Long-term trends in nitrogen oxides at different types of monitoring stations in the Czech Republic

Journal Pre-proof Long-term trends in nitrogen oxides at different types of monitoring stations in the Czech Republic

Iva Hůnová, Vít Bäumelt, Miloslav Modlík PII:

S0048-9697(19)34369-4

DOI:

https://doi.org/10.1016/j.scitotenv.2019.134378

Reference:

STOTEN 134378

To appear in:

Science of the Total Environment

Received date:

14 June 2019

Revised date:

16 August 2019

Accepted date:

8 September 2019

Please cite this article as: I. Hůnová, V. Bäumelt and M. Modlík, Long-term trends in nitrogen oxides at different types of monitoring stations in the Czech Republic, Science of the Total Environment (2019), https://doi.org/10.1016/j.scitotenv.2019.134378

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© 2019 Published by Elsevier.

Journal Pre-proof Long-term trends in nitrogen oxides at different types of monitoring stations in the Czech Republic Iva Hůnová, Vít Bäumelt, Miloslav Modlík

Czech Hydrometeorological Institute, Na Sabatce 17, 13406 Prague 4 - Komorany, Czech Republic

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Corresponding author: Iva Hůnová, [email protected], ++420244032426

Abstract

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Nitrogen oxides (NOx) are important in atmospheric chemistry and have substantial

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environmental impacts. This study provides a detailed data analysis on long-term changes in ambient NOx levels within the framework of their emission pattern. We examined the trends in NOx, NO, NO2

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and NO2/NOx ratio at 39 Czech sites representing different environments (urban, rural, mountain, industrial) in 1994–2016, i.e. a 23-year time series, using the non-parametric Man-Kendall test. The

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ambient air concentrations in NO, NO2 and NOx decreased significantly at most of the sites, as was assumed due to the substantial NOx emission decrease over the period under review. The largest decrease per year was detected at the urban site in capital Prague (decrease in the 98th percentile

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equal to 2.28 ppb, decrease in the annual median equal to 1.00 ppb). At some sites, however we observed a substantial equivocal temporal change in the NO2/NOx ratio. Whereas at some sites the NO2/NOx ratio was decreasing, at other sites the trend was increasing significantly. The highest increase per year in the annual median of the NO2/NOx ratio, equal to 0.0086 was detected at the urban Praha 1-nám-Rep. site; the highest increase per year in the 98th percentile, equal to 0.0126 was recorded at the rural Krupka site. Most sites (regardless of type) with generally increasing trends in the NO2/NOx ratio), are situated in the north-west portion of the Czech Republic The increasing NO2/NOx ratio detected at some sites is believed to implicate undesired changes in atmospheric chemistry, namely with respect to ambient O3 formation, promoting increasing O3 concentrations.

Keywords: nitrogen oxides, time trends, NO2/NOx ratio, emissions, 1994–2016

1. Introduction

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Journal Pre-proof Nitrogen oxides (further NOx), i.e. a mixture of nitrogen oxide (NO) and nitrogen dioxide (NO2) as denoted in atmospheric chemistry, play an important role in the atmosphere and have strong environmental impacts. Ambient NOx, per se, are toxic both for human health (NO2 in particular) and ecosystems (WHO, 1987, 2006; Chossière et al., 2017) and therefore they are criteria pollutants with limit values set up for human health and ecosystem protection (EC, 2008). For the protection of human health, European legislation (EC, 2008) requires the ambient levels to adhere to the NO2 annual limit value of 40 µg.m-3 and the NO2 hourly limit value of 200 µg.m-3, not to be exceeded on more than 18 occasions per year. For vegetation and ecosystem protection, an NOx annual limit value of 30 µg.m-3 is to be attained. The highest concentrations as well as limit violations are observed at

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traffic sites. In European conditions, road traffic is by far the major source of NOx which are emitted close to the ground, directly into the ‛breathing zone’, thus contributing relatively more to NOx

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ambient levels than high industrial and power plant stacks (EEA, 2016). An array of NOx abatement

extents worldwide (Skalska et al., 2010).

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techniques for anthropogenic NOx emissions decrease is available, though applied to different

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As fairly reactive compounds, NOx are involved in numerous atmospheric reactions (Seinfeld, Pandis, 1998; Hertel, 2011). In the ambient ozone (O3) chemistry NOx behaviour is ambivalent: NO2

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acts as a precursor of O3, whereas NO decrease O3 concentrations via a titration reaction (Sillman, 2002). NOx are important contributors to the dry deposition of reactive nitrogen, Nr (Wesely and

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Hicks, 2000). Moreover, NO2 is a precursor for NO3- particles (Lonsdale et al., 2012), which form an important fraction of PM2.5 mass and thus add to human ambient air exposure in cities worldwide (e.g. Cheng et al., 2016). Furthermore, NO3- particles are deliquescent and may perform (apart from

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SO42-) as cloud or fog condensation nuclei (Swietlicki et al., 2008). NO3- in precipitation contributes to pH decrease and acidification and eutrophication of the environment (e.g. Vet et al., 2014). Though reduced nitrogen (in the form of NH4+) exhibits an increasing trend in atmospheric wet deposition in many regions (e.g. Li et al., 2016, Hůnová et al., 2017), NO3- still contribute significantly to wet deposition, in spite of substantial NOx emission reductions both in Europe and North America (EMEP, 2017). NO3- particles play a role in the radiative and absorption properties of the atmosphere and thus contribute to climate change. Furthermore, NOx influence the climate in a very complex way, through a variety of opposing influences, such as NOx involvement in the atmospheric chemistry of methane CH4 and O3 (von Schneidemesser et al., 2015). In the stratosphere, NOx (together with halogenated hydrocarbons) are responsible for stratospheric O3 destruction (Sagi et al., 2017). Though in a strictly chemical sense other nitrogen oxides exist as well, such as nitrogen monoxide (N2O), an important greenhouse gas; liquid N2O3 or solid N2O5, it makes sense to distinguish the NOx group including just these two - NO and NO2 - in air pollution chemistry, as these two have similar properties (characteristics), similar lifetimes and originate in a single process of burning. In contrast

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Journal Pre-proof to SO2 emitted in the burning of fossil fuels containing sulphur, NOx are always emitted in a burning process, regardless of the fact whether nitrogen (N) is incorporated in the fuel or not, as N is plentiful in the air. According to their origin, we then distinguish between ʽfuel NOx’ and ʽthermal NOx’ (Hobbs, 2000). NO is a product of incomplete combustion, whereas NO2 is emitted via high-temperature incineration. NOx emitted from combustion processes tends to be emitted in the majority as NO (ca 95%) with only a small portion of primary NO2 (von Schneidemesser et al., 2015). In addition, NO2 is also produced in the air by the oxidation of NO through oxidation agents, such as O3 or OH· radical (Seinfeld and Pandis, 1998). Furthermore, NOx are emitted by natural sources, including soil processes driven by microbial activity, and lightning (Fowler et al., 2009). As compared to

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anthropogenic sources, however the natural sources are of lesser importance. On a global scale, the contribution of natural sources to the NOx burden is estimated to be less than 30% (Delmas et al.,

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1997). The lifetime of NO2 derived through field measurements varies in boundary layer from several hours to 1–2 days, depending on the atmospheric conditions (Warneck, 2000). The atmospheric

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mixing ratios of NOx are usually < 1 ppb in remote marine or forest areas, < 1 – 10 ppb in rural areas

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and 10 – 1,000 ppb in urban areas (von Schneidemesser et al., 2015). Recently, studies focused on changing the NO2/NOx ratio in ambient air have been published, revealing the changes in primary NO2

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emissions and implications for the regional photochemistry of secondary pollutants, including ozone (Carslaw and Carslaw, 2001; Monks et al., 2009; Kurtenbach et al, 2012; He et al., 2015).

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In the Czech Republic ambient NOx concentrations are measured regularly within a national monitoring network (CHMI, 2018) both as a criteria pollutant and as a substantial part of dry nitrogen

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deposition (Hůnová et al., 2016, 2017). In order to achieve permissible concentrations of NOx in the atmosphere, the Czech Republic has implemented an air quality management system applying a number of instruments (legislative, economic, informational etc.) at various levels. In this way, the Czech Republic met the emission ceilings under Directive 2001/81/EC for SO2, NOx, VOC and NH3 emissions by 2010 and is currently introducing further measures to meet the 2020 and 2030 commitments (European Parliament and Council Directive [EU] 2016/2284). Primary measures, such as adjusting and controlling air/fuel ratios and fuel change, are no longer sufficient to meet the NOx emission limit values. Due to the tightening of NOx emission limits as a result of Industrial Emissions Directive 2010/75/EU, it was necessary to introduce secondary measures to reduce NOx emissions based on the chemical removal of NOx from flue gas (e.g. selective non-catalytic reduction method) for many stationary sources (Muzio et al., 2002). The emission limits for NOx in flue gasoline and diesel engines for mobile sources are prescribed by the EURO standards, which are issued in gradually tightened numbered versions (EURO I-VI). The application of some denitrification methods may result in a change in the NO2/NOx ratio in flue gas (Rößler et al., 2017) or the formation of undesirable pollutants (Javed et al., 2007).

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Journal Pre-proof The aim of this paper is to analyse the trends in ambient NO, NO2 and NOx concentrations at long-term monitoring sites representing different environments in a Central European country. Furthermore, the ratio of NO2/NOx at different types of sites and its changes over time are examined. We assumed that there are significant time trends in ambient NO, NO2 and NOx concentrations and NO2/NOx due to emission reduction and to changes in the relative contribution of different emission sectors.

2. Methods 2.1. Sites and period under review

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We performed our analysis for selected measuring sites in the Czech Republic, a country with a long history of air pollution affecting both human society and the environment adversely (Moldan

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and Schnoor, 1992; Mejstřík, 1993; Zimmermann et al., 2003; Frantál and Nováková, 2014), where the ambient air pollution has improved considerably but still is far from satisfactory, in particular

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with regard to particulate matter, ozone and benzo(a)pyrene (CHMI, 2017; EEA, 2016).

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From all sites monitoring NOx, we selected these meeting the criteria of (1) measuring NO, NO2, and NOx simultaneously, (2) having long-term records and (3) having minimum failures and missing

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values throughout the period under review. These sites were categorised into three distinctive groups according to the environments they represent: urban, rural and mountain. We aimed to have

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approximately even representation in all site categories. Finally we arrived at 16 urban sites (including suburban and urban sites), 12 rural sites and 11 mountain sites. We opted for this simplified categorisation into three groups for practical reasons, to address the

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overall situation in NOx levels in the country. We are aware, however that there might be substantial differences in NOx behaviour at various sites even within the selected categories, namely urban sites, in particular with respect to traffic sites (Pandey et al., 2008; Cyrys et al., 2012; Mavroidis and Ilia, 2012). Hence, we carried out the analysis of trends in detail for individual sites (subchapter 2.3), but for practical reasons, to show the overall trends, we also plotted trends averaged for individual station categories, i.e. urban, rural and mountain. All the sites under study were included in the nation-wide ambient air quality monitoring network. Most of them (36) were operated by the Czech Hydrometeorological Institute; a few (3) were operated by Czech Power Company CEZ (2) and Public Health Service (1). The sites used for the analysis are presented in Table 1.

2.2. Ambient NOx data

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Journal Pre-proof We used NO, NO2 and NOx data measured continuously, year-round, within the national ambient air quality monitoring network by chemiluminiscence, the EC reference method (EC, 2008). The analysers used were Thermo Environmental Instruments (TEI), M42 until March 2015 and Teledyne Advanced Pollution Instruments (TAPI), T200 since March 2015. The total combined expanded measurement uncertainty, including sampling, was less than 13% (CHMI, 2018 b). The samplers were set up within the breathing zone, some 2 metres above the ground. The equipment and procedures did not change over the entire time series. Standard quality assurance/quality control (QA/QC) procedures were applied (EC, 2008). The input data into our analysis were 1-hour mean concentrations, the basic values stored in the nation-wide ambient air pollution database, ISKO

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requirement as required by European legislation (EC, 2008).

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(CHMI, 2017). For computation of the annual characteristics we applied a 90% data coverage

2.3. Statistical analysis

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For trend analysis we used the non-parametric Mann-Kendall test, recommended by the WMO for this kind of data and used in similar studies (Hůnová and Bäumelt, 2018). We applied software

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prepared by the Finnish Meteorological Institute (Määttä et al., 2002). All trends were detected for

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the entire measuring period, which differed among individual sites. For our analysis we considered the 10th, 20th, 50th and 98th percentiles, summer and winter medians for NO, NO2, NOx and NO2/NOx ratio. We used medians, as they are more appropriate for

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data with non-normal distribution. This is the case for air pollution data generally, as they are usually left-skewed, with numerous low concentrations close to zero, blocked on the left, with a few very

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high concentrations (Larssen, 1971). Additionally, we used annual means, as annual means are set up as limit values for ambient air quality by legislation (EC, 2008). We also checked the limit value exceedances, i.e. the compliance with the NO2 annual limit value of 40 µg m-3 (21 ppb) for human health protection, the NOx annual limit value of 30 µg m-3 (15.7 ppb) for vegetation and ecosystem protection and the NO2 daily limit value of 200 µg m-3 (104.5 ppb) for human health protection (EC, 2008).

2.4. Emissions of NOx The NO2 emission inventory for the period 1990–2016 presented in the NFR (Nomenclature for Reporting Codes) format was compiled from the REZZO database, a Czech emission inventory database operated by the Czech Hydrometeorological Institute (CHMI, 2018 a), and the NOx emission inventory reported under CLRTAP on 31 August 2018. The main NFR groups of emission sources (EMEP/EEA, 2019) consist in combustion sources including transport (NFR 1), technological sources without combustion, termed process emissions (NFR 2), sources using solvents (NFR 2D), agricultural

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Journal Pre-proof activities including livestock farming (NFR 3B), and waste management (NFR 5). The NOx inventory methodology combines the direct collection of data reported by source operators with model calculations from data reported by source operators or determined in the context of statistical surveys (CHMI, 2018 a). The REZZO database contains NOx emissions data from stationary sources of air pollution from which NO2 emissions were calculated according to the technology-specific emission factors expressed as the NO2/NOx ratio. The NO2/NOx ratio in the combustion of fuels in boilers is about 5%, in combustion turbines up to 10% and in stationary combustion engines up to 15%. The NO2/NOx ratio in emissions from the production of nitric acid, fertilizer production and chemical production of explosives can be 100% (Neužil, 2012). The Transport Research Centre has

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estimated the NO2 emissions from road transport using the COPERT 5 methodology (Pelikán, 2017). The estimate of NO2 emissions from non-road transport (estimated based on the values for road

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transport) were carried out in the case of combustion of petrol with an NO2/NOx ratio of 4%, and in

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case of combustion of diesel with an NO2/NOx ratio of 12.5%.

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3. Results

3.1. Averaged NOx trends for different types of monitoring sites

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The overall situation in averaged trends in NOx, NO2 and NO for different types of sites, i.e. urban, rural and mountain is presented in Figs. 1–3. The urban sites exhibit much higher

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concentrations as compared to rural and mountain locations, the differences between the rural and mountain sites are smaller. As expected, the differences in ambient NOx, NO2 and NO concentrations between the different types of sites were largest for the 98th percentiles, whereas they were minute

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for the 10th percentiles. The year-to-year variability in the 98th percentiles in NOx, NO2 and NO is high for all types of sites. All types of measuring sites, as averaged per individual category, are well under the NO2 limit value (Fig. 4).

3.2. Detailed NOx trends at individual monitoring sites Our analysis showed that at most of the sites under review the concentrations of NO, NO2 and NOx were decreasing significantly over the period under review. The largest decrease per year in the annual median of NOx, equal to 1.00 ppb was detected at the urban Praha 1-nám-Rep. site (representing the very central area of the capital Prague); the largest decrease per year in the 98th percentile, equal to 2.28 ppb was recorded at the same site. Of the rural sites, the largest decrease per year in the annual median of NOx equal to 0.28 ppb and in the 98th percentile equal to 0.89 ppb was recorded at the Krupka site, whereas of the mountain sites it was 0.23 ppb and 0.97 ppb, respectively, at the Rudolice v Horách site. Both of these sites are situated in north-west portion of the Czech Republic. The decrease in NOx concentrations was generally more pronounced in winter

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Journal Pre-proof than in summer. With regard to NO, clearly the largest decrease was observed at the urban Praha 1nám-Rep. site (decrease per year in the 98th percentile equal to 1.98 ppb, decrease per year in the annual median equal to 0.63 ppb) with more pronounced decrease detected in winter (0.93 ppb per year) than in summer (0.54 ppb per year). With regard to NO2, the largest decrease was again detected at the urban Praha 1-nám-Rep. site (decrease per year in the 98th percentile equal to 0.58 ppb, decrease per year in the annual median equal to 0.35 ppb). In contrast to NO, for which the difference between this site and other sites was substantial, for NO2 some rural and mountain sites exhibited very similar decrease, such as Tušimice, Krupka, Souš and Rudolice v Horách (decrease per year in the 98th percentile equal to 0.54–0.57 ppb, decrease per year in the annual median equal to

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0.16–0.19 ppb). A detailed analysis of the trends at individual measuring sites is shown in Tables 2–4. The only

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exception is an increasing trend in NO at a few sites. This course generally reflects the decreasing trends in NOx emissions from Czech sources (both fixed and mobile), which were fairly steep until

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1998 - when new legislation with a useful tool of emission limits forced operators to modernise or

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close their industries, and have been much gentler recently (Fig. 5). A similar decrease in NOx emissions, which are also likely to impact Czech ambient air quality, is evident for the neighbouring

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countries of Germany, Austria, Poland and Slovakia (EMEP, 2017). With regard to the NO2/NOx ratio, the situation is completely different. In spite of a general

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decrease in NO, NO2 and NOx concentrations recorded at individual sites, we can see that the decrease of the individual NOx component is not uniform with the resulting increase in the NO2/NOx ratio at many sites (Table 5). Several sites exhibit clearly statistically significant increasing trends in

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the NO2/NOx ratio for most or even for all examined percentiles. These are four urban sites (Praha 1nám. Rep., Praha 4-Libuš, Ústí n.L.-Kočkov, and Sokolov), four rural sites (Frýdlant-Údolí, Valdek, Krupka, Sněžník) and two mountain sites (Rudolice v Horách and Přebuz). The highest increase per year in the annual median of NO2/NOx ratio equal to 0.0086 ppb was recorded at the urban Praha 1nám. Rep. site. Of the rural sites, the highest increase per year in the annual median of the NO2/NOx ratio equal to 0.0081 was recorded at the Valdek site, whereas of the mountain sites it was 0.0051 at the Přebuz site. The highest increase per year in the annual 98th percentile of the NO2/NOx ratio equal to 0.0126 was measured at the rural Krupka site, of the urban sites it was 0.0068 at the Praha 1-nám. Rep. site and 0.0067 at the Ústí n.L.-Kočkov site, and of the mountain sites it was 0.0090 at the Přebuz site. The spatial pattern for trends in the NO2/NOx ratio revealing some geographical connotations is presented in Fig. 6. Most sites (regardless of type) with generally significantly increasing trends in the NO2/NOx ratio (marked in red on the map), are clustered in the north-west portion of the Czech Republic, a region with many energy-producing large emission sources. These thermal power plants,

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Journal Pre-proof traditionally burning poor-quality lignite of local provenience, have been modernised and partly denitrified recently. The remaining two sites are urban (in Prague). On the other hand, significantly decreasing trends in the NO2/NOx ratio (marked in green on the map) were detected at one urban site (Plzeň-Doubravka), two rural (Mikulov-Sedlec and Petrovice u Karviné) and three mountain (Šerlich, Jeseník-lázně and Bílý Kříž) sites. The check of compliance with the current NO2 and NOx limit values at the individual sites included in our analysis revealed that no exceedance was recorded over the time period under review.

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3.3. NOx emissions The development of the total NOx and NO2 emissions over the period 1990–2016 reflects diverse

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trends (Fig. 7). The reason for this difference lies in the country’s effort to target emission limits for NOx as a whole, whilst the proportion of NO and NO2 components varies depending on the type of

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emission source. The total NOx emissions in the period of 1990–2016 have a decreasing trend resulting in particular from the modernisation of combustion sources in connection with the

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introduction of new emission limits and their gradual tightening, fleet renewal, etc. Total NO2

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emissions in 1990–1999 were decreasing due to a decrease in NOx emissions. A significant increase in total NO2 emissions as compared to the trend of total NOx emissions has occurred since 1999 with a peak in 2007. The main causes of this unfavourable trend change include a significant increase in the

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performance of passenger cars and light duty vehicles with diesel engines, which are characterised by a higher proportion of the NO2 component in exhaust gases compared to gasoline engines. A

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moderate decrease in total NO2 emissions has occurred since 2008 and was due mainly to a reduction in diesel consumption in road transport and a marked downward trend in total NOx emissions. However, the NO2 emissions from passenger cars and light duty vehicles continue to rise even though new vehicles meet the stricter EURO emission limits. According to data in the COPERT 5 emission factor database, the proportion of the NO2 component in NOx emissions from diesel engines for passenger cars and light duty vehicles has risen in EURO 3 (2000) vehicles. Furthermore, it has peaked for vehicles complying with EURO 4 (2005) due to the mandatory introduction of oxidation catalysts that create more favourable conditions for NO2 formation. For EURO 5 (2009) and EURO 6 (2014) vehicles, the NO2 component in NOx emissions has started to decrease slightly again. The share of road transport: passenger cars (1A3bi) and road transport: light duty vehicles (1A3bii) on total NO2 emissions rose from 5.1% in 1990 (Fig. 8 – year 1990) to 14.9% in 2000 (Fig. 8 – year 2000). Moreover, in 2010 this share was 43.4% (Fig. 8 – year 2010), and reached a full 54.8% in 2016 (Fig. 8 – year 2016). An adverse effect on the trend of total NO2 emissions has occurred in recent years with the increase in fuel consumption in stationary combustion engines (cogeneration units) in relation

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Journal Pre-proof with the development of biogas stations that are included in the emission inventory into the agriculture/forestry/fishing: stationary (1A4ci), and commercial/institutional: stationary (1A4ai) sectors. The share of these sectors in total NO2 emissions rose from 1.9% in 2010 (Fig. 8 – year 2010) to 3.9% in 2016 (Fig. 8 – year 2016). 4. Discussion The decreasing trend in Czech NOx emissions is in line with the overall decline in NOx emissions of more than 50% throughout Europe, EU28 (EMEP, 2017). In spite of such a profound change in NOx emissions it has been shown that this is still not enough in Europe to allow for recovery from

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chronically high N deposition (Dirnböck et al., 2018) and that ecosystems are still threatened by excess nitrogen (Serrano et al., 2019). Though nitrogen deposition in the CR decreased, it still

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represents a considerable stress in Czech forests (Hůnová et al., 2014; 2016). Interestingly, the decrease in NOx and subsequently in NO3- in wet nitrogen deposition (Hůnová et al., 2014) changes

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the proportion of oxidized and reduced form of nitrogen in favour of reduced form. The increasing trend in the precipitation and wet-only deposition of N_NH4+/N_NO3- ratio in Czech forests was

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reported by Hůnová et al. (2017). This finding is in line with similar observations elsewhere (Du et al.,

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2014; Vet et al., 2014). The increasing N_NH4+/N_NO3- ratios in atmospheric deposition may negatively affect ecosystems as was suggested by some authors (Fangmeier et al., 1994; Britto and Kronzucker, 2002). It appears, actually, that nitrogen impacts on ecosystems might be more related

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to relative share of reduced and oxidised form of nitrogen than to total nitrogen load (BassiriRad, 2015; van den Berg et al., 2016).

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Furthermore, the annual limit value for NO2 is still widely exceeded across Europe. According to the latest EEA report, around 12% of all reporting stations in 2016 recorded concentrations exceeding the EU limit, with the vast majority – a full 88% – observed at traffic sites (EEA, 2018). This statement is in accordance with routine Czech evaluations, which indicate that NO2 limit values are exceeded sporadically, exclusively at the urban sites categorised as transport-oriented (CHMI, 2018). However, these sites were not included in our analysis as our site selection was based exclusively on the length and completeness of their records. We have found the most pronounced decrease in the 98th percentiles as compared to the other studied percentiles and mean. This behaviour is due to the fact that ambient air pollutant concentrations commonly follow a log-normal, left-skewed statistical distribution (Larssen, 1971), hence the decrease in emissions results in a decrease in very high concentrations encountered much less frequently than lower concentrations. We have reported similar behaviour with other pollutants, such as O3 (Hůnová and Bäumelt, 2018) and it was also indicated by other studies (Paoletti et al., 2014).

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Journal Pre-proof The changing proportion of NO2 in NOx in ambient air, namely its increase, is likely due to two factors: (1) changes in fleet primarily in big towns and cities and (2) reductions in NOx emissions in industrial regions. It is a reasonable assumption that the increase of NO2 in NOx reflects the increase in the NO2/NOx emission ratio from passenger diesel cars, offsetting the decrease in NOx emissions, which is large (39% in total and 44% for EU-28 traffic emissions) as reported by EEA (2016). Carslaw et al. (2006) reported the increasing amount of primary NO2 emitted by traffic in London, ranging between 30%–50% depending on conditions. Kurtenbach et al. (2012) have observed the increasing NO2/NOx emission ratio for traffic environments in Germany. We assume that this might be a factor driving the increasing trends in the NO2/NOx ratio at two urban sites included in our analysis, Prague

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sites (Praha 1-nám. Republiky and Praha 4-Libuš), although not truly traffic sites as defined by EoI 97/101/EC (EC, 1997), but under the indisputable impact of traffic emissions.

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The red marks indicating the increasing trends in the NO2/NOx ratio in the north-western Czech Republic both at urban and rural sites (Fig. 6) likely reflect the changes in emissions from high stacks

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of several large coal-burning power plants concentrated in the Podkrušnohorská pánev basin facing

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the Krušné hory (Erzgebirge) Mountains (Mejstřík, 1993). Some of the large coal-burning power plants of this formerly infamous polluted ʽBlack Triangle’ area (the border between the Czech

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Republic, the former East Germany and Poland) have been closed (CarbonBrief, 2019); some have been partly denitrified, which resulted in a regional increase in the ambient NO2/NOx ratio (CHMI,

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2017). The decrease in Czech NOx emissions on a regional basis in a spatial resolution of 5 km, published regularly by the CHMI and available at the CHMI website (CHMI, 2019), show clearly a substantial decrease just in the North-west region. It is a well-known fact, actually, that a substantial

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portion (as much as 95%) of NOx emissions is emitted as NO, whereas primary NO2 emissions constitute only about 5% (Miller, 2017; Glarborg et al., 2018). Thus we can reasonably assume that a certain decrease in NOx emissions resulted in a proportionally larger reduction of NO emissions than NO2 emissions from large coal-burning power plants with a subsequent increase in NO2/NOx in the ambient air. And indeed, eight sites situated in this area showed an increase in the ambient NO2/NOx ratio (Fig. 6). Nevertheless, this interesting finding would deserve thorough analysis in future research. Additionally, we should consider another factor. Ambient NOx-O3 chemistry is strongly interlinked and temperature-dependent (Monks et al., 2009). This non-linear association was recently confirmed on daily data from Czech monitoring sites from the time period of 1992–2016 (Hůnová et al., 2019). We can reasonably assume that, with ongoing climate change (i.e. with increasing solar radiation, increasing temperature, decreasing humidity, lack of precipitation and increasing frequency of heat waves) and changing photochemistry, secondary NO2 formation will increase (Jacob, Winner, 2009). An increasing NO2/NOx trend attributable to increased secondary NO2 formation has already been

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Journal Pre-proof reported by Mavroidis and Chaloulakou (2011) for Athens. In some regions, secondary NO2 forms a substantial part of NO2. For example, for three Spanish cities, Casquero-Vera et al. (2019) reported that even ca 70% of the total ambient NO2 was of secondary origin. The temperature has been on a significant, steady rise in the CR in recent decades according to the CHMI records (Brázdil et al., 2012; Tolasz et al., 2017); its impact, however, is likely to be much milder than in the Mediterranean region and so the observed changes in NO2/NOx ratio at Czech sites are most likely to be attributable to changes in primary NOx emissions. The NO2/NOx ratio is known to affect the formation and destruction of ambient O3. An increase in the NO2/NOx ratio has important implications in atmospheric chemistry, such as an increased

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formation of ambient O3 (Carslaw and Carslaw, 2007, Han et al., 2011, Yang et al., 2018). This was also indicated by our earlier study on long-term observation-based trends of ambient O3 in the CR

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(Hůnová and Bäumelt, 2018).

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4.1. Strength, weaknesses and limitations of our study

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In this paper we show the time changes in ambient NO, NO2 and NOx concentration, and the NO2/NOx ratio within the framework of emission changes throughout the CR. Analysing the trends

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since 1994, we cover the period after a substantial emission decrease in the mid-1990s not only in the CR but also in neighbouring counties. The major advantage of the presented study lies in using

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the high-quality observation-based data obtained within a nation-wide ambient air quality monitoring network on a routine and long-term basis. Moreover, the concentrations at all sites were measured by one and only one method over the entire period under examination, so the results are

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well comparable over both space and time.

The NOx emissions reported directly by the resource operator are usually measured directly (onetime or continuous measurement) and their uncertainty is estimated to be fairly low, i.e. up to 5%. The uncertainty of NOx emissions as determined by model calculations is expected to be somewhat higher, ranging between 25%–30% (CHMI, 2018). The uncertainty estimation for NO2 emissions is likely to be higher for stationary sources, as only limited data are available to determine the NO 2/NOx ratio. The emission inventory for NO2 includes all significant groups of emission sources. However, some groups of sources with an NO2/NOx ratio higher than previously assumed may exist. These are, for example, condensing boilers for which the NO2/NOx ratio could be higher than for atmospheric boilers (Neužil, 2012).

5. Conclusions We carried out a thorough analysis on ambient NOx concentrations recorded in a long-term monitoring run operated by the Czech Hydrometeorological Institute. Altogether, we checked the

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Journal Pre-proof time trends for data series from 39 sites of different types, i.e. urban, rural and mountain, reflecting different regions across the country, in 1994–2016. We used the non-parametric Mann-Kendall test to examine the trends in NOx, NO2, NO and the NO2/NOx ratio. We checked the 10th, 20th, 50th and 98th percentiles and annual, summer and winter means. We found that both NOx and its two individual components, NO2 and NO were decreasing significantly in the examined characteristics at an overwhelming majority of Czech sites. This decreasing trend was the most pronounced for the peak concentrations represented in our study by the 98th percentiles, whereas the trends in the medians and lower percentiles (10th, 20th percentiles) were subtle. This behaviour was evident for all types of sites and is attributable purely to the

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commonly encountered log-normal, left-skewed distribution of ambient air pollutant concentrations, including NOx. The decrease in concentrations was most pronounced at urban sites.

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We have found a changing proportion of NO2 and NO reflected in our study by a change in the NO2/NOx ratio on an equivalent basis. The spatial patterns of the NO2/NOx ratio were equivocal, the

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significantly increasing trend in NO2/NOx was recorded at sites in the north-western region of the CR,

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regardless of station type. We relate this change in ambient NO2/NOx concentrations to a decrease in NOx primary emissions from coal-burning thermal power plants cumulated in this region.

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Furthermore, an increasing trend in NO2/NOx was found in some cities, most likely attributable to changing primary emissions from traffic. The above trends in ambient NOx, NO2, NO and the NO2/NOx

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ratio reflect the trends in overall Czech NOx and NO2 emissions. Our results compare well with similar studies elsewhere.

The generally decreasing trends in ambient NOx concentrations have implications for

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atmospheric nitrogen deposition: for favourable decreasing of the total nitrogen load, but unfavourable change in relative proportion of oxidized (N_NO3-) and reduced (N_NH4+) forms of nitrogen in favour of the reduced form, with anticipated impacts on ecosystems. Based on our results we can conclude that in spite of the substantial decrease in NOx emissions, and consequent substantial decrease in ambient NOx and its individual components, i.e. NO and NO2, a significant increase in the NO2/NOx ratio at some sites representing different environments (urban, rural, background) was detected. This change is likely to contribute to changes in atmospheric chemistry, namely with respect to complicated highly non-linear formation and the destruction of ambient O3, promoting unintended and undesired increasing ambient O3 levels.

Acknowledgements The input data for our analysis were retrieved from the ISKO (Information System of Air Quality) database operated by the Czech Hydrometeorological Institute. We thank our colleague Jana Schovánková for preparing the figures and Erin Naillon for proofreading our manuscript.

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Figures: Fig. 1. The 10th, 50th and 98thpercentiles of NOx concentrations Fig. 2. The 10th, 50th and 98thpercentiles of NO2 concentrations Fig. 3. The 10th, 50th and 98thpercentiles of NO concentrations Fig. 4. The average NO2 levels at different types of measuring sites vs. the NO2 limit value Fig. 5. Trends in Czech NOx emissions from stationary and mobile sources, Czech Republic, 1990–2016 Fig. 6. Spatial pattern in NO2/NOx trends at the sites under review Fig. 7. Trends in Czech NO2 emissions from different sources, Czech Republic, 1990–2016 Fig. 8. Total emissions of NOx sorted by NFR sectors, 1990, 2000, 2010 and 2016

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Graphical abstract

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Journal Pre-proof Highlights We examined trends in NOx at 39 Czech sites (urban, rural, mountain) in 1994–2016



Trends in NOx, NO, NO2 are generally decreasing in line with decreasing emissions



The trends in NO2/NOx are significantly increasing at some sites regardless of type



NO2/NOX increase in north-west due to reduced NOx from coal-burning power plants

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Journal Pre-proof Measuring sites selected for analysis (ranked within individual categories according to increasing altitude)

Věřňovice Petrovice u Karviné Mikulov-Sedlec Blažim Tušimice Frýdlant-Údolí Valdek Ondřejov Krupka Košetice Sněžník Svratouch

203 243 245 261 322 381 438 514 533 535 590 735

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Jeseník-lázně Přimda Souš Hojná Voda Měděnec Rudolice v Horách Bílý Kříž Přebuz Krkonoše-Rýchory Šerlich Churáňov

Urban sites 1992–2006/2007-2016 1994 1998 1994 1992 1981 1993 1994 1985 1994 1999–2015 1973 1994 1993 1992 1995 Rural sites 1994 1995 1994 1996 1968 1992–2015 1993 1993 1992 1985 1992–2004–2016 1979 Mountain sites 1994 1998 1970 1994 1992 1995 1988 1993 1994 1994–2013 1988

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190 210 217 218 220 233 238 241 301 348 350 367 383 476 500 583

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Praha 1-Nám. Rep. Přerov Pardubice- Rosice Prostějov Ostrava-Fifejdy Hr. Král.-Sukovy Sady Karviná Brno-Tuřany Praha 4-Libuš Plzeň-Doubravka Liberec-město Ústí n.L.-Kočkov České Budějovice Sokolov Jablonec-město Prachatice

Measurement period

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Altitude [m a.s.l.]

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Station

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Table 1.

625 740 771 818 827 840 890 904 1001 1011 1118

Category

B/U/C (T/U/C) B/U/CR B/S/RI B/U/R B/U/R T/R/RCI B/U/R B/S/R B/S/R B/S/A B/U/RC B/S/RN B/U/R B/S/R B/U/R B/S/R B/R/AI-NCI I/S/C B/R/A-REG I/R/A B/R/IA-NCI B/R/AN-NCI B/R/AN-NCI B/R/N-REG B/R/N-NCI B/R/AN-REG B/R/N-REG B/R/AN-REG B/R/N-NCI B/R/N-REG B/R/N-REG B/R/N-REG B/R/ANI-NCI B/R/N-REG B/R/N-REG B/R/AN-REG B/R/N-REG B/R/N-REG B/R/N-REG

Note.: Sites are classified according to EC Decision EoI 97/101/EC (EC, 1997): B/S/R – background / suburban / residential, B/U/R – background / urban / residential, B/S/RN - background / suburban / residential, natural, B/R/A - background / rural / agricultural, B/R/N-REG - background / rural / natural-regional, T/U/RCI – traffic / urban / regional, commercial, industrial, I/S/C – industrial / suburban / commercial, I/R/A – industrial / regional / agricultural

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Journal Pre-proof Table 2.

Trends in NOx at individual selected sites, 1994–2016 (Mann-Kendall test results)

Station

10%

20%

50%

Trend

Δ

Trend

Δ

Praha1-nám Rep. Přerov Pardubice-Rosice Prostějov Ostrava-Fifejdy Hr. Král.-Suk. sad Karviná Brno-Tuřany Praha-Libuš Plzeň-Doubravka Liberec-město Ústí n L.-Kočkov České Buděj Sokolov Jablonec-město Prachatice

-0.57 -0.01 -0.09 -0.11 -0.03 -0.16 -0.10 -0.08 -0.23 -0.03 -0.12 -0.21 -0.11 -0.23 -0.18 -0.02

D***

-0.66 -0.05 -0.10 -0.13 -0.06 -0.34 -0.13 -0.10 -0.26 -0.06 -0.17 -0.25 -0.16 -0.26 -0.20 -0.02

D*** D+ D** D* D* D* D*** D** D*** D** D** D*** D*** D*** D***

-1.00 -0.08 -0.13 -0.15 -0.13 -0.43 -0.24 -0.11 -0.27 -0.10 -0.16 -0.32 -0.21 -0.33 -0.23 -0.03

Věřňovice Petrovice u Karviné Mikulov-Sedlec Blažim Tušimice Frýdlant-Údolí Valdek Ondřejov Krupka Košetice Sněžník Svratouch

0.03 0.19 -0.08 -0.03 -0.20 -0.18 -0.20 -0.04 -0.20 -0.002 -0.13 -0.06

I+ I*** D***

D** D*** D** D*** D***

0.02 0.17 -0.08 -0.07 -0.21 -0.20 -0.22 -0.06 -0.21 -0.01 -0.14 -0.07

D*** D*** D*** D* D***

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D*** D*** D*** D* D***

I** D***

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D*** D**

0.01 0.22 -0.08 -0.04 -0.24 -0.18 -0.26 -0.12 -0.28 -0.01 -0.18 -0.08

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D*** D** D***

-p

D* D*

D*** D**

98% Trend Urban sites

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Δ

D*** D* D+ D* D* D*** D* D** D** D* D*** D** D*** D***

Δ

Trend

-2.28 -0.53 -0.21 -0.58 -0.51 -0.71 -0.51 -0.38 -0.92 -0.53 -0.33 -0.91 -0.58 -0.96 -0.45 -0.35

D***

-0.55 -0.15 -0.41 -0.80 -0.74 -0.67 -0.83 -0.37 -0.89 -0.11 -0.84 -0.33

D*

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D* D* D*** D***

-1.10 -0.15 -0.14 -0.18 -0.17 -0.37 -0.27 -0.15 -0.36 -0.15 -0.20 -0.37 -0.26 -0.39 -0.25 -0.02

Rural sites I** D* D*** D*** D*** D* D*** D*** D*

Mountain sites D** D*** D*** -0.04 -0.06 -0.14 -0.27 D* D** D* -0.07 -0.10 -0.24 -0.44 D*** D*** D*** -0.11 -0.15 -0.22 -0.76 -0.02 -0.02 -0.02 -0.28 D*** D*** D* -0.16 -0.16 -0.21 -0.83 D*** D*** D** -0.19 -0.22 -0.23 -0.97 D*** D*** D*** -0.04 -0.04 -0.07 -0.09 D*** D*** D*** -0.14 -0.16 -0.22 -0.49 0.04 -0.003 0.07 0.16 -0.01 -0.003 0.01 -0.04 0.01 -0.009 0.02 -0.04 Note: D – statistically significant decreasing trend, I - statistically significant increasing trend, p – significance level, + p < 0.1, * p < 0.05, p < 0.01, *** p < 0.001, Δ – NOx concentration change [ppb] per year Jeseník-lázně Přimda Souš Hojná Voda Měděnec Rudolice v Horách Bílý Kříž Přebuz Krkonoše-Rýchory Šerlich Churáňov

D+

Annual M Δ

D** D*** D*** D*** D** D* D* D*** D+ D** D** D*** D+ D** D** D***

**

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-0.04 0.19 -0.13 -0.11 -0.30 -0.28 -0.33 -0.13 -0.36 -0.02 -0.28 -0.10 -0.09 -0.16 -0.25 -0.04 -0.27 -0.32 -0.08 -0.21 0.04 0.03 0.003

Journal Pre-proof Table 3.

Trends in NO2 at individual selected sites, 1994–2016 (Mann-Kendall test results)

Station

10%

20%

50%

Trend

Δ

Trend

Δ

Praha1-nám Rep. Přerov Pardubice-Rosice Prostějov Ostrava-Fifejdy Hr. Král.-Suk. sad Karviná Brno-Tuřany Praha-Libuš Plzeň-Doubravka Liberec-město Ústí n L.-Kočkov České Buděj Sokolov Jablonec-město Prachatice

-0.28 -0.03 -0.06 -0.10 -0.05 -0.15 -0.10 -0.07 -0.16 -0.06 -0.11 -0.16 -0.09 -0.15 -0,12 -0.03

D***

-0.33 -0.04 -0.06 -0.09 -0.06 -0.44 -0.11 -0.07 -0.17 -0.08 -0.14 -0.19 -0.12 -0.18 -0.14 -0.03

D*** D* D* D** D** D* D*** D** D*** D*** D*** D*** D*** D*** D***

-0.35 -0.07 -0.08 -0.13 -0.08 -0.20 -0.16 -0.10 -0.17 -0.11 -0.14 -0.26 -0.16 -0.22 -0.15 -0.04

Věřňovice Petrovice u Karviné Mikulov-Sedlec Blažim Tušimice Frýdlant-Údolí Valdek Ondřejov Krupka Košetice Sněžník Svratouch

0.03 0.06 -0.09 -0.05 -0.15 -0.10 -0.12 -0.04 -0.12 -0.01 -0.10 -0.06

re

D*** D*** D*** D* D***

I* D*** D+ D*** D*** D*** D* D***

D*** D*

-0.04 0.03 -0.11 -0.08 -0.18 -0.14 -0.19 -0.09 -0.19 -0.02 -0.14 -0.07

ro

I+ D***

-p

0.02 0.07 -0.09 -0.07 -0.15 -0.12 -0.14 -0.05 -0.15 -0.02 -0.10 -0.07

D*** D*

Urban sites D*** D+ D* D* D+ D+ D*** D** D** D*** D** D*** D*** D*** D***

lP

na

AM

Δ

Trend

Δ

-0.58 -0.24 -0.26 -0.10 -0.29 -0.36 -0.39 -0.35 -0.13 -0.25 -0.13 -0.44 -0.33 -0.47 -0.21 -0.11

D***

-0.36 -0.10 -0.08 -0.13 -0.08 -0.17 -0.17 -0.13 -0.18 -0.14 -0.14 -0.27 -0.17 -0.25 -0.17 -0.03

-0.54 -0.22 -0.38 -0.44 -0.54 -0.43 -0.51 -0.36 -0.54 -0.08 -0.40 -0.29

D**

D* D+ D** D* D** D*** D* D***

Rural sites

D*** D*** D*** D*** D* D**

D*** D* Mountain sites D*** D* D***

D*** D*** -0.05 -0.06 -0.07 -0.20 D** D*** -0.05 -0.07 -0.10 -0.32 D*** D*** -0.08 -0.12 -0.19 -0.55 -0.01 -0.02 -0.01 -0.23 D*** D*** D* -0.12 -0.13 -0.16 -0.50 D*** D*** D** -0.15 -0.17 -0.16 -0.57 D*** D*** D*** -0.04 -0.05 -0.07 -0.09 D*** D*** D*** -0.11 -0.11 -0.13 -0.33 0.01 -0.004 0.06 0.09 -0.03 -0.04 -0.05 -0.03 -0.06 -0.01 -0.004 -0.04 Note: D – statistically significant decreasing trend, I - statistically significant increasing trend, p – significance level, + p < 0.1, * p < 0.05, p < 0.01, *** p < 0.001, Δ – NO2 concentration change [ppb] per year

Jo ur

Jeseník-lázně Přimda Souš Hojná Voda Měděnec Rudolice v Horách Bílý Kříž Přebuz Krkonoše-Rýchory Šerlich Churáňov

D+ D** D** D+ D*** D** D*** D** D*** D*** D*** D*** D*** D+

98% Trend

of

Δ

D** D** D*** D*** D** D** D** D**

D* D* D*** D+ D** D*** D**

**

22

-0.08 0.02 -0.13 -0.14 -0.23 -0.18 -0.20 -0.13 -0.23 -0.03 -0.18 -0.09 -0.08 -0.13 -0.20 -0.05 -0.19 -0.26 -0.08 -0.16 0.01 -0.04 -0.01

Journal Pre-proof Table 4.

Trends in NO at individual selected sites, 1994–2016 (Mann-Kendall test results)

Station

10%

20%

50%

Δ

Trend

Δ

Trend

Δ

Praha1-nám Rep. Přerov Pardubice-Rosice Prostějov Ostrava-Fifejdy Hr. Král.-Suk. sad Karviná Brno-Tuřany Praha-Libuš Plzeň-Doubravka Liberec-město Ústí n L.-Kočkov České Buděj Sokolov Jablonec-město Prachatice

-0.34 -0.01 0.00 -0.03 -0.01 0.01 -0.02 -0.01 -0.04 0.01 -0.02 -0.01 0.00 -0.05 -0.03 0.02

D**

-0.04 -0.01 -0.02 -0.03 -0.01 -0.06 -0.03 -0.01 -0.05 0.01 -0.03 -0.02 -0.01 -0.06 -0.03 0.02

D***

-0.63 -0.03 -0.04 -0.04 -0.05 -0.15 -0.05 -0.01 -0.12 0.02 0.00 -0.04 -0.03 -0.10 -0.04 0.01

Věřňovice Petrovice u Karviné Mikulov-Sedlec Blažim Tušimice Frýdlant-Údolí Valdek Ondřejov Krupka Košetice Sněžník Svratouch

-0.01 0.11 0.00 0.05 -0.05 0.00 0.00 0.00 -0.02 0.00 -0.00 0.01

-0.01 0.12 0.00 0.03 -0.06 -0.02 -0.02 0.00 -0.03 0.00 -0.004 0.02

I** I* D***

D*** D** I**

I**

D*** D* D**

D+ I+

D* I*

Urban sites D*** D*

D* D+ D*** D** I+ D*** D*** D*

of

D*** D** I**

D***

0.01 0.13 0.00 0.01 -0.06 -0.03 -0.05 0.00 -0.05 0.00 -0.04 0.02

ro

D**

D*** D* D**

-p

D** D* D**

D+ D*

re

Jeseník-lázně Přimda Souš Hojná Voda Měděnec Rudolice v Horách Bílý Kříž Přebuz Krkonoše-Rýchory Šerlich Churáňov

D** D*

98% Trend

Trend

Δ

-1.98 -0.05 0.10 -0.28 -0.43 -0.57 -0.33 -0.11 -0.79 -0.24 -0.36 -0.51 -0.26 -0.49 -0.19 -0.13

D***

-0.77 -0.06 -0.06 -0.07 -0.11 -0.17 -0.09 -0.02 -0.18 0.00 -0.07 -0.11 -0.07 -0.13 -0.07 0.01

lP

na

D+ D**

D*** D***

Rural sites I***

D*** D** D*** D* D*** I* Mountain sites D*

-0.05 0.17 -0.08 -0.26 -0.40 -0.23 -0.35 -0.11 -0.54 -0.06 -0.51 -0.05

-0.001 -0.01 -0.01 -0.06 0.01 0.01 0.01 -0.06 I* 0.001 0.01 -0.01 -0.17 0.00 0.00 0.01 -0.01 D* D*** D*** -0.04 -0.05 -0.07 -0.36 D*** D*** -0.02 -0.04 -0.07 -0.24 I** I* 0.004 0.004 0.01 -0.02 D* D** D*** -0.002 -0.01 -0.05 -0.12 I* I** 0.02 0.02 0.04 0.19 I* I** I** 0.04 0.04 0.04 0.16 I+ I+ I* 0.01 0.02 0.03 0.01 Note: D – statistically significant decreasing trend, I - statistically significant increasing trend, p – significance level, + p < 0.1, * p < 0.05, p < 0.01, *** p < 0.001, Δ – NO concentration change [ppb] per year

Jo ur

AM

Δ

D+ D** D** D** D*** D* D+ D***

D** D** D** D** D***

**

23

0.01 0.15 0.00 0.01 -0.09 -0.06 -0.08 -0.02 -0.13 0.00 -0.09 0.01 -0.02 -0.02 -0.03 -0.01 -0.09 -0.08 0.00 -0.04 0.04 0.06 0.03

Journal Pre-proof Table 5. results)

Trends in NO2/NOx ratio at individual selected sites, 1994–2016 (Mann-Kendall test

Station

10%

20%

50%

Δ

Trend

Δ

Trend

Δ

Praha1-nám Rep. Přerov Pardubice-Rosice Prostějov Ostrava-Fifejdy Hr. Král.-Suk. sad Karviná Brno-Tuřany Praha-Libuš Plzeň-Doubravka Liberec-město Ústí n L.-Kočkov České Buděj Sokolov Jablonec-město Prachatice

0.0075 0.0016 0.0002 0.0014 0.0031 0.0015 0.0014 -0.0011 0.0051 -0.0044 -0.0008 0.0019 0.0010 0.0020 0.0018 -0.0031

I*

0.0077 0.0014 0.0013 0.0011 0.0043 0.0021 0.0016 -0.0003 0.0054 -0.0052 -0.0015 0.0021 0.0001 0.0024 0.0014 -0.0038

I*

0.0086 0.0008 0.0029 0.0036 0.0012 0.0021 0.0017 -0.0007 0.0047 -0.0039 -0.0004 0.0037 0.0013 0.0027 0.0000 -0.0019

Věřňovice Petrovice u Karviné Mikulov-Sedlec Blažim Tušimice Frýdlant-Údolí Valdek Ondřejov Krupka Košetice Sněžník Svratouch

-0.0041 -0.0056 -0.0074 -0.0035 0.0011 0.0087 0.0085 -0.0013 0.0052 -0.0008 0.0066 -0.0012

I* I+ D* D* D***

I** I**

I** D* D+ D* D***

I** I** I*

re

I**

-0.0026 -0.0064 -0.0056 -0.0025 0.0010 0.0079 0.0082 -0.0013 0.0042 -0.0010 0.0061 -0.0010

I*

lP

I***

of

I*

I*** D**

-0.0008 -0.0070 -0.0026 -0.0026 0.0025 0.0075 0.0081 0.0010 0.0045 -0.0004 0.0044 0.0003

ro

I*** D**

I*

-p

I*

I***

98% Trend Urban sites I*** I+

I** I*** D* I** I**

AM

Δ

Trend

Δ

0.0068 -0.0006 -0.0004 0.0026 -0.0051 -0.0072 0.0011 0.0009 0.0010 -0.0010 0.0018 0.0067 0.0002 0.0034 0.0010 -0.0005

I**

0.0069 0.0007 0.0017 0.0025 0.0003 0.0001 0.0012 -0.0002 0.0036 -0.0039 -0.0002 0.0033 0.0006 0.0023 0.0012 -0.0028

I* D*** D+ I+

I*** I+

Rural sites D*** D** D+ I*** I** I* I***

0.0010 -0.0053 -0.0001 -0.0008 0.0044 0.0031 0.0061 0.0024 0.0126 -0.0003 0.0038 0.0002

D**

I** I+ I* I*** I**

Jo ur

na

Mountain sites D* D** D+ -0.0031 -0.0027 -0.0017 -0.0014 0.0035 0.0020 0.0017 0.0006 0.0024 0.0032 0.0027 -0.0024 0.0042 0.0039 0.0027 0.0007 I* 0.0019 0.0016 0.0029 0.0045 I* I* 0.0026 0.0030 0.0034 -0.0010 D+ D+ -0.0026 -0.0020 -0.0011 -0.0014 I* I* 0.0032 0.0042 0.0051 0.0090 -0.0147 -0.0054 0.0001 0.0004 D* -0.0134 -0.0118 -0.0113 -0.0046 -0.0060 -0.0067 -0.0051 -0.0006 Note: D – statistically significant decreasing trend, I - statistically significant increasing trend, p – significance level, + p < 0.1, * p < 0.05, ** p < 0.01, *** p < 0.001, Δ - NO2/NOx ratio change per year Jeseník-lázně Přimda Souš Hojná Voda Měděnec Rudolice v Horách Bílý Kříž Přebuz Krkonoše-Rýchory Šerlich Churáňov

24

-0.0011 -0.0069 -0.0034 -0.0026 0.0027 0.0068 0.0075 0.0008 0.0054 -0.0008 0.0047 -0.0002 -0.0020 0.0014 0.0021 0.0023 0.0017 0.0020 -0.0016 0.0055 -0.0029 -0.0093 -0.0055

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8