Abstract
Abstract. As one common precursor for both PM2.5 and O3 pollution, NOx gains great attention because its controls can be beneficial for reducing both PM2.5 and O3. However, the effectiveness of NOx controls for reducing PM2.5 and O3 are largely influenced by the ambient levels of NH3 and VOC, exhibiting strong nonlinearities characterized as NH3-limited/NH3-poor and NOx-/VOC-limited conditions, respectively. Quantification of such nonlinearities is a prerequisite for making suitable policy decisions but limitations of existing methods were recognized. In this study, a new method was developed by fitting multiple simulations of a chemical transport model (i.e., Community Multiscale Air Quality Modeling System, CMAQ) with a set of polynomial functions (denoted as “pf-RSM”) to quantify responses of ambient PM2.5 and O3 concentrations to changes in precursor emissions. The accuracy of the pf-RSM is carefully examined to meet the criteria of a mean normalized error within 2 % and a maximal normalized error within 10 % by using 40 training samples with marginal processing. An advantage of the pf-RSM method is that the nonlinearity in PM2.5 and O3 responses to precursor emission changes can be characterized by quantitative indicators, including (1) a peak ratio (denoted as PR) representing VOC-limited or NOx-limited conditions, (2) a suggested ratio of VOC reduction to NOx reduction to avoid increasing O3 under VOC-limited conditions, (3) a flex ratio (denoted as FR) representing NH3-poor or NH3-rich conditions, and (4) enhanced benefits in PM2.5 reductions from simultaneous reduction of NH3 with the same reduction rate of NOx. A case study in the Beijing–Tianjin–Hebei region suggested that most urban areas present strong VOC-limited conditions with a PR from 0.4 to 0.8 in July, implying that the NOx emission reduction rate needs to be greater than 20–60 % to pass the transition from VOC-limited to NOx-limited conditions. A simultaneous VOC control (the ratio of VOC reduction to NOx reduction is about 0.5–1.2) can avoid increasing O3 during the transition. For PM2.5, most urban areas present strong NH3-rich conditions with a PR from 0.75 to 0.95, implying that NH3 is sufficiently abundant to neutralize extra nitric acid produced by an additional 5–35 % of NOx emissions. Enhanced benefits in PM2.5 reductions from simultaneous reduction of NH3 were estimated to be 0.04–0.15 µg m−3 PM2.5 per 1 % reduction of NH3 along with NOx, with greater benefits in July when the NH3-rich conditions are not as strong as in January. Thus, the newly developed pf-RSM model has successfully quantified the enhanced effectiveness of NOx control, and simultaneous reduction of VOC and NH3 with NOx can assure the control effectiveness of PM2.5 and O3.
Highlights
Tropospheric ozone (O3) and fine particulate matter (PM2.5) are two major air pollutants that exert significant effects on human health (Forouzanfar et al, 2015; GBD-MAPS, 2016; Cohen et al, 2017) and the global climate (Myhre et al, 2013)
The recent observation data suggested a continued increasing trend of 8 h maxima O3 in Zhuhai and Shenzhen in the Pearl River Delta from 2013 to 2016. Such an increase in O3 is likely to be associated with the NOx reductions in the area that are located in the volatile organic compound (VOC)-limited conditions, implying the disbenefit of NOx controls for O3 reduction under VOC-limited conditions
This study proposed a new method by fitting multiple simulations of a chemistry–transport models (CTMs) with a set of polynomial functions, called “pfRSM”
Summary
Tropospheric ozone (O3) and fine particulate matter (PM2.5) are two major air pollutants that exert significant effects on human health (Forouzanfar et al, 2015; GBD-MAPS, 2016; Cohen et al, 2017) and the global climate (Myhre et al, 2013). The reason might be associated with the increases in NH3, which has not been well controlled to date in China and exhibits an increasing trend of nearly 20 % from 2011 to 2014 observed from satellite retrievals (Fu et al, 2017) Such increases in NH3 weakened the control effectiveness of SO2 and NO2 in PM2.5 reduction (Wang et al, 2011; Fu et al, 2017). The recent observation data suggested a continued increasing trend of 8 h maxima O3 in Zhuhai (from 128 to 142 μg m−3) and Shenzhen (from 122 to 134 μg m−3) in the Pearl River Delta from 2013 to 2016 Such an increase in O3 is likely to be associated with the NOx reductions in the area that are located in the volatile organic compound (VOC)-limited conditions (i.e., decreased NOx leads to increased O3), implying the disbenefit of NOx controls for O3 reduction under VOC-limited conditions. How to assure the effectiveness of NOx controls for reducing O3 and PM2.5 becomes a difficult challenge for policy design (Cohan et al, 2005; Tsimpidi et al, 2008)
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