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Many efforts have been made to control PM 2.5 pollution in China during the National 13th Five-Year Plan and PM 2.5 concentrations significantly declined nationwide. Meteorological conditions and pollution emissions play the most important roles in PM 2.5 pollution. However, extant quantitative estimations of the contributions of meteorological conditions and anthropogenic emission reductions during the period were mostly based on surface observations data. Using a reanalysis dataset from 2016 to 2020, our results reveal that the annual average PM 2.5 concentrations have been reduced yearly in the four key regions: the Beijing-Tianjin-Hebei (BTH) region, Yangtze River Delta (YRD), Pearl River Delta (PRD) and central China. The frequency of stagnation days decreased in 2020 in the YRD and PRD, while it did not show significant fluctuations in the BTH region and central China. Through the sensitivity simulation experiments in 2016 and 2020, the implementation of the emission regulations led to PM 2.5 pollution reductions of 6.59 and 5.12 μg m−3, respectively in the BTH region and central China, of which the worsening meteorological conditions offset 61.46% (4.05 μg m−3) and 19.53% (1.00 μg m−3) respectively. However, the impacts of favorable weather conditions and effective anthropogenic restrictions jointly contributed to the improvement in air quality in the YRD and PRD. Anthropogenic controls explained 81.95% (3.75 μg m−3) of the total 4.58 μg m−3 reduction in the YRD and 52.10% (1.86 μg m−3) of the total 3.57 μg m−3 reduction in the PRD. The findings provide a perspective on the causes of pollution events and would help formulate more effective emission reduction policies in the future. • PM 2.5 concentrations decreased across China from 2016 to 2020. • The dominant factor of the improvement in PM 2.5 air quality is the reduction in anthropogenic emissions. • Meteorological changes offset emission control in the BTH region and central China. [ABSTRACT FROM AUTHOR]

Chemistry-transport models are increasingly used in Europe for estimating air quality or forecasting changes in pollution levels. But with this increased use of modeling arises the need of harmonizing the methodologies to determine the quality of air quality model applications. This is complex for planning applications, i.e. when models are used to assess the impact of realistic or virtual emission scenarios. In this work, the methodology based on the calculation of potencies proposed by Thunis and Clappier (2014) to analyze the model responses to emission reductions is applied on three different domains in Europe (Po valley, Southern Poland and Flanders). This methodology is further elaborated to facilitate the inter-comparison process and bring in a single diagram the possibility of differentiating long-term from short-term effects. This methodology is designed for model users to interpret their model results but also for policy-makers to help them defining intervention priorities. The methodology is applied to both daily PM 10 and 8 h daily maximum ozone. [ABSTRACT FROM AUTHOR]

Air quality in cities has been extensively studied due to the high population density potentially exposed to high levels of pollutants. The main problems in urban areas have been related to particulate matter (PM) and NO 2 . Less attention has been directed towards O 3 because urban levels are generally lower than those recorded in rural areas. The implementation of air quality plans, together with technological improvements, have resulted in reductions of PM and NO 2 levels in many European cities. In contrast, urban O 3 levels have experimented increases which may respond to declining NO x emission trends. It is therefore necessary to intensify the study of urban O 3 and its potential relation with NO x variations. In the agglomeration of Zaragoza (NE Spain), traffic circulation through the centre has dropped by 28.3% since 2008 due to several factors such as the implementation of a mobility plan, the completion of major construction projects and the economic crisis in Spain. The study of this case offers a unique opportunity to evaluate the impact of reductions in NO x emissions on the levels of O 3 in a characteristic Mediterranean city. This work analyses the variability and trends of ambient air levels of O 3 and NO x in Zaragoza and the Ebro valley from 2007 to 2012. Results demonstrate that, although the main factor explaining O 3 variability is still linked to meteorology, changes in NO x emissions strongly influence O 3 variability and trends, mainly due to interaction with fresh NO. Specific analysis of the O 3 “weekend effect” show a significant correlation ( r 2 = 0.81) between the drop of NO concentrations (associated to emissions) and the increment of O 3 levels during weekends. Moreover, trend analyses reveal that the decline in NO x emissions in Zaragoza from 2007 to 2012 can be associated with significant increments in O 3 levels. [ABSTRACT FROM AUTHOR]

Abstract: This study is an attempt to quantify the relation between changes in NO x emissions and nitric acid (HNO3) in the northeastern USA. From this relation, and previous work relating NO x emission changes and wet NO3 − deposition, we can estimate how changing NO x emissions may impact total (wet+dry) measured nitrogen (N) deposition. Electric utility emissions account for , and vehicle emissions account for over of the total NO x emissions in the eastern USA. Canadian NO x emissions from the seven easternmost provinces (Manitoba and east) represent less than 10% (1.2 teragrams (Tg) NO x ) of the NO x emissions compared with those from the eastern USA. Emissions from eastern Canada are dominated by vehicle NO x emissions, which account for of the total NO x emissions from eastern Canada. Data from the EPA National Emissions Inventory show, for the period 1991–2001, that nitrogen oxide (NO x ) emissions in the eastern USA have declined by 17% to from 16.1 to 13.1Tg. Large declines in vehicle emissions in 2001 may be questionable. If 2001 data are excluded the decline in total NO x is only 7%. A recent assessment of EPA''s emissions estimates suggest that vehicle NO x emissions may be underestimated, and total NO x emissions reductions may be less than what is reported by the EPA. The CASTNet (Clean Air Status and Trends Network) measurements of N dry deposition include HNO3, particulate NO3 − and NH4 +. The dominant N dry deposition product measured is HNO3, which represents 80% of measured N dry deposition for the sites used in this study. Amounts of NH3, NO2, organic nitrate and PAN dry deposition are not measured by CASTNet. The NH3 and NO2 deposition are probably significant, and may be major N dry deposition components in some areas. Random coefficient models with total NO x emissions as the independent variable, and HNO3 concentrations as the dependent variable, show that reducing total NO x emissions by 50% should reduce HNO3 concentrations by 36%. The average efficiency (the ratio of % change in HNO3 to % change in NO x emissions) is 72%. Random coefficient models with non-vehicle NO x emissions as the independent variable, and HNO3 concentrations as the dependent variable, show a 50% decline in non-vehicle NO x emissions (which is a 23% decline in total NO x emissions) should reduce HNO3 by 17–20%. The average efficiency in this case is 81%. Because non-vehicle NO x emissions data are more reliable than vehicle NO x emissions, non-vehicle NO x models are likely more accurate than the total NO x models. Combining the results of this study with previous work, which examined the relation between NO x emissions and wet NO3 concentrations, show that reducing total NO x emissions by 50% should reduce total NO3 − deposition by 37% (wet+dry combined efficiency is 74%), and total N deposition (as measured by CASTNet sites in the northeastern USA) by 25%. A decline in total NO x emissions of 23%, from a 50% reduction in non-vehicle NO x emissions should, on average, reduce total NO3 − deposition by 20% (wet+dry combined efficiency is 87%), and total N deposition, as measured by CASTNet sites, by 15%. [Copyright &y& Elsevier]

Hungarian air pollution levels are studied by using the Danish Eulerian Model. The air pollution levels under consideration include (i) concentrations of SO2, NO2, NH3 and O3 and (ii) four quantities related to critical ozone levels (accumulated exposure over threshold of 40 ppb values for crops and forests, the number of days when 8-h rolling averages of ozone concentrations exceed 60 ppb, and averaged over a given period daily maxima of the ozone concentrations). Setting all the Hungarian emissions to zero, we estimate the contributions from the SO2, NOx, VOC and NH3 emissions in the other European countries to the Hungarian pollution levels. An attempt to evaluate the Hungarian pollution levels for year 2010 is made under the assumption that the European emissions will, in general, be reduced considerably before year 2010. [Copyright &y& Elsevier]

The surface ozone (O 3) spatiotemporal distribution, variations and its causes in Beijing in 2014–2020 are revealed by quantitative estimates of the trends of 12 ozone metrics relevant to human health and tropospheric NO 2 and formaldehyde (HCHO) column concentration, based on the air quality monitoring network data and satellite retrievals from the Ozone Monitoring Instrument (OMI). Results showed that the maximum O 3 concentrations decreased slightly since 2014. The annual 90th percentile of the daily maximum 8-h average (MDA8) O 3 and the "ozone-season" (April–September) MDA8 decreased by 0.32 and 0.19 μg m−3 year−1, respectively. The changes in Beijing contrast the widespread and significant O 3 increases in the Beijing–Tianjin–Hebei region in recent years. However, significant positive trends in MDA8-90th and ozone-season MDA8 were observed in 7% and 14% of monitoring sites in Beijing (p < 0.05), most of which are located in urban areas and suburbs close to urban areas. The spatial distribution of high O 3 values in Beijing has changed significantly. The areas with high O 3 concentrations shifted from the northern suburbs (Changping and Miyun districts) to the northeastern and southwestern suburbs and the east of the central urban area. The ozone-season NO 2 and HCHO tropospheric columns showed reductions of 4.2% and 0.4% year−1, respectively, from 2014 to 2019 in Beijing, which suggested that the decrease in NO X emissions was much greater than the decrease in volatile organic compound (VOC) emissions. Such an extremely inappropriate control ratio of ozone precursor NO X /VOCs led to an overall trend of slow declining fluctuations of O 3 and an increasing trend of individual sites in Beijing. Obviously, the reduction of VOCs in Beijing is far from sufficient, and the significant decrease of Beijing's O 3 concentration can only be achieved by a drastically reduction in VOC emissions. [Display omitted] • The high O 3 concentrations decreased slightly during 2014–2020 in Beijing. • The spatial distribution of high O 3 values in Beijing has changed. • Changes in O 3 precursors emissions were uncovered by satellite data. • The O 3 increase in individual sites were due to an improper NO X /VOCs control ratio. • The reduction of VOCs should be greater than the reduction of NO X in the near future. [ABSTRACT FROM AUTHOR]