Air Quality outside Schools in Newcastle upon Tyne, UK: An Investigation into NO 2 and PM Concentrations and PM Respiratory Deposition

: Air pollution is the principal environmental threat to public health in the UK. Ever-increasing evidence links ambient air pollutants, preventable diseases, and health inequalities. Children are particularly vulnerable to harmful effects due to their short height, developing lungs, and higher rate of respiration. Using data from air quality monitors around schools, we investigated 2018–2019 ambient NO 2 , PM 10 , PM 2.5 , and PM 1 concentrations at 12 schools in Newcastle upon Tyne, UK. We compared ﬁndings with EU/UK air quality regulations and guidelines, identiﬁed patterns, and calculated PM respiratory deposition doses (RDDs). The range of annual average (AA) concentrations across the schools for the two-year period was 23.7–39.2 µ g/m 3 for NO 2 , 7.4–22.2 µ g/m 3 for PM 10 , 3.5–11.6 µ g/m 3 for PM 2.5 , and 1.7–9.0 µ g/m 3 for PM 1 . The highest PM RDD children were exposed to at school was 30 µ g/h. One school’s AA NO 2 , two schools’ hourly PM 2.5 averages, and one school’s 24-h PM 10 averages exceeded EU/UK regulations. All schools exceeded WHO 2005 24-h PM 10 and PM 2.5 guidelines in 2018, less in 2019. All 12 schools would have exceeded WHO 2021 NO 2 AA guidelines (10 µ g/m 3 ), 2 the WHO 2021 PM 10 AA (15 µ g/m 3 ), and 10 the WHO 2021 PM 2.5 AA (5 µ g/m 3 ). Evidence-based policy is required to improve school ambient air quality and reduce children’s exposure.


Introduction
Ambient air pollution is a public health issue of global concern and the principal environmental threat to public health in the United Kingdom (UK) [1]. An ever-increasing body of evidence demonstrates both associative and causal relationships between ambient air pollutants, preventable diseases, and health inequalities [2,3]. Air quality is an area of rapidly evolving policy; however, effecting change presents a significant challenge as ambient air pollutants are by-products of processes that are fundamental to how we currently live our lives [1].
Ambient air pollutants adversely affect human health and children are among the most vulnerable to these harmful effects due to their short stature, developing lungs, and higher rate of respiration [1,4,5]. The effects of ambient air pollutants on children's respiratory systems include suppression of lung growth, an increased risk of new-onset asthma and wheeze, an increased risk of bronchitis, and an increased risk of problematic respiratory symptoms [5][6][7]. Considering the impacts on children's other systems, exposure to ambient air pollution is associated with decreased concentration and alertness in children and may There are 92 government-funded schools and 9 independent schools in Newcastle [28]. In total, 22 schools in Newcastle have static air quality monitors (AQMs): 21 government-funded schools and 1 independent nursery. These AQMs have been provided to the schools through a partnership between NCC and the Newcastle Urban Observatory (UO) as part of a project called 'The Healthy Schools Project' [29]. The aim of the air quality work within this project is to raise awareness and engage schools, parents, and children in monitoring school ambient air quality [29].
This study aimed to investigate ambient air quality for 12 Newcastle schools. Specific objectives were to: (1) determine concentrations of NO2, PM10, PM2.5, and PM1 outside schools in Newcastle and investigate temporal, geographical, and meteorological patterns; (2) determine whether concentrations of these pollutants regularly exceed the WHO guideline values or EU/UK regulations; and (3) estimate students' exposure to inhalable particles.

Data Collection
Our study used Newcastle UO data [29,30], which are publicly available on the UO website. The UO collects ambient air quality data for the city of Newcastle and surrounding areas [31,32]. These data had not yet been formally analysed at the inception of this study, presenting an invaluable opportunity to do so.
Eleven schools in Newcastle had AQMs installed in 2017 or early 2018 after being identified by NCC as being located in potential pollution hotspots, one school already had an AQM in place, and a further 10 schools applied for an AQM as part of NCC's Healthy Pupil Capital fund and had AQMs installed in 2019. The UO's Healthy Schools Project uses fixed-location AQMesh pod AQMs to collect ambient air quality data. AQMesh pods are low-cost indicative AQMs manufactured in the UK that provide real-time ambient air quality data [33]. Electrochemical sensors in the monitors measure NO2 and light-scattering optical particle counters measure PM [34]. AQMesh pods are continuous monitors and generate an average reading every 1-15 min [34]. Technical information on AQMesh pod Quality Assurance can be found in the Supplementary Materials. Placement of the monitors outside the schools in the Healthy Schools Project is carefully considered by There are 92 government-funded schools and 9 independent schools in Newcastle [28]. In total, 22 schools in Newcastle have static air quality monitors (AQMs): 21 governmentfunded schools and 1 independent nursery. These AQMs have been provided to the schools through a partnership between NCC and the Newcastle Urban Observatory (UO) as part of a project called 'The Healthy Schools Project' [29]. The aim of the air quality work within this project is to raise awareness and engage schools, parents, and children in monitoring school ambient air quality [29].
This study aimed to investigate ambient air quality for 12 Newcastle schools. Specific objectives were to: (1) determine concentrations of NO 2 , PM 10 , PM 2.5 , and PM 1 outside schools in Newcastle and investigate temporal, geographical, and meteorological patterns; (2) determine whether concentrations of these pollutants regularly exceed the WHO guideline values or EU/UK regulations; and (3) estimate students' exposure to inhalable particles.

Data Collection
Our study used Newcastle UO data [29,30], which are publicly available on the UO website. The UO collects ambient air quality data for the city of Newcastle and surrounding areas [31,32]. These data had not yet been formally analysed at the inception of this study, presenting an invaluable opportunity to do so.
Eleven schools in Newcastle had AQMs installed in 2017 or early 2018 after being identified by NCC as being located in potential pollution hotspots, one school already had an AQM in place, and a further 10 schools applied for an AQM as part of NCC's Healthy Pupil Capital fund and had AQMs installed in 2019. The UO's Healthy Schools Project uses fixed-location AQMesh pod AQMs to collect ambient air quality data. AQMesh pods are low-cost indicative AQMs manufactured in the UK that provide real-time ambient air quality data [33]. Electrochemical sensors in the monitors measure NO 2 and light-scattering optical particle counters measure PM [34]. AQMesh pods are continuous monitors and generate an average reading every 1-15 min [34]. Technical information on AQMesh pod Quality Assurance can be found in the Supplementary Materials. Placement of the monitors outside the schools in the Healthy Schools Project is carefully considered by experts at the UO to ensure optimum data quality and they are commonly attached to lampposts at school entrances (Supplementary Materials Figure S3).
This study used meteorological data collected at the Albemarle Airfield weather station, west of the city. This station collects hourly data on wind speed, wind direction, temperature, atmospheric pressure, and humidity.

Data Analysis
Due to the varying installation dates of AQMs and start dates of data collection, only schools with sufficient data to provide annual averages were included in each analysis. This study included 12 schools and further information on the schools can be found in the Supplementary Materials Table S3. Specialist air quality data analysis was performed using "openair", an R package for air quality data analysis [35]. Data flagging was conducted to highlight outliers based on the Breathe London study [36]. Breathe London's NO 2 data flags use parts per billion (ppb), but the UO Healthy Schools project collects NO 2 data in µg/m 3 ; therefore, the NO 2 conversion factor of 1 ppb = 1.1925 µg/m 3 was used in this study [37]. Data flagging values can be found in Supplementary Materials Table S4. The EU Air Quality Directive 2008 outlines data quality objectives and states that the minimum data capture from AQMs should be 90% for NO 2 , PM 2.5 , and PM 10 ; therefore, this value was adopted as a data quality measure [11].
Fixed monitoring site air quality data at the urban background and roadside locations in Newcastle were obtained from the UK-AIR database to compare school air quality with the air quality of the wider city (Figure 1). At urban background sites, ambient pollutants are not influenced by a single source but instead represent city-wide background concentrations [38]. The Newcastle urban background air quality monitoring site collects NO 2 , PM 10 , and PM 2.5 data. Roadside air quality monitors are located at sites where concentrations of ambient pollutants are determined by nearby traffic emissions [39]. The Newcastle roadside air quality monitoring site collects NO 2 and PM 10 data.

Respiratory Deposition Dose (RDD)
Aerosolised particles, such as PM, become harmful to health when they are inhaled and deposited (remain after expiration) in the respiratory tract. Understanding the dose of PM deposited in different settings helps us to understand the health risk posed by the PM concentrations in these settings [40]. RDD estimates provide an indication of PM 10 , PM 2.5 , and PM 1 deposition in the respiratory tract [41]. RDD is calculated using Equation (1) [41,42]: where DF is the deposition fraction of particles in the respiratory, V T is the volume of air inhaled per breath (m 3 ), f is the respiratory rate (breaths/minute), C is the concentration of particles in the air (µg/m 3 ), and T is the exposure time, the amount of time spent in an activity or setting (minutes) [42][43][44]. DF is the fraction of inhaled particles that deposit in the respiratory tract from the extrathoracic region to deep in the thoracic region [45,46]. RDD regional depositions are calculated (head airways, tracheobronchial region, and alveolar region) and the total deposition in the respiratory tract is the sum of the regional depositions [40]. The deposition of aerosolised particles within the respiratory system is complex and determined by the characteristics of both the exposed individual and the inhaled particle [43][44][45]. Characteristics of the particle that impact on DF include its shape, density, chemical composition, and size [43]. DF is estimated using Equation (2): where d p is the particle size (µm) and IF is the inhalable fraction (fraction of ambient particles present in the volume of air before inspiration that enter the nose and mouth) used in the International Commission on Radiological Protection (ICRP) model [47]. IF is estimated using Equation (3): where (V T × f × T) for light activity was calculated as 0.90 m 3 /h for school students in this study [41,42,48].

Ambient Air Quality in Newcastle during the Study Period
Data from the two precision air quality monitoring sites in Newcastle were analysed to understand urban background and roadside air quality in the city (see Supplementary Material Table S5). Data captured at the two precision monitoring sites ranged from 83-99%. In 2018, the annual mean NO 2 concentration (±standard deviation) at the urban background site was 28.6 ± 16.4 µg/m 3 and at the roadside site, it was 39.0 ± 26.2 µg/m 3 . In 2019, the annual mean NO 2 concentration at the urban background site was 32.1 ± 15.8 µg/m 3 and at the roadside site, it was 38.3 ± 27.7 µg/m 3 . In 2018, the annual mean PM 10 concentration at the urban background site was 12.4 ± 12.7 µg/m 3 and at the roadside site, it was 15.5 ± 11.8 µg/m 3 . In 2019, the annual mean PM 10 concentration at the urban background site was 15.3 ± 30.1 µg/m 3 and at the roadside site, it was 16.4 ± 12.2 µg/m 3 . In 2018, the annual mean PM 2.5 concentration at the urban background site was 9.1 ± 7.8 µg/m 3 and in 2019, it was 8.9 ± 7.8 µg/m 3 .
When comparing pollutant concentrations at the two sites, these data indicate that in both 2018 and 2019, NO 2 and PM 10 concentrations were higher at the roadside site than at the urban background site. When comparing pollutant concentrations by year, these data suggest that concentrations of PM 10 increased between 2018 and 2019 at both the urban background site and the roadside site. Daily average NO 2 , PM 10 , and PM 2.5 concentrations at the urban background and roadside sites from 1 January 2018-31 December 2019 are shown in Figure 2. The deposition of aerosolised particles within the respiratory system is complex and determined by the characteristics of both the exposed individual and the inhaled particle [43][44][45]. Characteristics of the particle that impact on DF include its shape, density, chemical composition, and size [43]. DF is estimated using Equation (2) where dp is the particle size (μm) and IF is the inhalable fraction (fraction of ambient particles present in the volume of air before inspiration that enter the nose and mouth) used in the International Commission on Radiological Protection (ICRP) model [47]. IF is estimated using Equation (3): where for light activity was calculated as 0.90 m 3 /h for school students in this study [41,42,48].

Ambient Air Quality in Newcastle during the Study Period
Data from the two precision air quality monitoring sites in Newcastle were analysed to understand urban background and roadside air quality in the city (see Supplementary Material Table S5). Data captured at the two precision monitoring sites ranged from 83-99%. In 2018, the annual mean NO2 concentration (±standard deviation) at the urban background site was 28.6 ± 16.4 μg/m 3 and at the roadside site, it was 39.0 ± 26.2 μg/m 3 . In 2019, the annual mean NO2 concentration at the urban background site was 32.1 ± 15.8 μg/m 3 and at the roadside site, it was 38.3 ± 27.7 μg/m 3 . In 2018, the annual mean PM10 concentration at the urban background site was 12.4 ± 12.7 μg/m 3 and at the roadside site, it was 15.5 ± 11.8 μg/m 3 . In 2019, the annual mean PM10 concentration at the urban background site was 15.3 ± 30.1 μg/m 3 and at the roadside site, it was 16.4 ± 12.2 μg/m 3 . In 2018, the annual mean PM2.5 concentration at the urban background site was 9.1 ± 7.8 μg/m 3 and in 2019, it was 8.9 ± 7.8 μg/m 3 .
When comparing pollutant concentrations at the two sites, these data indicate that in both 2018 and 2019, NO2 and PM10 concentrations were higher at the roadside site than at the urban background site. When comparing pollutant concentrations by year, these data suggest that concentrations of PM10 increased between 2018 and 2019 at both the urban background site and the roadside site. Daily average NO2, PM10, and PM2.5 concentrations at the urban background and roadside sites from 1 January 2018-31 December 2019 are shown in Figure 2. This figure demonstrates that temporal variations exist in the NO 2 concentrations at the two precision sites in Newcastle over the course of a year. NO 2 concentrations are at their highest at both sites between November and January (late autumn to mid-winter), gradually decline to their lowest in July (mid-summer), and then gradually increase again through autumn and winter months. This apparent decline in the urban background and roadside NO 2 concentrations during the spring and summer months in Newcastle could be attributable to the increased temperatures in these months leading to less personal vehicle use, greater use of active travel (such as walking or cycling), and lower heating requirements.
PM concentrations show less noticeable variation over the course of a year; however, they do show occasional spikes in concentration, with the most obvious spike between January and July 2019 at the urban background site. Spikes in PM concentrations could be attributable to specific events, such as occasions when fireworks displays occur or emissions of dust from construction works. Spikes in the winter months could be attributable to increased fuel usage for heating and increased use of personal vehicles.

Ambient Air Quality Outside the 12 Schools in the Study Period
Data capture and data flagging of school data are reported in the Supplementary Materials (Tables S6-S8).

NO 2 Concentrations
In 2018, annual mean NO 2 Table S5 and Figure 3a.
The monthly mean NO 2 concentrations for all 12 schools combined can be seen in Figure 4a. The overall mean NO 2 concentrations are lowest in the late spring, summer, and early autumn months (May-September) and highest in the winter months (October-February). When considering monthly NO 2 concentrations at an individual school level, consistently high monthly mean NO 2 concentrations were observed from April to November 2019 (range 40-53.7 µg/m 3 ) at Sacred Heart Catholic High, and NO 2 concentrations remained >40 µg/m 3 throughout the winter months at St Gabriel's Children's Day Nursery. The high monthly average NO 2 concentrations in January and February 2018 were due to high NO 2 concentrations at St Teresa's Primary School (60.5 and 78.5 µg/m 3 in January and February 2018, respectively). Early morning peaks at this school reached around 100 µg/m 3 . This school's AQM is located near a busy road with a large church, a row of shops and cafes, and a traffic light-controlled pedestrian crossing next to the AQM. Overall, the higher NO 2 concentrations seen in the winter highlight the impact that the burning of fossil fuels for heating and energy and increased vehicle usage have on ambient NO 2 levels in Newcastle in the colder months.
No schools exceeded the WHO2005 guideline or EU/UK regulations for the annual mean NO2 concentration in 2018. However, all schools would have exceeded the updated WHO2021 guideline for the annual mean NO2 concentration if it had been in place. In 2019, Sacred Heart Catholic High met the WHO2005 guideline and EU/UK regulations for the annual mean NO2 concentration (40 μg/m 3 ), but no other schools met or exceeded this value in this year. The 2018 and 2019 annual mean NO2 concentrations outside the 12 schools are shown in Table S5 and Figure 3a.  The monthly mean NO2 concentrations for all 12 schools combined can be seen in Figure 4a. The overall mean NO2 concentrations are lowest in the late spring, summer, and early autumn months (May-September) and highest in the winter months (October-February). When considering monthly NO2 concentrations at an individual school level, consistently high monthly mean NO2 concentrations were observed from April to November 2019 (range 40-53.7 μg/m 3 ) at Sacred Heart Catholic High, and NO2 concentrations remained >40 μg/m 3 throughout the winter months at St Gabriel's Children's Day Nursery. The high monthly average NO2 concentrations in January and February 2018 were due to high NO2 concentrations at St Teresa's Primary School (60.5 and 78.5 μg/m 3 in January and February 2018, respectively). Early morning peaks at this school reached around 100 μg/m 3 . This school's AQM is located near a busy road with a large church, a row of shops and cafes, and a traffic light-controlled pedestrian crossing next to the AQM. Overall, the higher NO2 concentrations seen in the winter highlight the impact that the burning of fossil fuels for heating and energy and increased vehicle usage have on ambient NO2 levels in Newcastle in the colder months.   ber 2019 (range 40-53.7 μg/m 3 ) at Sacred Heart Catholic High, and NO2 concentrations remained >40 μg/m 3 throughout the winter months at St Gabriel's Children's Day Nursery. The high monthly average NO2 concentrations in January and February 2018 were due to high NO2 concentrations at St Teresa's Primary School (60.5 and 78.5 μg/m 3 in January and February 2018, respectively). Early morning peaks at this school reached around 100 μg/m 3 . This school's AQM is located near a busy road with a large church, a row of shops and cafes, and a traffic light-controlled pedestrian crossing next to the AQM. Overall, the higher NO2 concentrations seen in the winter highlight the impact that the burning of fossil fuels for heating and energy and increased vehicle usage have on ambient NO2 levels in Newcastle in the colder months.

Jan-18
Feb  In 2018, the annual mean PM 2.5 concentrations ranged from 3.8 ± 5.3 µg/m 3 (Cragside Primary) to 11.0 ± 10.2 µg/m 3 (Atkinson Road Primary Academy) (Table S5 and Figure 3c). In 2019, the annual mean PM 2.5 concentrations ranged from 3.1 ± 4.7 µg/m 3 (Westerhope Primary) to 12.2 ± 15.8 µg/m 3 (St Alban's RC Primary). These are the same two schools that had the lowest and highest annual mean PM 10  The average monthly PM 2.5 concentrations were lowest in the late spring, summer, and early autumn months (May-October) and highest in the late autumn and winter months (November-February) (Figure 4c). Over the two-year study period, no month exceeded the EU/UK annual mean PM 2.5 regulations but three months exceeded the WHO 2005 annual mean PM 2.5 guideline. These three months were November 2018 (14.2 ± 5.6 µg/m 3 ), February 2019 (11.9 ± 5.6 µg/m 3 ), and April 2019 (11.2 ± 5.9 µg/m 3 ). The school with the highest PM 2.5 in these months was St Alban's RC Primary, with a monthly mean of 23.  Figure S4c).
There are currently no guidelines or regulations for PM 1 concentrations. The ratio of PM 1 :PM 2.5 in the 12 school sites (range 0·61 to 0·95) indicates higher PM 1 fractions in the PM 2.5 concentrations at these sites. Given the lack of current guidelines or regulations on PM 1 , and its potential for increased health risk, we chose to set our short-term exceedance limit to half that of the WHO 2005 Figure S4d).

Diurnal Variation
We produced time variation plots to investigate the diurnal variation of NO 2 , PM 10 , PM 2.5 , and PM 1 at schools in Newcastle. Only three schools were chosen to produce the time variation plot for each pollutant. These 3 schools were the school with the highest annual mean concentration, the school with the lowest, and the school with the annual mean concentration closest to the mean of all of the 12 schools.
The NO 2 levels outside the 3 chosen schools show a distinct pattern over 24 h (Figure 5a). Their distribution is bimodal, with noticeable morning and afternoon/evening peaks. The morning peak occurs at around 07:00-09:00 and the afternoon/evening peak occurs at around 16:00-18:00. The afternoon/evening peak is generally higher and has fewer steep gradients than the morning peak. The morning peak could be attributable to morning rush hour traffic and the afternoon/evening peak could be attributable to evening rush hour traffic. The greater height and less steep gradient of the afternoon/evening peak could be attributable to the fact that baseline NO 2 concentrations are higher before the evening rush hour begins than they are before the morning rush hour begins due to the accumulation of NO 2 during the day from ongoing traffic emissions.
PM2.5 concentrations at these sites. Given the lack of current guidelines or regulations on PM1, and its potential for increased health risk, we chose to set our short-term exceedance limit to half that of the WHO2005 guidelines for PM2.5 at 12.5 μg/m 3 . During the 2-year study period, 5 schools had >50 days where PM1 concentrations exceeded 12.5 μg/m 3 . Broadway East exceeded this value on 55 days, St Alban's RC Primary exceeded it on 99 days, St Mary's Catholic on 55 days, St Teresa's Primary on 66 days, and West Jesmond Primary on 63 days. Over the 2-year study period, the highest average PM1 concentration was 19.1 ± 6.9 μg/m 3 at Broadway East, 23.5 ± 10.9 μg/m 3 at St Alban's RC Primary, 18.2 ± 7.6 μg/m 3 at St Mary's Catholic, 18.8 ± 9.5 μg/m 3 at St Teresa's Primary School, and 19.1 ± 8.8 μg/m 3 at West Jesmond Primary ( Figure S4d).

Diurnal Variation
We produced time variation plots to investigate the diurnal variation of NO2, PM10, PM2.5, and PM1 at schools in Newcastle. Only three schools were chosen to produce the time variation plot for each pollutant. These 3 schools were the school with the highest annual mean concentration, the school with the lowest, and the school with the annual mean concentration closest to the mean of all of the 12 schools.
The NO2 levels outside the 3 chosen schools show a distinct pattern over 24 h ( Figure  5a). Their distribution is bimodal, with noticeable morning and afternoon/evening peaks. The morning peak occurs at around 07:00-09:00 and the afternoon/evening peak occurs at around 16:00-18:00. The afternoon/evening peak is generally higher and has fewer steep gradients than the morning peak. The morning peak could be attributable to morning rush hour traffic and the afternoon/evening peak could be attributable to evening rush hour traffic. The greater height and less steep gradient of the afternoon/evening peak could be attributable to the fact that baseline NO2 concentrations are higher before the evening rush hour begins than they are before the morning rush hour begins due to the accumulation of NO2 during the day from ongoing traffic emissions. PM10 levels outside of the 3 chosen schools show an almost opposite shape to NO2 concentrations over 24 h (Figure 5b). Rather than two distinct peaks, PM10 concentrations show a "U-shape" and are at their highest from midnight until early morning and their The time variation plots of PM 2.5 and PM 1 show similar patterns to that of PM 10 (Figure 5c,d). Of note, variation in the PM concentrations over 24 h appear more marked outside of schools with the highest overall PM concentrations.

Exposure Assessment
The AQMesh pod AQMs that collected the data used in this study are commonly attached to lampposts at school entrances. Therefore, when children are outside during the school day (while arriving at and leaving school, while on outdoor breaks, or while undertaking outdoor physical education (PE) lessons), it is likely that they are exposed to PM concentrations that are similar to those of the AQMesh pod microenvironment.
During the study period, the highest total PM 10 RDDs (µg/h) occurred outside Atkinson Road Primary (12.5 Figure 6. The estimated RDD is an indicator of the health risk posed by ambient PM concentrations for the children attending those schools [9].
PM10 concentrations at both the urban background and roadside monitoring sites and 2019.
The time variation plots of PM2.5 and PM1 show similar patterns to that of PM ure 5c,d). Of note, variation in the PM concentrations over 24 h appear more mark side of schools with the highest overall PM concentrations.

Exposure Assessment
The AQMesh pod AQMs that collected the data used in this study are com attached to lampposts at school entrances. Therefore, when children are outside the school day (while arriving at and leaving school, while on outdoor breaks, o undertaking outdoor physical education (PE) lessons), it is likely that they are exp PM concentrations that are similar to those of the AQMesh pod microenvironmen During the study period, the highest total PM10 RDDs (μg/h) occurred outsid son Road Primary (

Discussion
Overall, the findings of this study indicate that the annual mean ambient N centrations outside schools in Newcastle do not regularly exceed the WHO2005 gu and EU/UK regulations. In contrast, modelling studies conducted in London ha sistently found that around 25% of all schools in London are in areas where the W guideline and EU/UK regulations for annual mean NO2 are regularly exceeded [2 This suggests that the annual mean NO2 levels outside schools in Newcastle may a lower risk to children's health when compared to those in the country's capital

Discussion
Overall, the findings of this study indicate that the annual mean ambient NO 2 concentrations outside schools in Newcastle do not regularly exceed the WHO 2005 guidelines and EU/UK regulations. In contrast, modelling studies conducted in London have consistently found that around 25% of all schools in London are in areas where the WHO 2005 guideline and EU/UK regulations for annual mean NO 2 are regularly exceeded [20,49,50]. This suggests that the annual mean NO 2 levels outside schools in Newcastle may present a lower risk to children's health when compared to those in the country's capital city.
Annual mean NO 2 concentrations outside schools in this study ranged from 21.7 ± 11.2 µg/m 3 [49]. They found that the annual mean NO 2 concentrations outside these 30 schools ranged from 6.2-56.5 µg/m 3 and that AURN stations marked as "traffic urban" generally had the highest NO 2 concentrations [49].
The patterns of NO 2 concentrations outside schools in Newcastle are consistent with those of other studies investigating schools' ambient air quality. This study found evidence for a relationship between NO 2 concentrations outside schools in Newcastle and rush hours (and therefore small vehicle traffic volumes). The Breathe London Wearables study found that children's exposure to ambient pollutants was high during the morning rush hour and that main roads and busy junctions were associated with higher NO 2 concentrations due to traffic density [51]. Studies conducted outside of Europe also demonstrate the relationship between traffic density, particularly small vehicle traffic density in streets surrounding schools, and ambient NO 2 concentrations outside schools [52,53].
Overall, PM levels outside schools in Newcastle exceed the WHO 2005 guidelines relatively frequently, particularly short-term (24-h) exceedances. This indicates that children at schools in Newcastle may be at greater risk of harm from PM than NO 2 . Furthermore, the patterns of PM concentrations outside schools are consistent across years and the size of particles. There is evidence that some schools experience noticeably worse ambient PM concentrations than other schools and this would benefit from both further monitoring and action to reduce the risk to children at these schools.
The annual mean PM 10  also found that high PM 2.5 concentrations existed at both urban traffic and urban background monitoring sites, in keeping with this study's findings that ambient PM concentrations are less closely associated with small vehicle traffic density than ambient NO 2 concentration [49]. Tofful and Perrino (2015) reported that outdoor PM 2.5 concentrations in Rome vary from 17 to 56 m 3 and some schools' indoor PM 2.5 concentrations are higher than ambient concentrations [54]. A further study reported very high PM 10 concentrations outside 39 schools in Barcelona (Spain) of around 50.1 µg/m 3 [55]. Janssen et al. (2001) also found that the closer a school is to a motorway, the higher its ambient PM 2.5 concentrations [53]. Furthermore, Patel et al. (2009) found that school ambient PM 2.5 levels in New York City were 1.8 times higher in dense urban settings than in suburban settings due to increased truck and bus volumes [56]. These findings help to explain the differences in the diurnal variations of PM and NO 2 concentrations and may provide a basis upon which to investigate why some schools in Newcastle experience noticeably worse ambient PM concentrations than others.
This study provides evidence upon which policy may be based to improve school ambient air quality. Given the association between NO 2 concentrations outside schools, small vehicle traffic density, and rush hours, measures, such as road closures outside schools (school streets initiative), anti-idling campaigns, staggered drop-off and pick-up times, relocating drop-off and pick-up sites away from school entrances and playgrounds, and active travel programmes, should be introduced (or continued). Action taken to reduce NO 2 concentrations outside schools will also help to reduce PM concentrations. Specific policy measures to reduce ambient PM concentrations outside schools highlighted by the findings of this study could include mitigation of schools' proximity to sources of PM, such as motorways or roads with high volumes of large vehicles, and being aware of sporadic weather events or anthropogenic events that may generate PM, putting mitigation measures in place where these are identified. Lastly, education for children, parents, and teachers on the health benefits of clean air and measures they can take to reduce schoolchildren's exposure to ambient air pollutants, particularly during the winter months, is essential.
Despite having some limitations (reported in the Supplementary Materials), this study addressed a gap in the understanding identified in the literature. In their systematic review,  highlighted that only 3 of the 14 UK-based studies investigating school air quality in their review used fixed location air quality monitoring; the remaining 11 used modelling to estimate concentrations of pollutants [17]. This study presents air quality data from fixed location continuous AQMs over two years and therefore addresses these identified gaps and adds value to the existing literature.

Conclusions
As evidence for the harmful effects of air pollution in children increases and becomes ever more robust, this study provides yet unseen insight into the ambient air quality of 12 schools in Newcastle Upon Tyne, UK, and highlights the need to act urgently to protect children's health. It provides an understanding of key ambient pollutant concentrations outside schools in urban areas, particularly urban areas that are outside of capital cities in developed countries with temperate climates. The findings in this study agree with other recent similar studies investigating school ambient air quality. This study, therefore, provides evidence upon which organisations can plan and implement policy at both a local and national level to protect children from the harmful effects of ambient air pollution during their day at school.
This study provides opportunities for further research that could both enhance and expand on the current findings. Further research suggestions include, but are not limited to, investigating Newcastle school indoor air quality, expanding this study to include more schools, repeating this study in a different area of the UK, and widening this study to include additional harmful pollutants, such as volatile organic compounds (VOCs), including BTEX.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/atmos13020172/s1, Study location description, Urban design, Meteorology, Technical information AQMesh pod Quality Assurance, Study limitations. Table S1. Comparison of WHO air quality guideline values (2005 and 2021) and EU/UK pollutant limit values for NO 2 , PM 2.5 , and PM 10 [11,12,16]. Table S2. Previous school air quality studies conducted in the UK. Please note: Much of this data (study numbers 1-14 in italics) has been taken from a table in Osborne et. al (2021) with kind permission from the authors [17]. Please refer to Osborne et. al for the original table and further detail. Table S3. Participating schools, pupil ages, air quality data collection start date and air quality monitor location information. Table S4. Data flagging values, adapted from the Breathe London project [36]. Table S5. Annual average and standard deviations of PM 10 , PM 2.5 , PM 1 , and NO 2 concentrations at each of the participating schools during 2018 and 2019. Table S6. NO 2 data capture (%), and data flagging by school in 2018 and 2019. Table S7. PM 10 data capture (%), and data flagging by the school in 2018 and 2019. Table S8. PM 2.5 data capture (%), and data flagging by school in 2018 and 2019. Figure S1. 2015 Index of Multiple Deprivation by ward in the city of Newcastle Upon Tyne [25]. Figure S2. Monthly wind rose profile for 2018 (bottom) and 2019 (top) in Newcastle Upon Tyne. Figure S3. Examples of Healthy Schools project air quality monitor locations in Newcastle Upon Tyne. Figure S4. Scatterplots of (a) 1-h average NO 2 concern, (b) 24-h average PM 10