High NO 2 Concentrations Measured by Passive Samplers in Czech Cities: Unresolved Aftermath of Dieselgate?

: This work examines the effects of two problematic trends in diesel passenger car emissions— increasing NO 2 /NO x ratio by conversion of NO into NO 2 in catalysts and a disparity between the emission limit and the actual emissions in everyday driving—on ambient air quality in Prague. NO 2 concentrations were measured by 104 membrane-closed Palmes passive samplers at 65 locations in Prague in March–April and September–October of 2019. NO 2 concentrations measured by city stations during those periods were comparable with the average values during 2016–2019. The average measured NO 2 concentrations at the selected locations, after correcting for the 18.5% positive bias of samplers co-located with a monitoring station, were 36 µ g/m 3 (range 16–69 µ g/m 3 , median 35 µ g/m 3 ), with the EU annual limit of 40 µ g/m 3 exceeded at 32% of locations. The NO 2 concentrations have correlated well (R 2 = 0.76) with the 2019 average daily vehicle counts, corrected for additional emissions due to uphill travel and intersections. In addition to expected “hot-spots” at busy intersections in the city center, new ones were identiﬁed, i.e., along a six-lane road V Holešoviˇck á ch. Comparison of data from six monitoring stations during 15 March–30 April 2020 travel restrictions with the same period in 2016–2019 revealed an overall reduction of NO 2 and even a larger reduction of NO. The spatial analysis of data from passive samplers and time analysis of data during the travel restrictions both demonstrate a consistent positive correlation between trafﬁc intensity and NO 2 concentrations along/near the travel path. The slow pace of NO 2 reductions in Prague suggests that stricter vehicle NO x emission limits, introduced in the last decade or two, have so far failed to sufﬁciently reduce the ambient NO 2 concentrations, and there is no clear sign of remedy of Dieselgate NO x excess emissions.

• NO 2 measured by 104 passive samplers at 65 places in Prague, corrected mean 36 µg/m 3 • NO 2 increases with traffic intensity corrected for intersections and hills • High NO 2 /NO x ratios and excess NO x emissions from diesel cars a culprit • Not much improvement after "Dieselgate" • Reductions below 40 µg/m 3 suggested based on health evidence literature review

Introduction
Mobile sources, including on-road vehicles, remain to be one of the largest contributors to the air pollution in most metropolitan areas in Europe, with particulate matter and nitrogen oxides (NO x , defined as a sum of nitric oxide NO and nitrogen dioxide NO 2 ) being of highest concern. Outdoor air pollution is now being considered one of the leading causes of premature death [1], with estimated tolls of approximately half a million premature deaths annually in the EU [2], and associated economic damage around 5% of HDP in Central Europe [3]. At the same time, the state-of-the art technology of the internal combustion engine has improved considerably over the last decades. Very low levels of sulfur and metals in the fuel have allowed the introduction of three-way catalysts on spark ignition engines, a common technology used throughout the U.S. over the last four decades with a somewhat delayed deployment in Europe, and the introduction of diesel particle filters on virtually all on-road diesel engines manufactured in the last decade. The emissions of nitrogen oxides, primarily NO, on engines operating with excess air remained a challenge, being ultimately resolved about a decade ago with selective catalytic reduction (SCR) systems on heavy-duty vehicles [4] and more recently also on light-duty vehicles.
In the EU, the concentrations of NO 2 , deemed to be more detrimental to human health than NO, are limited and monitored in the ambient air. Overall, the concentrations of NO 2 have not been decreasing as fast as those of other key pollutants. In the Czech Republic, the concentrations of NO 2 at most air quality monitoring stations have been, according to the data in [5], decreasing by on the order of 1% a year over the last two decades. A gradual decrease of NO 2 concentrations in the overall atmosphere above the Czech Republic over the last decade has been also reported from remote sensing satellite measurements [6].
NO 2 in ambient air originates both from direct (primary) emissions and from gradual conversion of NO into NO 2 [7]. While the total emissions of NO x have been gradually decreasing, there is no apparent trend of a decrease in NO 2 primary emissions over the last 15 years [6]. One of the culprits of high primary NO 2 emissions are diesel vehicles, which have been, over the last two decades, equipped with oxidation catalysts, which convert a considerable portion of NO into NO 2 . In the U.S., average NO 2 /NO x ratio in vehicle exhaust (all vehicles, including predominantly gasoline cars and light trucks and predominantly diesel heavy trucks) was 5.3% [8], compared to approximately 15% in Europe [9]. This paper explores a hypothesis that the observed decrease in NO 2 concentrations falls short of that expected based on order-of-magnitude decrease in vehicle NO x emissions limits and that non-compliant diesel cars could substantially contribute to this shortfall. The underlying aspects of NO x emissions and the adverse health effects of NO 2 are summarized. The results of a monitoring NO 2 with passive samplers are reported and discussed in light of these findings. As an additional insight, the effects of coronavirus related restrictions on NO and NO 2 concentrations in Prague are reported and discussed.

Review of Trends and Shortcomings in NO 2 and NO x Emissions from Vehicles
Nitrogen oxide (NO) is formed in combustion processes from atmospheric nitrogen and oxygen at high temperatures [10,11], which are generally associated both with efficient combustion and with high thermal efficiency of the engine. Subsequent oxidation of NO in the atmosphere yields primarily nitrogen dioxide (NO 2 ), a brownish irritant gas. Other oxides of nitrogen-N 2 O 2 , N 2 O 3 , N 2 O 4 , N 2 O 5 -are generated in small concentrations, are unstable and short-lived in the atmosphere. The oxides of nitrogen are summarily referred to as NO x , although there is no precise definition. Often, NO x is evaluated as the sum of NO and NO 2 . Technically, the sum of NO x also includes nitrous oxide (N 2 O), which is, however, not hazardous to human health, but is a potent greenhouse. NOx leads to the formation of nitrous acid (HNO 2 ) [12,13], nitric acid (HNO 3 ) and a variety of salts such as ammonium nitrate, present in the atmosphere as particulate matter [14]. Photodissociation of NO 2 under the presence of sunlight produces NO and atomic oxygen, which reacts with molecular oxygen to form ozone [15], a highly reactive compound generally harmful to human health, organisms and plants. NO x and ground-level (tropospheric) ozone are, together with particulate matter, the principal part of urban air pollution.
On spark ignition engines, CO and VOC, principally a product of incomplete oxidation of fuel and to a lesser extent engine lubricating oil, and NO x have been successfully abated by the combination of three-way catalysts [16] and by maintaining stoichiometric air-fuel ratio through closed-loop control of the quantity of fuel injected [17]. This technology has proven to be remarkably efficient.
On diesel engines, the emissions of NO x have been, at first, controlled through delayed combustion timing and exhaust gas recirculation, both associated with a slight fuel penalty, and at a later time, with NO x storage and reduction catalysts and selective reduction catalysts (SCR). The reduction of NO x has historically come at an expense of both capital and operating costs, with operating costs including either fuel (notably on older vehicles using delayed combustion, exhaust gas recirculation, NO x storage and reduction catalysts) or a reducing agent used in SCR (mostly aqueous solution of urea, known as diesel exhaust fluid or "AdBlue"). These costs have motivated, over the last few decades, many manufacturers and vehicle users to circumvent NO x reduction efforts, as the savings were realized by them directly, while considerably larger overall damage to human health was born by the society, a problem known as the Tragedy of the Commons [18]. A widespread practice of dual engine mapping in the U.S. in the 1990s [19,20] has led to the gradual extension of vehicle emissions limits to ordinary on-road operation first of heavy-duty and later of light-duty vehicles [21][22][23]. In the heavy-duty vehicle engine sector, many recent studies now show that on-road NO x emissions of newer heavy-duty vehicles have been successfully reduced by an order of magnitude except for low-load operation typical for congested urban areas. Quiros et al. [24] reports NO x emissions of 2013 and 2014 model year heavy trucks of 0.36 g/km during motorway operation in California. Jiang et al. [25] reports, for similar conditions, 0.3 g/km NO x during extraurban and motorway operation. Grigoratos et al. [26] reports NO x emissions during motorway operation in Europe of 0.07, 0.08, 0.17 and 0.24 g/kWh for four trucks and 0.80 g/kWh for a bus. Giechaskiel et al. [22] reports NO x emissions of a garbage collection truck of less than 0.4 g/kWh during extraurban operation (note: for heavy vehicles, emissions per kWh roughly correspond to emissions per km).
Unfortunately, this has not been the case with light-duty vehicles with diesel engines, highly prevalent in Europe, where they account for several tens of percent of vehicle registration and in Prague, for about two thirds of vehicles counted on the road [27]. Large portion of European automobile diesel engines produced over the last one to two decades have been reported to emit substantially, often by an order of magnitude, more NO x on the road than during the type approval test [28][29][30][31][32]. Weiss et al. [29] reports on-road NOx emissions factors 0.76 ± 0.12 g/km for Euro 4, 0.71 ± 0.30 g/km for Euro 5 and 0.21 ± 0.09 for Euro 6. In a more recent study by Suarez-Bertoa et al. [23], NO x emissions from Euro 6 diesel cars varied substantially from mid tens to mid hundreds of milligrams of NO x per kilometer, with a median value of about 0.2 g/km NO x during the city-motorway test.
At the same time, on nearly all light-vehicle diesel engines of the last decade or so, oxidation catalysts are used to convert NO into NO 2 , as higher concentrations of NO 2 , around 10%, are beneficial both for the combustion of soot in DPF and for the "fast" reduction of NO x in SCR catalysts. As a result, NO 2 from newer engines accounts for 10% of NO x [33,34]. On passenger cars and light-duty trucks, NO 2 /NO x ratios of around 10-15% up to Euro 3 and 25-30% for Euro 4 and 5 were found in a London remote sensing study [35]. In the U.S., NO 2 /NO x ratio from heavy duty diesel trucks have doubled from around 7% in 2010 (average of trucks passing on the road in a given year, not a model year of the vehicles) to around 15% in 2018 [36]. This increase, however, did not result in an absolute increase in NO 2 emissions, as total NOx emissions have decreased dramatically due to the widespread use of SCR catalysts. According to Preble [36], "Fleet-average NO 2 emission rates remained about the same, despite the intentional oxidation of engine-out NO to NO 2 in DPF systems, due to the effectiveness of SCR systems in reducing NO x emissions and mitigating the DPF-related increase in primary NO 2 emissions".
In Europe, NO x emissions from diesel cars have not, however, decreased in proportion to the decreasing emissions limits. A recent on-road study in Prague reports the mean emissions of Euro 5 and 6 diesel cars and vans of over 0.1 g/km NO 2 and over 0.5 g/km NO x [37], while a recent study of one of the most common diesel cars (Euro 6) reported about 0.15 g/km over WLTC cycle, and about 0.4 g/km over the Artemis driving cycle [38], which is more than the 0.08 g/km Euro 6 limit for total NO x (with which the vehicle reasonably complied over the NEDC cycle).
The presumption of the regulators that increased the NO 2 /NO x ratio after the oxidation catalyst and before the DPF, highly beneficial both for DPF and SCR operation, will be mitigated by the rather high efficiency of the NO x aftertreatment, envisioned in both U.S. EPA and EU emissions standards, which has been compromised by intentional acts resulting in diminished, or even zero, efficiency of the NO x aftertreatment. Examples of such acts include dual-mapping of the engines by the manufacturers (a prime example of which is "Dieselgate") and disabling of the SCR (and emulating its proper functioning to the on-board diagnostics by "SCR emulators") by vehicle operators. Under such conditions, relatively high amounts of NO 2 , intended to be reduced in NO x aftertreatment, are emitted out of the tailpipe. Logically, this results in very high, and much higher than intended, primary emissions of NO 2 in the streets. This finding is consistent with the rather slow decrease in NO 2 concentrations.

Review of the Impact of NO 2 to Central Nervous System in Children and Adults
The first experimental data were obtained several decades ago, indicating that air pollution may induce behavioral changes. Singh [39] studied the effect of NO 2 exposure on pregnant mice, exposed during gestation day 7-18. Prenatal exposure significantly altered the righting reflex and aerial righting score. These results suggest that maternal NO 2 exposure produce deficits in the functional capability of the offspring.
Wang et al. [40] was the first one, who studied the impact of NO 2 exposure to children's neurobehavioral changes. They studied this effect in the year 2005 on two groups of children (A N = 431, B N = 430) in the age of 8-10 years using neurobehavioral testing. Group A was exposed to 7 µg NO 2 /m 3 , group B to 36 µg NO 2 /m 3 . Children from the polluted area showed poor performance in all tests: visual simple reaction time, continuous performance, digit symbol, pursuit aiming and sign register, This study found a significant relationship between chronic low-level traffic related air pollution and neurobehavioral function in exposed children.
Guxens et al. [41] analyzed the association between prenatal exposure, diet and infant mental development in four regions in Spain, in 1889 children, who were exposed to 29.0 ± 11.2 µg NO 2 /m 3 (20.1-36.8). Infant mental development was evaluated at 14 months by Bailey Scales of Mental Development. Exposure to NO 2 did not show a significant association with mental development. Inverse association was observed in infants whose mothers reported low intake of fruit/vegetables during pregnancy (−4.13 (−7.06, −1.21)). This study suggests that antioxidants in fruits and vegetables during pregnancy may modulate an adverse effect of NO 2 on infants' mental development.
A similar study was organized in Spain on 438 mother-child pairs by Lertxundi et al. [43] at 15 months of age, using the Bailey scales of mental development. A 1 µg NO 2 /m 3 increase during pregnancy decreased the mental score (β = −0.29; 90% CI: −0.47; −0.11). Prenatal residential exposure to NO 2 adversely affects infant motor and cognitive development.
A prospective cohort study was conducted with 2715 children aged 7-10 years in Barcelona, Spain, as a part of the BREATHE project (brain development and air pollution ultrafine particles in school children [44]). Children were tested every 3 months with a computerized test. Cognitive development was assessed with the n-back and the attentional network test as working memory and inattentiveness. NO 2 exposure was completed in the outdoors in a low traffic region 40.5 ± 9.6 µg/m 3 and high traffic region 56.1 ± 11.5 µg/m 3 . Children attending schools with higher NO 2 pollution had an 11.5% (95% CI 8.9%-12.5%) slower working memory and slower growth in all cognitive measurements, which means a smaller improvement in cognitive development.
Pujol et al. [45] selected from this cohort 263 children, aged 8-12 years, for magnetic resonance investigation (MRI) to analyze brain volumes, tissue composition, myelination, cortical thickness, neural tract architecture, membrane metabolites and functional connectivity. Outdoor NO 2 exposure was 46.8 ± 12.0 µg/m 3 /year and indoor NO 2 exposure was 29.4 ± 11.7 µg/m 3 /year. Higher NO 2 exposure was associated with slower brain maturation with changes specifically concerning the functional domain.
Forns et al. [46] evaluated 2897 children from the Barcelona cohort within the BREATHE project. NO 2 exposure in schools was 29.82 µg/m 3 (11.47-65.65) and outdoor was 48.46 µg/m 3 (25.92-84.55). Behavioral development was assessed using the strengths and difficulties questionnaire (SDQ), which was filled out by parents. NO 2 exposure was positively associated with SDQ total difficulties scores, suggesting more frequent behavioral problems. This study was understood as the first one to evaluate the impact of air pollution on behavioral development in schoolchildren using both indoor and outdoor air pollution levels measured at schools. NO 2 outdoor levels (IQR = 22.26 µg/m 3 ) significantly increased total difficulties score (1.07, 95% CI: 1.01, 1.14, p < 0.05). NO 2 exposure at school is associated with worse general behavioral development in schoolchildren.
Min and Min [47] studied in Korea 8936 children born in the year 2002 and followed them for the next 10 years, investigating the relationship between exposure to NO 2 and attention-deficit hyperactive disorder (ADHD). They diagnosed 313 children with ADHD. The hazard ratio (HR) associated with the increase in 1 µg of the NO 2 /m 3 was 1.03 (95% CI: 1.02-1.04). Comparing infants with lowest tertile of NO 2 exposure with the highest tertile of NO 2 , HR = 2.10 (95% CI: 1.54-2.85), exposure had a 2 fold increased risk of ADHD. The study showed a significant association between exposure to NO 2 and the incidence of ADHD in children.
Sentis et al. [48] evaluated prenatal and postnatal exposure to NO 2 and attentional function in children at 4-5 years of age in four regions of Spain (N = 1298). The attentional function was evaluated by the Conners kiddie continuous performance test (K-CPT). The prenatal NO 2 level was 31.1 µg/m 3 (18.4-37.9). Higher exposure to prenatal levels of NO 2 was associated with a 1.12 ms (95% CI; 0.22, 2.02) increase in hit reaction time and 6% increase in the number of emission errors (95% CI: 1.01, 1.11) per 10 ug/m 3 increase in prenatal NO 2 . Higher exposure to NO 2 during pregnancy is associated with impaired attentional function, especially increased inattentiveness in children aged 4-5 years. This reduced attentional function in population could lead to poor educational indicators. It seems to be important that this effect was observed with NO 2 concentrations lower than EU standard 40 µg/m 3 .
Sunyer et al. [49] followed in 2012-2013 2687 school children from Barcelona, assessing children s attention process 4 times every three months, using the attention network test (ANT). NO 2 indoor pollution was 30.09 ± 9.51 µg/m 3 and ambient air pollution was 37.75 ± 18.41 µg/m 3 . Daily ambient levels were negatively associated with all attention processes (children in the bottom quartile of daily exposure to NO 2 had a 14.8 ms (95% CI: 11.2, 18.4) faster response time than those in the top quartile, which corresponds to a 1.1 month delay (95% CI: 0.84, 1.37) in natural development). Short-term exposure to NO 2 is associated with potential harmful effects on neurodevelopment.
Working memory was estimated by a computerized n-back test. Exposure to NO 2 was related to the slower development of working memory (β = −4.22, 95% CI: −6.22, −2.22). These reductions corresponded to a −20% (95% CI: −30.1, −10.7) change in annual working memory development associated with one interquartile range increase in outdoor NO 2 . Forns et al. [50] observed a persistent negative association between NO 2 levels at school and cognitive development over a course of 3.5 years. Therefore, they suggested that highly exposed children might face obstacles to fully achieve their academic goals.
Vert et al. [51] analyzed association between exposure to NO 2 and mental disorders on 958 residents from Barcelona (45-74 years old). Long-term residential exposure (period 2009-2014) was related to patients' self-reported history of anxiety and depression disorders. NO 2 exposure corresponded to 57.3 µg/m 3 (50.7-62.7). NO 2 increased the odd ratio for depression of 2.00 (95% CI: 1.37, 2.93) for each 10 µg NO 2 /m 3 increase. The study shows that long-term exposure to NO 2 may increase the incidence of depression.
Alemany et al. [52] analyzed on the group of children from the BREATHE project (N = 1667 at the age of 11 years), if there is any association between traffic-related air pollution and the ε4 allele of the apolipoprotein E gene, which is understood as a genetic risk factor for Alzheimer s disease.
Carey et al. [53] investigated the incidence of dementia to residential level of NO 2 in London. Among 130,978 adults aged 50-79 years was, in the period 2005-2013, 2181 subjects diagnosed with dementia (39% Alzheimer s disease and 29% vascular dementia). The average annual concentration of NO 2 was 37.1 ± 5.7 µg/m 3 . Higher risk of Alzheimer s disease was observed in subjects exposed to the highest concentrations of NO 2 (>41.5 µg/m 3 ) vs. subjects with the lowest concentrations of NO 2 (<31.9 µg/m 3 ) (HR = 1.40, 95% CI 1.12-1.74). These associations were more consistent for Alzheimer s disease than vascular dementia. Study found evidence of a positive association between residential level of NO 2 across London and being diagnosed with dementia.
Roberts et al. [54] explored the effect of NO 2 exposure to mental health problems in children in London, U.K. (N = 284). Symptoms of anxiety, depression, conduct disorder and ADHD were assessed at ages 12 and 18. NO 2 concentration in the year 2007 was 37.9 ± 5.5 µg/m 3 (IQR 34.1-41.7). They did not observe any association between NO 2 exposure in childhood and mental health problems at age 12. However, they detected association between NO 2 exposure and subsequent development of symptoms and clinically diagnosable depression and conduct disorders at age 18. They demonstrated that NO 2 exposure at age 12 years was significantly associated with major depressive disorder at age 18.
Prenatal exposure to NO 2 and sex dependent infant cognitive and motor development was analyzed by Lertxundi et al. [55] in children at 4-6 years of age, in four regions in Spain (N = 1119). Infant neuropsychological development was assessed by McCarthy scales: verbal, perceptive-manipulative, numeric, general cognitive, memory and motor. NO 2 exposure during pregnancy was from 18.7 ± 6.1 to 41.8 ± 10.7 µg/m 3 . The majority of cognitive domains were negative for NO 2 , associations were more negative for boys, statistically significant for memory, global cognition and verbal. These findings indicate a greater vulnerability of boys in domains related to memory, verbal and general cognition.
Jorcano et al. [56] assessed association between NO 2 and depressive and anxiety symptoms, and aggressive symptoms in children of 7-11 years, related to their prenatal and postnatal exposure. Data were analyzed in 13,182 children from eight European population-based cohorts. Prenatal NO 2 levels ranged from 15.9 to 43.5 µg/m 3 , postnatal levels ranged from 14.0 to 43.5 µg/m 3 . A total of 1108 (8.4%) and 870 (6.6%) children were classified as having depressive and anxiety symptoms, and with aggressive symptoms. Obtained results suggest that prenatal and postnatal exposure to NO 2 is not associated with depressive and anxiety symptoms or aggressive symptoms in children of 7-11 years old.
Loftus et al. [57] used the mother-child cohort from the CANDLE study and analyzed the impact of prenatal NO 2 exposure (22.3 ± 7.1 µg/m 3 ) and postnatal exposure (16.2 ± 4.7 µg/m 3 ) on childhood behavior (N = 975). In the sample 64% were African American, 53% had a household annual income below USD 35,000 and the child's age was 4.3 years. Mothers completed the child behavior checklist, a measure of problem behaviors in the past two weeks. The 4 µg/m 3 higher prenatal NO 2 was positively associated with externalizing behavior (6%, 95% CI: 1, 11%) and the effect of postnatal exposure was stronger (8%, 95% CI: 0, 16%). Prenatal NO 2 exposure was also associated with significant internalizing and externalizing behaviors. NO 2 exposure is positively associated with child behavior problems and African American and low SES children may be more susceptible.
Kulick et al. [58] examined in 5330 participants from the Northern Manhattan area of New York City the effect of long-term exposure to NO 2 (annual estimates 57.4 ± 22.1 µg/m 3 ) and PM 2.5 (annual estimates 13.1 ± 4.8 µg/m 3 ), predominantly in women, with a median age of 75.2 (±6.46) years. A + IQR increase of residential NO 2 was predictive of a 22.SD (95% CI, 0.30, −0.14) low global cognitive score at baseline and a more rapid decline (−0.06 SD; 95% CI −0.08, −0.04) in global cognitive function between biennial visits.
Erikson et al. [59] studied the association between NO 2 exposure and total gray matter and total white matter volumes in adults, using sample from UK Biobank. Participants were recruited from 2006 to 2010, a subset with magnetic-resonance brain imaging (MRI) included 18,292 participants, with an average age of 62  and NO 2 levels were 25.61 ± 6.86 µg/m 3 . The mean total gray-matter volume was 708,111 mm 3 (±47,940), the mean total white-matter volume was 708,111 mm 3 (±40,696). The total gray-matter volume was inversely associated with NO 2 (b = −103, p < 0.01). The effect of NO 2 on gray-matter volume was more pronounced in females (b = 161, p < 0.05). Obtained findings suggest that NO 2 concentrations lower than EU standard could be associated with reduced total gray-matter.
All reviewed studies indicate a significant health risk of NO 2 exposure at concentrations lower than the EU annual limit of 40 µg/m 3 : • Prenatal exposure impaired attentional function at the age of 4-5 years; • Induce neurobehavioral changes in children at the age of 8-10 years; • Affect attention process in children aged 8-12 years and induced changes are persistent for another 3.5 years; • Increase major depressive disorder at age 18; • Increase the incidence of dementia; • Exposure to NO 2 is associated with reduced total gray-matter.
The overall evidence presented in the mentioned studies suggests that attainment of the current EU annual limit for NO 2 of 40 µg/m 3 may not be sufficient for the protection of human health and further reductions of NO 2 concentrations would be beneficial and should be considered. In Switzerland, the current limit for the annual average of NO 2 is 30 µg/m 3 .

Measurement of NO 2 in Prague by Passive Samplers
To build up on this hypothesis, the measurements of NO 2 concentrations at various locations by passive samplers are examined. Some of the results were presented by Deutsche Umwelthilfe [60] as preliminary data; in this study, the results from Prague were examined in a greater detail.
For passive monitoring, membrane-closed Palmes tube [61] passive samplers (Passam, Switzerland [62]) were used. Several hundreds of samplers were placed at selected locations in the Czech Republic, out of which 65 were in Prague, during spring and fall of 2019 (46 and 58 samplers, respectively, a total of 104 samplers), each time for a period of approximately one month. The placement of the tubes generally followed the requirements set in the EU air quality directive (2008/50)-placement away from buildings at a breathing height 1.5-4 m, away from larger obstructions, and for traffic sites, within 10 m of curbside and, in most cases, over 25 m from intersections. In some cases, the samplers were placed closer to intersections, and in some cases, the samplers were placed in less conspicuous places such as behind a traffic sign (see photo in Figure 1), to reduce the chances of tampering. The expanded uncertainty (95% confidence) of the measurement given by the manufacturer is 18.3% for a concentration range 20-40 µg/m 3 [62]. The location of samplers is shown on an overview map in Figure 1. The same map also shows the locations of the national air quality monitoring stations referred to in this study. The measured concentrations are given in Table 1. For the spring campaign, the dates of the sampling are listed in the "spring measurement period" column, while for the fall campaign, a value is given when a measurement has taken place during the three sampling periods, as some locations were sampled twice. The spring, fall and overall average concentrations, divided by a correction factor of 1.185 (will be explained later in the manuscript) are given. For each location, the average daily vehicle traffic counts reported by the City of Prague Highway Department for 2019 [63] are reported. This table also reports vehicle counts adjusted for additional emissions due to inclines and intersections, these adjustments are discussed later in the manuscript.

Validation by Comparison with the Air Quality Monitoring Network
According to [64], passive diffusion tubes for measuring NO 2 concentrations in air were originally developed in the late 1970s for personal monitoring. They have been widely used in Europe for spatial and temporal measurement of NO 2 concentrations. The method has been found to be cheap, simple, and "provides concentration data in most circumstances that are sufficiently accurate for assessing exposure and compliance with Air Quality criteria" [64]. Reporting on a series of comparison tests, Buzica et al. [65] have concluded that "In the case of NO 2 , all the results of the laboratory and field experiments respected the requirements necessary for the demonstration of equivalence" and that the MCPT are equivalent to the reference methods for assessment of NO 2 . Passive diffusion tubes were reported to show a positive bias when sampling close to sources of NO, such as roadside or street canyons [64]. At the same time, prolonged (several weeks) sampling periods were reported to lead to negative bias [64]. A review done by the Joint Research Center of the European Commission [66], done in part to assess the feasibility of using the samplers for the long-term monitoring of nitrogen dioxide, with the particular aim of checking compliance with the European Union annual limit value of 40 µg/m 3 , citing a range of previous studies, reports that the "precision of the sampler showed that it is usually better than 5% when using a barrier or shelter to reduce effects of wind-induced turbulence" and that "the relative expanded uncertainty of individual results was estimated to be 32% for worst-case conditions", with lower values, generally <25%, obtained, for example, by parallel measurements with a reference method, by direct approaches, concluding that overall, "the Palmes tube is at least suitable for performing long-term measurements of NO 2 for indicative purposes, and possibly even for fixed measurements". Recent review of biases associated with Palmes tube type passive samplers by Heal et al. [67] suggests that "The effect of net bias can be reduced by application of a local "bias adjustment" factor derived from colocations of PDTs with a chemiluminescence analyzer. When this is carried out, the PDT is suitable as an indicative measure of NO 2 for air quality assessments".
To evaluate the bias, the data from passive samplers were compared to the data from selected relevant stations of the national air quality monitoring network, listed in Table 2. The national network uses chemiluminescence analyzers capable of measuring both NO and total NO x , with NO 2 calculated as the difference of total NO x and NO. The uncertainty of the measurements is periodically determined through analysis of reference samples, repeated measurements of the same sample, interlaboratory exercises, and for 2019, was reported to be a combination of absolute uncertainty of 2.3 µg/m 3 and a relative uncertainty of 12.3% [68].
The results of this comparison are given in Figure 2. In each case, the value reported by the passive sampler was compared to the average of hourly values from the monitoring station over the period during which the sampler was exposed. The three larger points (in red/orange) represent two samplers colocated with the Karlín monitoring station over two separate one-month periods and one sampler colocated with the Vysočanská monitoring station, show a linear correlation with a slope of 1.185 (at zero intercept; standard error of slope 0.008; differences passive sampler vs. monitoring station of +20%, +17% and +18%). While it can be argued that a regression of three points has a limited meaning, in this case, it shows that three different samplers, each used in a different time period, has produced readings that are a consistent multiple of the monitoring station data. Additionally, two samplers placed at the city urban background reference station for particulate matter (Suchdol campus of the Czech Academy of Sciences, last two lines in Table 1) during the same time period show a relative difference of 6%. These findings are in line with the 5% precision of the Palmes tube samples reported in [66].  Smaller blue points in Figure 2 show additional locations. Two samplers were placed at an urban background monitoring station Suchdol, however, data from this station was not available, and the readings are compared with another background monitoring station in Kobylisy. Two samplers were placed near Náměstí Republiky monitoring station, but a few dozen meters away and near an exit/entrance ramp to a large shopping center underground parking garage. Two samplers were placed on the corner of Legerova and Rumunská, near the monitoring station but at an intersection controlled by a traffic light. The readings from these four samplers were higher than from the monitoring station, which can be reasonably expected as they were near stopped and accelerating vehicles. The slope for the additional samplers was 1.17 with a standard error of 0.09; it should be noted that differences between actual NO 2 concentrations at the sampler and at the monitoring station are most likely the largest source of uncertainty.
Additional samplers close to the Legerova station (about 150 m from a large intersection) were closer to intersections and therefore exposed to additional cross-traffic, in addition to the increase in emissions rates in the vicinity of intersections. Two samplers were also placed at the Legerova monitoring station (urban hotspot) in the spring of 2019, but both were stolen. Additional samplers were placed near the Karlín monitoring station and near the Náměstí Republiky monitoring stations, and in the general vicinity of the Legerova station. The NO 2 concentrations reported for the samplers were compared with the average NO 2 concentrations measured by the monitoring station, obtained by averaging data over the time the samplers were exposed on the site.
Additional samplers used in the comparison were at reasonably close locations with not overly dissimilar traffic, and were not too far from the 15% tolerance reported by the Defra report [64]. It should be noted that the tolerance is applicable to the deviation of the sampler-reported and reference value, and not to the differences due to the samplers being at different locations with different emissions characteristics.
For all subsequent data analysis, the concentrations from the passive samplers were divided by the regression slope of 1.185. It should be noted that while this correction represents the best judgment by the authors, it is based on limited data and could be viewed as arbitrary, as the difference could arise out of the 12.3% uncertainty of the reference measurement the manufacturer-reported 18% expanded uncertainty of the passive sampler.

Comparison of NO 2 during Passive Samplers Deployment with Long-Term Averages
The variation of climatic and weather conditions is an additional source of bias to consider when comparing passive samplers to annual mean values. Figure 3   The consistency of the measurement by passive samplers during spring and fall periods is shown, along with data from the reference monitoring stations, in Figure 4. The slope of regression (with intercept forced through zero) was 0.91 ± 0.05 for the monitoring stations and 0.92 ± 0.02 for the passive samplers, showing that the monitoring stations and the passive samplers reported the same overall trends in NO 2 concentrations.

Effects of Traffic
For further analysis, all passive sampler measurements were divided by a factor of 1.185 (the slope of regression of passive sampler vs. reference NO 2 , see Figure 1).
The relationship between the vehicular traffic intensity and the NO 2 concentrations measured by the passive samplers is given in Figure 5. As samplers were used over two different periods, they are plotted separately in two series, one for each period, along with the average values from Legerova and Náměstí Republiky monitoring stations. It appears that there is a moderate positive trend of NO 2 increasing with traffic. Additionally, samplers located next to an uphill section of a divided highway (or a one-way street with the traffic going in the uphill direction) and next to an intersection tend to exhibit higher NO 2 concentrations. It also appears that the NO 2 concentrations are higher in urban canyons and congested streets of the city center and near intersections. To assess whether high NO 2 are associated with truck traffic, samplers located in the area with limited access of vehicles over 6 tons gross weight (entry by permit only, restricted to local traffic) are plotted separately in Figure 6 (for locations where multiple samplers were used, average values are plotted). It is clear from the figure that the highest NO 2 were measured in areas where trucks over 6 tons are mostly excluded.
To account for additional emissions due to hills and intersections, the intensity of traffic traveling uphill was increased by 100% to account for additional fuel consumption, and for samplers located at intersections, the intensity of traffic was increased by 300% to account for fuel consumed at idle and when accelerating (where the intersection was without a major delay, such as time-synchronized signals at intersections of a larger oneway street with a side street or pedestrian crossing, the factor was reduced by one half). These adjustments factors were arbitrarily selected based on experience with vehicle emissions behavior (additional emissions due to climbing a hill, additional emissions due to idling at intersections and acceleration from intersections) and were independent of each other. (Note: as an example of rough calculation for a passenger car diesel engine, the acceleration of a 1500 kg car from 0 to 50 km/h requires a gain of kinetic energy of 145 kJ or 40 Wh, corresponding, at 250 g/kWh engine fuel consumption, to 10 g of fuel. The fuel consumption at idle is about 5 g/min. A one-minute stop and acceleration consumes 15 g of fuel. Driving at steady speed requires about 30 g of fuel per km, or 3 g per 100 m. If half of the cars stop and wait, the emissions in a 100 m segment around the intersection are 9 g, compared to 3 g in the case of free-flowing traffic. For simplicity, NO x emissions are assumed to be proportional to the fuel consumption.) The relationship between the adjusted vehicle volume and NO 2 concentrations is plotted in Figure 7.  The relatively strong correlation between the adjusted traffic volumes and NO 2 concentrations (R 2 = 0.78 for September-October data and 0.76 for spring-fall averages; slope 0.13 ± 0.01; intercept 27 ± 1 µg/m 3 ) suggests that "local" NO 2 , comprising of primary NO 2 emitted from the tailpipe and NO 2 formed locally from NO by reaction with ozone (i.e., [69]), is a considerable and in many locations dominant source of NO 2 . There is no observable difference between the sampling locations where truck traffic over 6 tons was excluded and the locations where it was not excluded. Overall, there seems to be a very strong correlation between the estimated relative intensity of mobile source emissions and the measured NO 2 concentrations. It is likely that the correlation could be further improved by taking into the account distance from the traffic, traffic on adjacent streets, tunnel exits and other compounding factors.
A similar plot of the regression of the dependency of NO 2 on adjusted traffic volumes is plotted separately for the spring and fall campaigns in Figure 8, with red line denoting the legal annual NO 2 limit of 40 µg/m 3 and green line the Swiss federal limit of 30 µg/m 3 (shown for illustration in support of the health review). The regression shows that NO 2 concentrations, in all cases, increased by 0.13 µg/m 3 per 1000 vehicles daily traffic volume, adjusted for uphill and intersections, where adjusted traffic count is traffic count multiplied by a factor of (1 + fraction of vehicles travelling uphill + 3 × fraction of vehicles stopping at an intersection). It should be noted that the intercept of the regression (25-28 µg/m 3 in Figures 7 and 8; (standard error of slope is 0.01; standard error of intercept is 1 µg/m 3 ) is higher than the "urban background" concentrations of 15-20 µg/m 3 , most likely due to accounting only for traffic on major roads and not for parking garages, taxi waiting areas, and similar locations. Even the urban background concentrations cannot be considered as NO 2 concentrations that would be theoretically be expected if no motor vehicles were operated in Prague, due to the dispersion and transport of the pollutants. Even at a rather conservative adjustment of the passive sampler readings (according to the regression, the sampler readings were 18% higher, however, this was, to a large extent, due to many samplers being at locations where the concentrations would reasonably be expected to be higher than at the corresponding monitoring station), it is clear from Figure  7 that the annual average limit of 40 µg/m 3 NO 2 is likely to be exceeded at numerous locations throughout Prague, generally, where the adjusted traffic volumes exceed the equivalent of 100 thousands of vehicles per day. This is, for example, the north-south passageway through the center city (Wilsonova, Sokolská and Legerova street) with many intersections, but also roads like V Holešovičkách (a six-lane road with 85-90 thousand vehicles per day, with a gradient of approximately 3%), a possible new hot-spot in Prague. In the worst case (intersection of two one-way streets with all vehicles traveling uphill), this limit could be reached already at 20 thousand vehicles per day, as also apparent from Figure 6.

Effects of Travel Restrictions on Ambient NO and NO 2 Concentrations
In order to assess the contribution of light and heavy vehicles to NO and NO 2 concentrations, hour-by-hour NO and NO 2 ambient air quality data from the national air quality monitoring network was analyzed for a period of 14 March-30 April 2020, during which travel restrictions were imposed, including the prohibition of all non-cargo international travel (truck traffic was exempted). For reference, the same period was assessed for four previous years.
A total of five stations in Prague were selected: a. Legerova street, considered an urban hotspot, with about 45 thousand vehicles traveling daily in one direction (with similar traffic volumes in the opposite direction on a parallel street), primarily (97-98%) light-duty vehicles (trucks over 12 tons are restricted from entering inner Prague and trucks over 6 tons are restricted in the Prague historical district); b.
Vysočanská street and Průmyslová street, two traffic stations located on heavily traveled main roads used by local and transit truck traffic; c.
Náměstí Republiky, urban background station in a historical city center, on the border of pedestrian area d.
Kobylisy, a station in a suburban residential neighborhood e.
For comparison, a rural background station in Košetice, serving as the Czech national reference station, was used as a reference.
Arithmetic and geometric means and the NO 2 /NO x ratios are plotted, for each station and all years, in Table 3. A single-factor analysis of variance (ANOVA) was performed to compare the variances among the five data sets (one for the year 2020, four for each of the reference years 2016-2019) with the differences within the sets. The associated p-value (p1) was compared to the p-value (p2) associated with the difference between mean for the year 2020 and the grand mean for all five years. The higher of the p2/p1 ratio and the p2 (ensuring that the significance of the difference of the year 2020 is much higher than the difference among the years) is then considered the resulting p-value of the test.
As an alternative analysis, the statistical difference of data from each year from the combined data set for all five years was evaluated using a t-test, and the p-value associated with the test for the year 2020 was divided by the average of the four p-values associated with each of the four reference years.
It is apparent from the Table 2 that NO concentrations significantly decreased at all three traffic stations, with a highest mean decrease of 46% at Legerova and at the Košetice rural background station. The decrease in NO 2 concentrations was lower than for NO at all Prague stations, highest at Legerova (20%), and even higher (40%) at the Košetice rural background station. As vehicles emit primarily NO, the NO 2 /NO x ratio tends to increase with the age of the emissions, being lowest (around 60%) at Legerova street, 65-70% at Vysočanská, Průmyslová and Náměstí Republiky, 80% at the Kobylisy residential background station and around 90% at the rural station in Košetice. One possible interpretation of the increase in the NO 2 /NO x ratio at Legerova could be that the primary emissions of both NO and NO 2 were reduced, with lower reduction in "background" NO 2 originating from NO x emitted elsewhere. Another possible explanation is the reaction of NO with ozone, yielding NO 2 [70]. Both March and April of 2020 were substantially sunnier than average-4 sunny days and 180 h of sunshine in March and 13 sunny days and 290 h of sunshine in April, compared to 1981-2010 average of about 3 sunny days and 120 h of sunshine for March and 3-4 sunny days and 180 h of sunshine for April [71]. Table 3. Comparison of NO and NO 2 concentrations at six monitoring stations during March-April 2020 travel restrictions with the same period during the prior four years.

Discussion
A detailed analysis of NO 2 concentrations measured by the passive samplers shows a clear correlation of NO 2 concentrations with daily traffic counts, adjusted for additional emissions due to uphill travel and stopping at intersections. This finding is in good agreement with the data from the monitoring stations, which, by themselves, are too sparse to make such inference. The correlation of NO 2 concentrations with vehicular traffic intensity is also apparent from the comparison of the data from state air quality monitoring stations during the period of 14 March-30 April 2020, during which travel restrictions were imposed, including the prohibition of all non-cargo international travel, with comparable periods of four previous years. Overall, the findings confirm that vehicular traffic, through primary NO 2 emissions (and possibly through fast reaction of primary NO with ozone), directly affects the NO 2 concentrations in the immediate vicinity.
This correlation, along with correlation of passive sampler readings and air quality monitoring stations, and good consistency of reported NO 2 concentrations among samplers used within the same location at different time periods, all suggest that passive samplers appear to provide, at a reasonable cost and effort, a fairly good image of the distribution of NO 2 concentrations. Judging from limited data, the passive samplers were found to measure about 18.5% higher values than the monitoring stations. Repeated-and most likely deliberate-removals of passive samplers from the immediate vicinity of the monitoring stations have prevented a more quantitative comparison. A comparison of a broader set of data reveals a slightly smaller bias, contributed to, in several cases, by the passive samplers being at more exposed locations (i.e., near the exit of a large underground parking garage) than the monitoring stations. The true bias could therefore be possibly even lower.
Since the trends are comparable within and outside the heavy truck exclusion area, this seems to be primarily an effect of cars and other lighter vehicles (per city statistics, about 90% of traffic is passenger cars [63]). Additionally, there is no correlation between the measured NO 2 concentrations and the heavy vehicle traffic count or between the measured NO 2 concentration and the fraction of heavy vehicles. This is in line with the findings that truck NOx emissions have decreased to a considerably higher extent than those of diesel cars in Europe.
The samplers at the locations with highest fraction of heavy vehicles (10-15%, vs. average for all locations 4%) and with the highest absolute heavy vehicle counts (7-16 thousands/day, vs. average 1.7 thousands/day) have measured 25-35 µg/m 3 NO 2 , which is in the second lowest quartile (median concentration is 35 µg/m 3 ). This may also be, in part, due to a dependent factor that heavy vehicle traffic is limited in the high population density city center.
The monitoring station at Legerova street is most likely not the absolute hot-spot-it is expected that the emissions of NO x would be higher on the parallel street where the vehicles travel uphill (Legerova is one-way street downhill) and at nearby intersections. The street V Holešovičkách, a six-lane road, which is, unlike most other roads of similar size, immediately bordered by residential neighborhoods, with a traffic intensity approaching 100 thousand vehicles per day, a major increase after the opening of a new complex of tunnels providing an alternative route through congested areas, further complicated by a 3% grade, could easily be the next traffic hot-spot.
Considering the finding that about half of the vehicles traveling on the road are not older than 7 years [27], and the several-fold decrease in NO x emissions standards over the last decade and half, a much sharper decrease of NO 2 concentrations would be expected than the approximately 1% annually reported by Hůnová [5]; a higher reduction of about 2.5% annually was observed in Western Europe, and about 4.7% annually in United States and Canada [74]. Given the decrease in the limit values of roughly two thirds from Euro 3 (0.50 g/km NO x , 2000) to Euro 5 (0.18 g/km, 2009-2010) and from Euro 4 (0.25 g/km NO x , 2005) to Euro 6 (0.08 g/km, 2014-2015), the introduction of Euro 5 in late 2009 and Euro 6 in late 2014 should have resulted in about a two thirds NO x reduction in at least half of the vehicles, or about one third reduction in NO x emissions in general. As learned from the analysis of the effects of traffic restrictions, the effect on NO 2 concentrations may be different, and possibly somewhat smaller than the reduction in NO x emissions, due to atmospheric chemistry. The effects of such a decrease could also have been diminished by an increase in traffic, however, in the center city, the intensity of automobile traffic has been stagnating, or even slightly decreasing.
The mediocre decrease in NO 2 concentrations, despite more dramatic reduction being expected from improving vehicle technology, is in line with earlier findings that the real NO x emissions of diesel vehicles did not decrease despite the decreasing emissions limits. The situation should have been, however, substantially remedied by "post-Dieselgate" vehicles and by repairs of vehicles affected by Dieselgate. Since it was not, a question therefore arises as to the possibility that Dieselgate relevant repairs were not done on a sufficient number of vehicles and/or were not sufficiently effective and/or were reversed to the "original factory conditions" by the vehicle owners. The authors do not have any reliable statistics on this matter. Furthermore, considering that all three mentioned situations could be associated with criminal offenses and/or considerable civil penalties, detailed investigation of the matter is likely to be considerably difficult.
If there is no assurance that the NO 2 concentrations will decrease dramatically due to a radical improvement in primary NO x emissions, the only other suitable strategy to improve the air quality is to reduce, to the extent required, the intensity of vehicular traffic. Contrary to the remote regions where automobiles are, in most cases, the only practical means of travel, Prague has an extensive network of public transit. According to the City of Prague statistics [63], only 29% of trips in Prague are done by automobile, 26% of trips are by walking and 42% of trips by public transit. Of the public transit, slightly over one third is done by subway, and another third by trams and commuter rail, which are, with the exception of a rather small number of diesel rail cars used on sparsely traveled rail lines, run on electric power, and therefore with very small effect on NO 2 emissions. The remaining third of trips is by diesel buses, the majority of which are equipped with SCR catalysts, and potentially reaching NO x emissions not much larger (and according to measurements possibly even smaller) levels, per kilometer and vehicle, than an average diesel car. It is therefore readily apparent that shifting from an average automobile to any other means of transport is likely to reduce the NO 2 concentrations. (Shift to electric power, compressed natural gas, or other "clean" propulsion is a gradual process and is unlikely to be done, within a few years, on a sufficiently large number of vehicles to make a difference throughout the city).

Summary and Conclusions
Despite massive reductions in diesel cars NO x emission limits, of about two thirds from Euro 3 to Euro 5 and from Euro 4 to Euro 6, NO 2 concentrations throughout the Czech Republic have been decreasing at a mediocre rate of 1% annually.
A review of the underlying engine emissions trends shows that the conversion of NO into NO 2 in diesel oxidation catalysts, beneficial for regeneration of diesel particle filters and for the functioning of the SCR systems for NO x reduction, did not, contrary to the intentions of the legislation, go hand in hand with a major reduction of NO x emissions in subsequent (downstream) NO x aftertreatment devices. As a result, primary NO 2 emissions from light duty diesel vehicles are in most cases considerably higher than intended in the emissions legislation due to non-adherence of many manufacturers to the primary intent of the legislation.
A review of the health effects on NO 2 on children shows that all reviewed studies indicate a significant effect of prenatal NO 2 exposure to children s neurobehavioral development, in adults to dementia at concentrations lower than EU standards of 40 µg/m 3 /year. These results should be understood as a strong recommendation to reduce the NO 2 concentrations below the current EU standard. All presented studies prove that NO 2 can significantly deteriorate CNS and therefore this knowledge should be used to improve the quality of our lives.
To elucidate the effects of motorized traffic on NO 2 concentrations, data from 104 passive NO 2 samplers deployed at 65 locations in Prague during March-April and September-October of 2019 were examined. Comparisons with the national monitoring network show a positive bias of 18.5% for colocated samplers and 17% for samplers nearby (or in similar settings as) the monitoring stations. There was a good correlation among repeated measurements at the same locations. The data from the national air quality monitoring network show that the average concentrations in both spring and fall sampling periods were consistent with 2016-2019 averages.
The average measured NO 2 concentrations at the selected locations, after correcting for the 18.5% bias, were in the range of 16-69 µg/m 3 , with a mean of 36 µg/m 3 and a median of 35 µg/m 3 , and were higher than the EU and national limit (annual average) of 40 µg/m 3 at 32% of locations. The NO 2 concentrations have correlated well with the intensity of traffic (average daily vehicle counts), corrected for additional emissions due to uphill travel and due to idling at, and accelerating from, intersections. Several additional "hot-spots" were identified, in addition to the "hot-spot" monitoring station at Legerova street (2016-2019 NO 2 average of 51 µg/m 3 ), where the vehicles travel on a slight decline on a one-way street: several intersections at Sokolská street, parallel with Legerova with uphill direction of travel, and emerging hot-spots along V Holešovičkách street, where the traffic intensity increased due to the opening of a new series of tunnels. Analysis of the effect of coronavirus related travel restrictions were evaluated by comparing the data from six monitoring stations (15 March-30 April 2020, relative to the same period during 2016-2019) reveal a reduction of NO, NO 2 and NO x (except for a small increase of NO 2 at one of the background stations), with NO reduction being, at high traffic locations, higher than that of NO 2 . The spatial analysis of data from passive samplers and time analysis of data during the travel restrictions both demonstrate a consistent positive correlation between traffic intensity and NO 2 concentrations along/near the travel path.
It appears that decreases in vehicle NO x emission limits, introduced in the last decade or two, have failed to sufficiently reduce the ambient NO 2 concentrations in exposed locations in Prague. This is in part due to increased fraction of NO 2 in NO x in newer vehicles, and in part due to "a major disparity between the numerical value of the emission limit and the actual emissions in everyday driving". Further, there is no apparent sign of, and it is far from clear that, the "excess emissions" of NO x , a problem known as Dieselgate, have been efficiently remedied. Data Availability Statement: Most of the relevant data is contained in the manuscript. Sampling and analytical protocols associated with passive samplers are available from Miroslav Šuta. Traffic volume data are publicly available, see the link in the reference list. Data from the national air quality monitoring network are a third-party data and must be requested directly from the Czech Hydrometeorological Institute.