Diurnal regulation of VOCs may not be effective in controlling ozone pollution in China

Highlights

• VOCs diurnal regulation may not be effective in controlling O₃ at the regional scale.

• VOCs diurnal regulation had minor influence on hourly peak O₃ concentration.

• Positive or negative impacts on O₃ concentrations were both found at the city level.

Abstract

Over the past few years, marked increases in ground-level ozone (O₃) concentration has been detected over much of eastern China. These high levels of O₃ pollution have been demonstrated to be detrimental to human health and plant growth, which requires urgent control on China's O₃ pollution. Among the various control measures, diurnal regulations of VOCs were adopted in some regions in China. For example, several local governments encourage factories with notable VOCs emissions shift their manufacturing processes to the nighttime; some governments also offer discount for night refueling. However, the effects of this diurnal regulation approach are still unclear. To evaluate the effectiveness of this measure, we proposed a quantitative assessment using WRF-CAMx air quality model. The assessment was systematically implemented at multiple time and spatial scales. Results at the regional scale revealed that the diurnal regulation of VOCs could slightly redistribute hourly O₃ concentrations and had minor influence on hourly peak O₃ concentration. The effects of this measure on hourly O₃ concentrations varied substantially across different cities: positive or negative impacts on O₃ concentrations were both found at the city level. At the daily scale, the variations of maximum daily 8-h average (MDA8) O₃ concentrations were distributed between −1% and 1%, accounting for about 88%, 76%, 65% of the entire days in key regions under S1, S2, S3; the average O₃-polluted days demonstrated an increasing trend in “2 + 26″cities and YRD, but for FWP and PRD, there were little changes in average number of O₃-polluted days. And at the monthly scale, the effects were not satisfactory, monthly average MDA8 O₃ demonstrated an increasing trend under VOCs regulation scenarios in “2 + 26” cities, FWP and YRD; Increasing or decreasing trends in monthly average MDA8 O₃ were both found at the city level, but most of the variations were less than 1%. In general, Diurnal regulation of VOCs may not be effective in controlling ozone pollution at the regional scale. Measures proposed to control O₃ pollution need more refined management and scientific evaluation before wide implementation.

Introduction

Rapid economic growth and associated emission increase in China have led to severe air pollution in recent decades (Zhang et al., 2019). From the National Air Pollution Prevention and Control Action Plan, to the Three-Year Action Plan to Win the Battle for Blue Skies (Ministry of Ecology and Environment; MEE, 2020), China has made great efforts to reduce severe pollution (Feng et al., 2019), resulting in significant reduction of fine particulate matter (PM_2.5) nationwide. However, O₃ situation in China continued to worsen in the past few years, the 90th percentile of maximum daily 8-h average (MDA8) O₃ concentration has increased by 20% continuously nationwide, from 123 μg/m³ in 2015 to 148 μg/m³ in 2019, with increased O₃ concentrations in the Beijing-Tianjin-Hebei (BTH) area and its surrounding regions (i.e., “2 + 26 cities”), Fen-Wei Plain (FWP), Yangtze River Delta region (YRD), Pearl River Delta region (PRD), of 36%, 40%, 27%, 26%,respectively (MEE, 2016; MEE, 2020; see map of these regions in Fig. 1). The O₃ has become a dominant pollutant during summertime in China. As a strong atmospheric oxidant, surface O₃ can be harmful to human health and plant growth. Recent reports from the Health Effects Institute (2017) highlight that ambient ozone contributes to the global health burden through its impact on premature deaths and disabilities from chronic obstructive pulmonary disease. In addition, as one of the six conventional pollutants (PM_2.5, PM₁₀, SO₂, NO₂, O₃, and CO), O₃ pollution are closely related with air quality index (AQI) in China, AQI is a dimensionless index that quantitatively describes air quality conditions based on the standards of the “Technical Regulation of Ambient Air Quality Index” released in 2012, the rate of good and moderate AQI was one of the binding control indicators during China's 13th Five-Year Plan. At present, air pollution control has become one of the top priorities of China's “Ecological Civilization”, great attentions from the public and governments have been paid to the trend of increased ozone pollution in China's environmental governance. Thus, how to slow down or even reverse the trend of O₃ accumulation has been a scientific and political question.

O₃ is secondarily formed in photochemical reactions of nitrogen oxides (NOx) and volatile organic compounds (VOCs) with the existence of sunlight, it is well known that the photochemical formation of O₃ is non-linearly dependent on its precursors NOx and VOCs (Xue et al., 2014). O₃ sensitivity to anthropogenic emissions depends on the photochemical regime for ozone formation (Li et al., 2018), In some cases, O₃ formation is controlled almost entirely by NOx and is largely independent of the amount of VOCs (NOx-sensitive), while it increases with increasing VOCs (VOCs-sensitive) in other cases. In urban polluted atmosphere, high O₃ levels are usually driven by the oxidation of high concentrations of VOCs (Wang et al., 2008). VOCs are crucial precursors in the formation of O₃ (Yuan et al., 2013; Zhao et al., 2013), especially in VOC-sensitive urban areas (Tang et al., 2012; Ding et al., 2013; Han et al., 2013; Lam et al., 2013). In addition, VOCs originate from both anthropogenic and biogenic sources, biogenic and anthropogenic VOCs in ambient air affect the air quality, mainly contributing to form the O₃ and the secondary organic aerosol (Wu and Xie, 2018). Moreover, some VOCs species and its oxidation products have adverse effects on human health (Garg and Gupta, 2019). So it is essential to control VOCs emissions.

In order to control severe O₃ pollution in summertime, many provinces and cities in China actively explore O₃ control strategies. Among the various control measures, diurnal regulations of VOCs had been adopted in some regions. For example, several local governments encourage factories with notable VOCs emissions to shift their manufacturing processes to the nighttime; some governments also offer discount for night refueling; the Department of Ecology and Environment of Shandong Province issued the notice on strengthening the management of VOCs emission reduction at gas stations, which required scientific arrangements of oil unloading time and introducing preferential policies for shifting peak refueling. Meanwhile, VOCs diurnal regulation became a hot topic and were copied by many cities, however, the effects of this diurnal regulation approach are still unclear. The greatest challenge of O₃ control is rooted in the complexity of both photochemistry formation and meteorological dependence (Li et al., 2019; Lu et al., 2020). The nonlinear relationships between O₃ and its precursors indicate VOCs reduction may increase O₃ concentrations in some cities. The establishment of an effective control policy for urban O₃ pollution requires a thorough understanding of the O₃-precursor relationships, local versus regional contributions, and the effects of isolated urban emission and regional biogenic sources. Up to now, there is still lack of assessment on the effects of VOCs diurnal regulation quantitatively.

Taking the above into consideration, the main objectives of this study are to (1) quantitatively evaluate the effectiveness of VOCs diurnal regulation on O₃ improvement; (2) analyze the possible effects of the measure on O₃ pollution prevention and control at the regional and city level. The study also identifies the key point of next steps for O₃ pollution control, which will be beneficial for future policymaking.