropriate for making OPS forecasts for O3 contingency control. OPS is not only influenced by local precursor emissions but is also closely related to meteorological conditions governed by large-scale circulation (LSC). In this study, a localized three-dimensional numerical modeling system was used to investigate the relationship between LSC and OPS in the Pearl River Delta (PRD) of China during September 2017, a month with continuous O3 pollution. Our results highlighted that there was a close relationship between LSC and OPS over the PRD, and the four dominant LSC patterns corresponded well to the NOx-limited, NOx-limited, VOC-limited, and transitional regimes, respectively. The clear linkage between LSC and OPS was mainly driven by the spatial heterogeneity of NOx and VOC emissions within and beyond the PRD along the prevailing winds under different LSC patterns. A conceptual model was developed to highlight the intrinsic causality between the LSC and OPS. Because current technology can accurately forecast LSC 48-72 h in advance, the LSC-based OPS forecast method provided us with a novel approach to guide contingency control and management measures to reduce peak O3 at a regional scale.

Emission source-based ozone isopleth and isosurface diagrams and their significance in ozone pollution control strategies.

In the past decade, ozone (O3) pollution has been continuously worsening in most developing countries. The accurate identification of the nonlinear relationship between O3 and its precursors is a prerequisite for formulating effective O3 control measures. At present, precursor-based O3 isopleth diagrams are widely used to infer O3 control strategy at a particular location. However, there is frequently a large gap between the O3-precursor nonlinearity delineated by the O3 isopleths and the emission source control measures to reduce O3 levels. Consequently, we developed an emission source-based O3 isopleth diagram that directly illustrates the O3 level changes in response to synergistic control on two types of emission sources using a validated numerical modeling system and the latest regional emission inventory. Isopleths can be further upgraded to isosurfaces when co-control on three types of emission sources is investigated. Using Guangzhou and Foshan as examples, we demonstrate that similar precursor-based O3 isopleths can be associated with significantly different emission source co-control strategies. In Guangzhou, controlling solvent use emissions was the most effective approach to reduce peak O3 levels. In Foshan, co-control of on-road mobile, solvent use, and fixed combustion sources with a ratio of 3:1:2 or 3:1:3 was best to effectively reduce the peak O3 levels below 145 ppbv. This study underscores the importance of using emission source-based O3 isopleths and isosurface diagrams to guide a precursor emission control strategy that can effectively reduce the peak O3 levels in a particular area.

Identification of close relationship between atmospheric oxidation and ozone formation regimes in a photochemically active region.

Understanding ozone (O3) formation regime is a prerequisite in formulating an effective O3 pollution control strategy. Photochemical indicator is a simple and direct method in identifying O3 formation regimes. Most used indicators are derived from observations, whereas the role of atmospheric oxidation is not in consideration, which is the core driver of O3 formation. Thus, it may impact accuracy in signaling O3 formation regimes. In this study, an advanced three-dimensional numerical modeling system was used to investigate the relationship between atmospheric oxidation and O3 formation regimes during a long-lasting O3 exceedance event in September 2017 over the Pearl River Delta (PRD) of China. We discovered a clear relationship between atmospheric oxidative capacity and O3 formation regime. Over eastern PRD, O3 formation was mainly in a NOx-limited regime when HO2/OH ratio was higher than 11, while in a VOC-limited regime when the ratio was lower than 9.5. Over central and western PRD, an HO2/OH ratio higher than 5 and lower than 2 was indicative of NOx-limited and VOC-limited regime, respectively. Physical contribution, including horizontal transport and vertical transport, may pose uncertainties on the indication of O3 formation regime by HO2/OH ratio. In comparison with other commonly used photochemical indicators, HO2/OH ratio had the best performance in differentiating O3 formation regimes. This study highlighted the necessities in using an atmospheric oxidative capacity-based indicator to infer O3 formation regime, and underscored the importance of characterizing behaviors of radicals to gain insight in atmospheric processes leading to O3 pollution over a photochemically active region.

Identification of long-term evolution of ozone sensitivity to precursors based on two-dimensional mutual verification.

Tropospheric ozone pollution has been continuously worsening in China during the past decade. Identification of long-term evolution of ozone sensitivity to precursors is essential to evaluating the impact of emission reduction measures on ozone pollution. Traditional observation-based model and 3-d numerical model are not suitable for analyzing long-term variation of ozone sensitivity to precursors. In this study, by transforming the conventional ozone isopleth plot into a VOCR isopleth plot in the functional space of NOx and ozone concentrations, we developed a novel approach to identify ozone sensitivity to precursors by simply using long-term monitoring data of ozone, NOx and temperature. This approach estimated ozone formation regimes (OFR) by ozone sensitivity to NOx and temperature separately, and the convergence of OFR serves as a way of mutual verification. We found that ozone formation was generally in the VOCR-limited or transitional regime in Shanghai, the largest metropolitan area in China. However, OFR was shifted to NOx-limited at Pudong station during 2017-19 due much to the stringent NOx emission control. OFR was also shifted to NOx-limited along with the increasing temperature. When temperature was over 30 °C, Shanghai was mostly in a NOx-limited OFR. This highlights that the NOx emission control measures need to be strengthened to reduce peak ozone levels more efficiently. Jinshan station exhibited a different trend with OFR shifted to VOCR-limited in 2017-19, which proved the effectiveness of VOCs emission control on petrochemical sector. However, OFR was shifted to NOx-limited when temperature was over 30 °C, suggesting more stringent VOCs emissions control should be targeted on days with higher temperature.

Synoptic condition-driven summertime ozone formation regime in Shanghai and the implication for dynamic ozone control strategies.

Regarding the continuous worsening of tropospheric ozone pollution, the scenario in Shanghai is a microcosm of the entire China. Understanding the ozone formation regimes (OFRs), their variations, and driving factors is a prerequisite for formulating effective ozone control strategies. Traditional OFR estimation by numerical model, which often involves sensitivity analysis on at least tens of scenarios, is labor-intensive and time-consuming; therefore, it is not appropriate to make OFR forecasts to guide ozone contingency control. In this study, by using a localized modeling system consisting of the Weather Research and Forecasting, Sparse Matrix Operator Kernel Emissions, and Community Multiscale Air Quality models and considering the latest emission inventory over the Yangtze River Delta of China, we discovered a strong connection between the variations of large-scale circulation (LSC) and OFRs over Shanghai in July 2017, thereby providing an alternative way to infer OFR. During the northward movement of Western Pacific Subtropical High from South China Sea, the wind field over Shanghai changed from weak westerly to moderate southwesterly and to one without a distinct direction. The local OFR shifted from anthropogenic volatile organic compounds (AVOCs)-limited to NOx-limited and ultimately to the transitional regime. Such a variation in OFR is essentially driven by the spatial heterogeneity of NOx and AVOC emissions in different directions of Shanghai, brought on by the wind under different LSC patterns. With the existing weather forecasting technology, the LSC patterns can be well-predicted 48-72 h in advance. Hence, we propose the adoption of a dynamic ozone control strategy for Shanghai with the priority control target on AVOC or NOx emission sources adjusted according to the LSC pattern and OFR forecasts in a forthcoming O3 pollution episode. This would serve to maximize the peak ozone reduction under varying pollution conditions.