Quantification of the enhanced effectiveness of NO<sub><i>x</i></sub> control from simultaneous reductions of VOC and NH<sub>3</sub> for reducing air pollution in the Beijing–Tianjin–Hebei region, China

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.

NH<sub>3</sub>-promoted hydrolysis of NO<sub>2</sub> induces explosive growth in HONO

“…The NH 3 concentration begins to increase in the morning (at approximately 08:00 LT) then reaches a plateau in the afternoon (8.5 to 15.5 ppb on average); the SO 2 concentrations show a similar diurnal variation to that of NH 3 . This type of diurnal variation for SO 2 was also found by Xu et al (2014); however, the cause of the common diurnal pattern between NH 3 and SO 2 during this field campaign requires further investigation. The NO * 2 concentration increases quickly in the afternoon and decreases in the evening.…”

Causes of a continuous summertime O<sub>3</sub> pollution event in Jinan, a central city in the North China Plain

“…The average overestimation of NO 2 was 22 % in Mexico City, which even increased to 50 % in the afternoon (Dunlea et al, 2007). Xu et al (2013) suggested that the chemiluminescence monitors overestimated NO 2 by less than 10 % in urban areas with fresh emission of NO x , but the positive bias went up to 30 %-50 % at the suburban sites. As described in Sect.…”

“…NO 2 was calculated from the difference between NO and NO x . Studies indicated that NO 2 monitored with chemiluminescence was generally overestimated due to the conversion of the total odd nitrogen (NO y ) to NO by molybdenum oxide catalysts (McClenny et al, 2002;Dunlea et al, 2007;Xu et al, 2013). The positive bias was more significant in more aged air masses, resulting from higher levels of NO z (NO z =NO y − NO x ) (Dunlea et al, 2007).…”

The acidity of atmospheric particles and clouds

“…Overall, the eastern US aerosol is predicted to be one of the most acidic locations, with average pH F near a value of 1 (Battaglia et al, 2017;Weber et al, 2016;Pye et al, 2018;Xu et al, 2015; and higher pH F , by about 1 unit, observed in locations of intensive agriculture with high NH 3 concentrations (Nah et al, 2018) and those influenced by larger particles . Higher aerosol pH (2-3 in pH F ) was estimated for Los Angeles in summer (Guo et al, 2017b), similar to observationally derived values for the eastern Mediterranean (0.5-2.8 pH F , Bougiatioti et al, 2016).…”

“…The studies employing proxies likely suffered from problems with the proxies; themselves (as discussed above); and confounding factors such as correlations between organic aerosol and sulfate, a major source of acidity, that often occur in regional pollution Nguyen et al, 2015). These seemingly contradictory results were resolved once pH F was used, and the conclusions reached in these prior studies have been revisited based upon detailed understanding of the underlying chemical mechanisms and additional insight suggesting that the aerosol acidity is frequently not a limiting factor in catalyzing SOA formation Xu et al, 2015;Weber et al, 2016). Acid-catalyzed isoprene SOA has now been implemented in a wide variety of box model and chemical transport model applications that now rely exclusively on thermodynamic models for acidity estimates Marais et al, 2016;Riedel et al, 2016;Budisulistiorini et al, 2017).…”