# An insight into recent PM1 aerosol light scattering properties and particle number concentration variabilities at the suburban site ATOLL in Northern France

Lenka Suchánková, Suzanne Crumeyrolle, Eric Bourrianne, Roman Prokeš, Ivan Holoubek, Vladimír Ždímal, Isabelle Chiapello

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## To cite this version:

Lenka Suchánková, Suzanne Crumeyrolle, Eric Bourrianne, Roman Prokeš, Ivan Holoubek, et al. An insight into recent PM1 aerosol light scattering properties and particle number concentration variabilities at the suburban site ATOLL in Northern France. Science of the Total Environment, 2025, 959, 10.1016/j.scitotenv.2024.178190. insu-05101312

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Submitted on 6 Jun 2025

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# An insight into recent PM₁ aerosol light scattering properties and particle number concentration variabilities at the suburban site ATOLL in Northern France

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Lenka Suchánková<sup>a,b,c,d,*</sup>, Suzanne Crumeyrolle<sup>d,**</sup>, Eric Bourrianne<sup>d</sup>, Roman Prokes<sup>b,c</sup>,
Ivan Holoubek<sup>b,c</sup>, Vladimír Ždímal<sup>a</sup>, Isabelle Chiapello<sup>d</sup>

## HIGHLIGHTS

* Over 5-years-based observation of aerosol scattering properties and PNC in PM₁.

* PM₁ aerosol scattering properties characterized in an urban environment.

* Significant decrease of σ<sub>sp</sub> at 525 and 635 nm, no significant change for σ<sub>bsp</sub>.

* Substantial increase in number concentration of N<sub>20-30 nm</sub>, N<sub>30-60 nm</sub> and N<sub>20-800 nm</sub>.

## GRAPHICAL ABSTRACT

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A decrease in aerosol light scattering in PM₁ fraction does not correlate with an increase in fine particle number concentration.

## ARTICLE INFO

Editor: Pavlos Kassomenos

Keywords:

Optical properties

Ultrafine particles

Urban environment

Temporal variability

Air quality

## ABSTRACT

Aerosol particles in the PM₁ fraction considerably influence the climate-related effects of aerosols and impact human health despite representing very variable fractions of the total aerosol mass concentration. Aerosol optical measurement techniques (aerosol light scattering) may not be sufficiently effective for detecting all particles in the PM₁ fraction, particularly regarding number concentration. The present study investigates temporal variations of aerosol light scattering properties and particle number concentration (PNC) at different size modes in the PM₁ fraction at the atmospheric site ATOLL (The Atmospheric Observations in Lille), Northern France from January 2018 to February 2023. The total scattering coefficient σ<sub>sp</sub> decreased annually by 6 % and 8 % at 525 and 635 nm, respectively. Maximum annual changes occur in winter and summer seasons with a decrease above 10 % per year. Although the backscattering coefficient (σ<sub>bsp</sub>) at 525 nm significantly decreased in winter, this did not result in a significant overall decline over time. Despite a decrease in aerosol light scattering, PNC exhibited a notable annual increase in concentration of N<sub>20-30 nm</sub> and N<sub>30-60 nm</sub>, which led to an increase in the total N<sub>20-800 nm</sub>.

<footnote><sup>a</sup> Institute of Chemical Process Fundamentals of the Czech Academy of Sciences, Prague 165 02, Czech Republic</footnote>

<footnote><sup>b</sup> Global Change Research Institute of the Czech Academy of Sciences, Brno 603 00, Czech Republic</footnote>

<footnote><sup>c</sup> RECETOX, Faculty of Science, Masaryk University, Brno 611 37, Czech Republic</footnote>

<footnote><sup>d</sup> UMR 8518 Laboratoires d'Optique Atmosphérique, CNRS, Université de Lille, Lille 596 55, France</footnote>

<footnote>* Correspondence to: L. Suchánková, Institute of Chemical Process Fundamentals of the CAS, Rozvojová 135, 165 02 Prague, 6, Czech Republic.</footnote>

<footnote>** Correspondence to: S. Crumeyrolle, LOA UMR 8518, UFR de Physique - Bâtiment P5, Université de Lille, 59655 Villeneuve d'Ascq Cedex, France.
E-mail addresses: suchankova@icpf.cas.cz (L. Suchánková), suzanne.crumeyrolle@univ-lille.fr (S. Crumeyrolle).</footnote>
nm size range. N<sub>20-30nm</sub> increased by 10 % annually, with the highest increase by 37 % in spring. Both traffic and photooxidative processes influenced PNCs, underscoring the need for a more comprehensive investigation of the detailed particle number size distribution to assess air quality and the health effects of increased ultrafine PNC at urban/suburban sites in Europe.

# 1. Introduction

Atmospheric aerosols are defined as solid or liquid particles of a size range of 1 nm to 100 µm suspended in the atmosphere. Elevated aerosol levels decrease visibility and impact human health (Bond et al., 2013; Hinds, 1999; Seinfeld and Pandis, 2006). In addition, aerosol particles influence the climate system of the Earth. IPCC (2021) estimated radiative forcing caused by the direct aerosol effect (scattering and absorption of light on particles) to be −0.22 Wm<sup>-2</sup> (95 % confidence interval: −0.47 to 0.04 Wm<sup>-2</sup>). The indirect effect of aerosols is related to alterations in the lifespan and development of clouds. It contributes significantly to the cooling effect of particles with a radiative forcing estimate of −0.84 Wm<sup>-2</sup> (95 % confidence interval: −1.45 to −0.25 Wm<sup>-2</sup>). However, due to the considerable spatial and temporal variability and the different nature of aerosol sources, the radiative forcing of aerosols remains one of the most uncertain factors regarding climate change modeling (Bellouin et al., 2020; IPCC, 2021; Shindell et al., 2013).

Over the last decade, several studies have characterized aerosol optical properties in different environments, especially over Europe (Ma et al., 2014; Pandolfi et al., 2018) and around the world (Collaud Coen et al., 2020; Laj et al., 2020; Li et al., 2023). Most of the studies analyzed and reported a decreasing trend in the aerosol light scattering properties of the PM₁ fraction over several European regions: in Southern Finland from 2006 to 2010 (Luoma et al., 2019); in Po Valley from 2004 to 2010 (Putaud et al., 2014); in the Czech Republic from 2012 to 2019 (Suchánková et al., 2024), omitting information on the fine particle mode (PM₁) climate effect. Black carbon, almost exclusively present in the PM₁ fraction, is the most effective aerosol light absorbent of both incoming and terrestrial radiation, thus contributing to the warming of the atmosphere. A significant PM₁ fraction consists of sulfate and secondary organic aerosols that have a predominantly scattering effect (Bond et al., 2013; Seinfeld and Pandis, 2006).

PM₁ particles are also highly effective cloud condensation nuclei (CCN) and outrange supermicron CCN by number concentration. Their activation into cloud droplets leads to an increase in cloud albedo and an extension of cloud lifetime, increasing the indirect aerosol effect. Mazoyer et al. (2019) showed an increased ability of particles within a range of 150 to 500 nm to serve as CCN from measurements performed over Paris. PM₁ particles remain suspended in the atmosphere longer than larger particles due to lower settling velocities and could be transported over long distances. Therefore, they could participate in chemical reactions in the atmosphere and influence the climate on a global scale (IPCC, 2021). Moreover, Spracklen et al. (2008) estimated that primary aerosols plays a minor role in the total particle number concentration on a global scale, whereas newly formed particles play a significant role. They also demonstrated that new particle formation (NPF) within the planetary boundary layer increases the concentration of cloud condensation nuclei (CCN) by 3–20 % at 0.2 % supersaturation. Merikanto et al. (2009) determined that particles originating from nucleation account for 45 % of the global CCN number concentration at 0.2 % supersaturation. The aerosol scattering properties are subdivided into intensive and extensive properties. The latter can be directly related to aerosol number concentration. Several studies reported that aerosol light scattering is proportional to atmospheric aerosol loading (Collaud Coen et al., 2020; Delene and Ogren, 2002; Laj et al., 2020; Pandolfi et al., 2018). The current light scattering measurements cannot capture particles of a specific size range, which results in a lack of information regarding the lower end of the fine-mode particles. This gap is deemed

necessary for studies on air quality.

The impact of aerosols on human health has been a topic of concern throughout history, with comprehensive efforts to improve air quality recognized since the last century (Fowler et al., 2020). In recent years, improvement in urban air quality in terms of particle loads has been observed both worldwide (Li et al., 2023) and over specific regions and countries, as in Europe (Jaume et al., 2022), in the US (EPA, 2022), in China (Yousefi et al., 2023) or in South Africa (Nyasulu et al., 2023). However, a considerable proportion of the global population lives in areas where air quality standards are exceeded, and exposure to ambient particulate matter (PM), ozone, and sulfur dioxide remains a significant environmental concern (WHO, 2021).

Regarding PM, most published health-related particles concentrate on trends in mass concentrations of PM₁₀ and PM₂.₅ (Belis et al., 2022; Canals-Angerri et al., 2024; Huis in't Veld et al., 2023; Mehmood et al., 2020; Pisoni et al., 2023; Rovira et al., 2020; Segersson et al., 2017; Sosa et al., 2017). However, particles within the PM₁ fraction generally contribute more significantly to the particle number concentration rather than to the mass concentration. Submicronic particles could penetrate deep into the lungs, while particles with a diameter of <0.1 µm into the bloodstream, causing cardiovascular, pulmonary, carcino-genic, or inflammatory diseases (Pateraki et al., 2014; Pope et al., 2002). Furthermore, these particles can contain a large amount of potentially toxic compounds due to their high surface/mass ratio and often not a well-known hazardousness of their mixtures (Mesquita et al., 2017; Pateraki et al., 2020; Trippetta et al., 2016).

Several studies reported decreasing trends in particle number concentrations (PNC) in urban and suburban areas. Asmi et al. (2013) found significantly decreasing trends of N<sub>20-500 nm</sub> at different stations worldwide, mainly due to decreasing concentrations of anthropogenic emissions. Mikkonen et al. (2020) reported decreasing N<sub>6-100 nm</sub>, N<sub>25-100 nm</sub>, and N<sub>100-1000 nm</sub> concentrations in Budapest, Hungary, due to decreased local emissions from road traffic, residential heating, and industrial activities. Lorelei de Jesus et al. (2020) observed a decreasing trend in PM₂.₅ mass and PNC in five urban agglomerations in Europe, Australia, and the US for over 15 years, until 2017. Carnerero et al. (2021) consistently observed a decline in the total number of concentrations of N<sub>15-25 nm</sub> (nucleation mode), N<sub>25-100 nm</sub> (Aitken mode), and N<sub>100-478 nm</sub> (accumulation mode) in Spain until 2019, despite an increase in secondary aerosol concentrations due to photonucleation. Hopke et al. (2024) found an apparent reduction in PNC related to motor vehicles and a reduction in S-rich fuels due to the pollution policies implemented in New York. However, they also reported increased nucleation-mode particle concentration over the period of 2014–2019, attributed to reduced condensational sink from accumulation-mode particles. The RI-URBANS project is a European research initiative that commenced in 2022 with the objective of developing an effective air quality monitoring system and mitigating the adverse effects of poor urban air quality on human health. Particular emphasis is placed on nanoparticles and particulate matter fractions (https://riurbans.eu/Accessed: 2024-11-18). In the course of this project, Trechera et al. (2023) conducted an evaluation of particle number size distributions at 26 sites in Europe and one in the USA (2017–2019). They identified a notable absence of systematic and harmonized ultrafine particle measurements, which they asserted is a significant obstacle to the efficient assessment of air quality levels. In a recent study, Garcia-Marés et al. (2024) identified the sources of particles of varying sizes within the ultrafine range (<100 nm) in 18 European urban locations and including ATOLL, one site in the USA. They observed a significant decrease in the
concentration of ultrafine particles at 38 % of monitored stations, mostly ones characterized as "traffic sites", which they attributed primarily to the stricter emission standards in Europe (UFP slopes for urban and traffic sites were −60 and −491 # cm⁻³ per year, respectively). However, the increasing tendency was observed at the urban site in Budapest and no significant trends were observed in some urban and suburban sites. Other RI-URBANS project studies focused on further source apportionment of ultrafine particles (Harni et al., 2024; Rowell et al., 2024; Sartelet et al., 2022; Via et al., 2023) but also modeling, simulations and machine learning in air quality assessment (Denby et al., 2024; Siouti et al., 2023; Vida et al., 2024; Zhong et al., 2023) or new particle formation (Aktypis et al., 2023; in't Veld et al., 2024; Kulmala et al., 2024; Zhang et al., 2024).

This study aims to characterize temporal variation of both the light scattering properties, particle number concentration, and size distribution in the PM₁ fraction at the suburban background site ATOLL in Lille, Northern France. The study covers a period of slightly over five years. This study is one of the first to report light scattering properties in the PM₁ fraction and fine particle number concentration in an urban/sub-urban European environment.

## 2. Materials and methods

The ATmospheric Observations in LiLLE (ATOLL, Fig. 1) station is located in Villeneuve d'Ascq, the metropolitan region Hauts-de-France, Northern France (50.6114 N, 3.1406 E, 60 m a.s.l.), 6 km away from the city center of Lille, which is the core of the metropolis (Métropole Européenne de Lille, with >1.1 million inhabitants) to which Villeneuve d'Ascq belongs. ATOLL is part of the Aerosols, Clouds, and Trace gases Research InfraStructure (ACTRIS, http://www.actris.net, station labeling status: initially accepted in August 2024 both for remote sensing and in-situ measurements), one of the historical AERONET stations, providing high-quality long-term atmospheric data in Northern France. This station is under the influence of many anthropogenic sources, e.g., road traffic, residential sector, agriculture, and industries (Chen et al., 2022), as well as maritime emissions, sea salt, and episodically under the influence of natural events such as aged volcanic plumes and Saharan dust (Boichu et al., 2016; Bovchaliuk et al., 2016; Mortier et al., 2013). Low single scattering albedo values (SSA<sub>525nm</sub>) (0.75 on average within the PM₁ fraction, Velazquez-Garcia et al., 2024) and high mean particle total number concentration (6140 cm⁻³ on average) suggest that aerosol measurements performed at ATOLL are comparable to Global Atmospheric Watch (GAW) sites classified as urban (Laj et al., 2020; Rose

<figure><img src="image_6.png" /><figcaption>**Fig. 1.** Location of the ATOLL station in Villeneuve-d'Ascq, near the city of Lille (France), with the frequency of counts by wind direction from 2018 to 2023 (bottom left) and by seasons (bottom right) with mean wind speed and percentage of calm days (0 ms⁻¹).</figcaption></figure>
et al., 2021).

A large set of in situ and remote sensing instruments is implemented at ATOLL to characterize the physical, chemical, optical, and radiative properties of particles and clouds. In situ, instruments have independent sampling stainless steel lines located at least 1 m above the roof and equipped with a PM₁ cyclone or PM₁₀ inlet, according to ACTRIS recommendations.

The measurements used for the present study were made between January 1, 2018, and February 28, 2023, from instruments described below. The dry total scattering ($\sigma_{sp}$) and backscattering ($\sigma_{bsp}$) coefficients are measured using two Aurora (ECOTECH) nephelometers (4000 and 3000) operating at 450, 525, and 635 nm. Both nephelometers run in series, with a Nafion dryer upstream of both instruments ($<$40 % RH) and a PM₁ cyclone before Aurora 3000. Therefore, dry scattering coefficients are measured in particles first within the PM₁₀ (A-4000) fractions, then within the PM₁ (A-3000) fractions. Data analysis for this study was performed solely on the PM₁ fraction. Both nephelometers are adjusted daily using filtered air to consider the daily variation of the scattering gas concentrations (such as NO₂). Furthermore, both nephelometers are calibrated monthly using CO₂. All scattering coefficients have been corrected from angular truncation errors and illumination intensity non-idealities based on Müller et al. (2009).

Climate-relevant variables were calculated from $\sigma_{sp}$, $\sigma_{bsp}$ and wavelength dependence: the ratio of backscattering to total scattering coefficient $b$ (Rama Gopal et al., 2014; Suchánková et al., 2024); the quantification of the preferential direction of aerosol light scattering, the asymmetry factor $g$ (Andrews et al., 2006; Wiscombie and Grams, 1976) and the dependence of scattering on wavelength of light, Scattering Ångström Exponent $SAE_{\lambda_1/\lambda_2}$, where $\lambda_1$ and $\lambda_2$ characterize selected wavelengths.

The Scanning Mobility Particle Sizer (SMPS) measures every 5 min the particle number size distribution between 20 and 800 nm (divided into >100 bins) downstream of a Nafion membrane as recommended by ACTRIS standards to keep the RH below 40 %. The SMPS system consisted of a condensation particle counter (TSI model 3775), a differential mobility analyzer (DMA, TSI 3081A) as described by Villani et al. (2007), and a nickel aerosol neutralizer (Ni-63 95MBq). The sheath flow rate was controlled with a critical orifice in a closed-loop arrangement (Jokinen and Mäkelä, 1997). The scan time was 300 s, and the particle concentrations were corrected by considering multiple charge and diffusion losses/broadening calculated using the manufacturer's software and algorithms (AIM 10.2.0.11).

Meteorological data, including temperature, water vapor mixing ratio, wind speed, and direction, were also measured every minute at the sampling site using a weather station (DAVIS Inc. weather station, Vantage Pro 2). The prevailing wind directions at the site are mostly SSW (highly predominant in winter and fall, with a significant NNE wind direction in spring, Fig. 1).

The supportive calculation of air mass back trajectories arriving to the ATOLL site were computed using the hybrid single-particle Lagrangian integrated trajectory model (HYSPLIT version 5.3.0) from the NOAA Air Resources Laboratory (Draxler and Hess, 1998; Rolph et al., 2017; Stein et al., 2015) and meteorological input from the global data assimilation system (GDAS) at 1° x 1° resolution.

# 3. Results and discussion

## 3.1. Temporal variability of aerosol light scattering properties (ASP)

Annual, seasonal, monthly, and diurnal variations of ASP in ATOLL have been investigated and reported at a specific wavelength ($\lambda$ = 525 nm), with the median as the primary statistical indicator if otherwise stated. The overall availability of the data is 83 % and differs slightly between seasons (in spring 89 %, in summer 79 %, in fall 86 %, and in winter 79 %). The medians of $\sigma_{sp}$ over the entire observed period were 31.50 Mm-1 24.75 Mm$^{-1}$, and 19.25 Mm$^{-1}$ followed by $\sigma_{bsp}$ medians

3.97 Mm$^{-1}$, 3.29 Mm$^{-1}$ and 2.76 Mm$^{-1}$ at 450, 525 and 635 nm, respectively (Supp. Mat., Table A.1). The ratio of backscattering to the total scattering coefficient $b$ (0.127) and the asymmetry factor $g$ (0.594) both at 525 nm correspond to typical values for polluted areas with significant influence of anthropogenic activity. $SAE_{450/635}$ showed a slight predominance of particles at the lower end of the fine mode in the average aerosol population (1.65).

The literature review on other European sites measuring ASP in the PM₁ fraction in the EBAS database (https://ebas.nilu.no/) found only two other stations: the continental rural K-Puszta station in the Great Hungarian Plain located 15 km NW of the closest city, surrounded by mixed forests and grasslands with sandy soil (dataset 2006–2008, Yli-Juuti et al., 2009) and the rural maritime El Arenosillo (ARN) station in Doñana National Park, Spain surrounded by pine forest, located 1 km away from the Atlantic Ocean coastline (data set 2018–2023, Adame et al., 2022).

The K-Puszta station exhibited almost double values of both $\sigma_{sp}$ and $\sigma_{bsp}$ at any wavelength (median 51.89 Mm$^{-1}$ and 6.45 Mm$^{-1}$ at 525 nm, respectively) but a close $b$ value of 0.120. On the other hand, approximately halved values of $\sigma_{sp}$ and $\sigma_{bsp}$ compared to ATOLL at the ARN station (median 14.56 Mm$^{-1}$ and 2.05 Mm$^{-1}$ at 525 nm, respectively) with a higher $b$ value of 0.139, probably due to the influence of marine air masses. In both cases, $SAE_{450/635}$ were higher than one at the ATOLL site (median 2.08 and 2.38 at K-Puszta and ARN, respectively).

In addition to previous stations, Luoma et al. (2019) studied aerosol optical properties in PM₁₀ and PM₁ fractions at the SMEAR II station in Southern Finland, a rural station surrounded mostly by Scots pine tree forest from 2006 to 2010. The station is 60–100 km from the closest large cities. As expected, $\sigma_{sp}$ and $\sigma_{bsp}$ were significantly lower than at ATOLL, medians 7.1 and 1.1 Mm$^{-1}$ at 550 nm, respectively. The backscattering coefficient $b$ showed a higher fraction of backscattering in total scattering than in ATOLL (0.15). The significantly higher $SAE_{450/700}$ in Finland (2.28) indicated a higher presence of finer particles than at the ATOLL.

Fig. 2 shows the interannual variation of selected ASP at 525 nm. 2023 includes only January and February, which naturally increased levels of $\sigma_{sp}$ and $\sigma_{bsp}$. Due to the non-normal distribution and high seasonality of observed data, we tested the significance of the evolution with the Mann-Kendall seasonal test (Supp. Mat., Table A.2). The significance was tested in monthly medians to avoid the influence of seasonal variation with trend directions as for annual variation. Since the study period is five consecutive years, slopes (also variables discussed) were calculated from median values of years to diminish the influence of outliers and extreme values.

A significant annual decrease ($\alpha$ = 0.05, $p$ < 0.05) was observed for $\sigma_{sp}$ at 525 and 635 nm over the studied period (by 1.50 Mm$^{-1}$ per year and 1.60 Mm$^{-1}$ per year which corresponds to 6 % per year and 8 % per year, respectively, Fig. 2A; $\sigma_{sp}$ at 635 nm is not displayed). No significant decrease was observed for $\sigma_{bsp}$ at any wavelength, leading to a relative increase in the backscatter fraction to total scattering (0.002 per year at all wavelengths) (Fig. 2C) along with a significant decrease in $g$ (–0.006 per year, –0.006 per year and –0.010 per year at 450, 525, 635 nm) (Fig. 2D). This result could lead to an increased potential for the effect of aerosol cooling on local climate; however, additional research on aerosol absorption properties is needed to fully assess the direct aerosol effect. The statistically significant reduction of $\sigma_{sp}$ implies the reduction of aerosol mass concentration at this site, which is consistent with Pandolfi et al. (2018) and imply effectiveness of European mitigation policies against particulate air pollution. The statistical test revealed no significant increase in $SAE_{450/635}$ over the years.

When it comes to seasonal variation, the highest median values of $\sigma_{sp}$ and $\sigma_{bsp}$ were observed in winter, 31.73 Mm$^{-1}$ and 4.31 Mm$^{-1}$ (at 525 nm, Fig. 3), respectively, mainly due to atmospheric stability and lower dilution due to the lower height of the planetary boundary layer (median 0.63 km in winter compared to median of 0.86 km in summer; Supp. Mat., Fig. A.1), combined with accumulated traffic emissions and
<figure><img src="image_7.png" /><figcaption>**Fig. 2.** Box plots representing the interannual variation of aerosol A) scattering (σsp), B) backscattering (σbsp) coefficients, C) ratio of both coefficients (b), D) asymmetry factor (g) at 525 nm and E) Scattering Ångström Exponent (SAE450/635). Green areas represent the interquartile range. The whiskers extend to the minimum and maximum values within 1.5 times the interquartile range. Orange lines indicate the direction of the trend. The width of the box plots corresponds to the availability of the data. An asterisk (*) indicates a statistically significant change.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 3.** Diurnal variation of $\sigma_{sp}$ (yellow) and $\sigma_{bsp}$ (red) at 525 nm A) throughout the period, B) in winter, C) in spring, D) in summer, and E) in fall. The colored areas show the 25th and the 75th percentiles, and the horizontal curves show the median values.</figcaption></figure>

increased emissions from residential heating. The second highest values were observed in spring (median 28.23 $Mm^{-1}$ and 3.86 $Mm^{-1}$, respectively) when the still present but weakening effect of atmospheric stability and heating emissions was counterbalanced by agricultural practice and pollen emissions, as well as the enhanced generation of secondary organic aerosols (SOA, Crumeyrolle et al., 2023). The lowest median values of $\sigma_{sp}$ and $\sigma_{bsp}$ were observed in summer, 20.22 $Mm^{-1}$ and 2.61 $Mm^{-1}$, respectively (Supp. Mat., Table A.3).

The median values of SAE<sub>450/635</sub> were the highest in spring and summer, >1.8, which was consistent with the seasonal variation observed in the occurrence of NPF at the ATOLL site (Crumeyrolle et al., 2023). The lowest SAE<sub>450/635</sub> values were observed in winter periods, indicating the highest share of larger particles (SAE<sub>450/635</sub> median value 1.50), possibly due to the accumulation of particles (worse atmospheric dispersion, Suchánková et al., 2024) along with the transport of sea salt and Saharan dust to the site (Bovchaliuk et al., 2016; Crumeyrolle et al., 2023). The total minimum SAE<sub>450/635</sub> values were detected in 2020, especially in winter. During this period, stronger winds were observed, compared to other years, with the strongest contribution from SW and SSW (Supp. Mat., Fig. A.4). Furthermore, the calculation of air mass back trajectories during the winter of 2020 utilising the HYSPLIT model (version 5.3.0) demonstrated a substantial influence of marine-derived air masses arriving at the site (Supp. Mat. Fig. A.5). This suggests a notable impact of sea salt particles on the local aerosol population and its number size distribution. Similar findings regarding the correlation between SAE and sea-salt particles have been documented in previous research. Smirnov et al. (2003) observed a SAE modal value below 0.50, which they concluded was indicative of maritime air mass transport over Midway Island in the central Pacific Ocean. Vaishya et al. (2012, 2011) reported a low SAE at the Mace Head Atmospheric Research Station, accompanied by the common presence of coarse mode sea salt particles during the winter season. Pandolfi et al. (2018) discovered that SAE is lower in the Nordic, Baltic, and western geographical regions compared to central and eastern regions, potentially due to the contribution of sea salt particles in the coarse mode.

Table 1 summarizes the annual and seasonal trends in the slopes of $\sigma_{sp}$, $\sigma_{bsp}$, and SAE<sub>450/635</sub>. Both $\sigma_{sp}$ and $\sigma_{bsp}$ demonstrated a consistent

<caption>**Table 1**</caption>

Annual and seasonal slopes and percentual changes per year of $\sigma_{sp}$, $\sigma_{bsp}$, and SAE<sub>450/635</sub> during 2018–2023 ($Mm^{-1}$ per year for $\sigma_{sp}$ and $\sigma_{bsp}$, SAE<sub>450/635</sub> is dimensionless). An asterisk (*) indicates a statistically significant change ($\alpha$ = 0.05, $p$ < 0.05). The percentage of change is the ratio of slope and median value of all years/seasons.

<table><thead><tr><th>ASP</th><th>Annual</th><th>Winter</th><th>Spring</th><th>Summer</th><th>Fall</th></tr></thead><tbody><tr><td>σ<sub>sp</sub> (450 nm)</td><td>-1.54/-5%</td><td>-4.70/-12%*</td><td>-0.48/-1%</td><td>-2.79/-11%</td><td>-1.37/-5%</td></tr><tr><td>σ<sub>sp</sub> (525 nm)</td><td>-1.50/-6%</td><td>-3.47/-11%</td><td>-0.78/-3%</td><td>-2.62/-13%*</td><td>-1.42/-7%</td></tr><tr><td>σ<sub>sp</sub> (635 nm)</td><td>-1.60/-8%</td><td>-2.49/-10%</td><td>-1.48/-7%</td><td>-2.61/-17%*</td><td>-1.84/-10%</td></tr><tr><td>σ<sub>bsp</sub> (450 nm)</td><td>-0.18/-5%</td><td>-0.49/-10%</td><td>-0.17/-4%</td><td>-0.34/-11%</td><td>-0.16/-5%</td></tr><tr><td>σ<sub>bsp</sub> (525 nm)</td><td>-0.16/-5%</td><td>-0.45/-10%</td><td>-0.13/-3%</td><td>-0.24/-9%</td><td>-0.10/-4%</td></tr><tr><td>σ<sub>bsp</sub> (635 nm)</td><td>-0.16/-6%</td><td>-0.33/-9%</td><td>-0.08/-3%</td><td>-0.33/-15%</td><td>-0.14/-6%</td></tr><tr><td>SAE<sub>450/635</sub></td><td>0.036/2%</td><td>0.024/2%</td><td>0.058/3%</td><td>0.066/3%</td><td>0.076/5%</td></tr></tbody></table>

decline across all wavelengths, albeit with varying degrees of statistical significance. Annual $\sigma_{sp}$ significantly decrease (except at 450 nm) whereas annual changes in $\sigma_{bsp}$ are not statistically robust at any wavelength. At 525 nm, $\sigma_{sp}$ exhibited a notable decrease during summer, while it did not show statistically significant changes during other seasons. On the other hand, $\sigma_{sp}$ at 635 nm decreased significantly in all seasons except spring (over -10% per year).

A closer look at the monthly variation (Supp. Mat., Fig. A.2B) showed the highest $\sigma_{sp}$ in March, 34.96 $Mm^{-1}$ (the second highest in January, 34.88 $Mm^{-1}$) and $\sigma_{bsp}$ in January, 4.69 $Mm^{-1}$ with a gradual decrease towards the lowest median values in August, 17.97 $Mm^{-1}$ and 2.20 $Mm^{-1}$, respectively.

Investigating the slope variability for separate months showed an interesting anomaly: an increase in $\sigma_{sp}$ and $\sigma_{bsp}$ in March, with a slope of 10.38 $Mm^{-1}$ per year and 0.99 $Mm^{-1}$ per year, respectively. Slightly negative slopes are observed for most other months, except December
(σ<sub>sp</sub> and σ<sub>bsp</sub> 4.49 Mm<sup>-1</sup> per year and 0.42 Mm<sup>-1</sup> per year, respectively). The highest monthly decrease is observed in May (–8.29 Mm<sup>-1</sup> per year and – 0.97 Mm<sup>-1</sup> per year, respectively). The monthly slope change from negative to positive in December may have been caused by the prevailing geopolitical situation in the region - the Russian invasion of Ukraine in February 2022 and resulting oil and gas crisis in Europe. The analysis of monthly December slope excluding 2022 showed only a slight increase of 2.25 Mm<sup>-1</sup> and 0.03 Mm<sup>-1</sup> per year for σ<sub>sp</sub> and σ<sub>bsp</sub>, respectively. Further analysis is provided in Section 3.3 Relationship between aerosol light scattering properties and particle number concentration.

The SAE<sub>450/635</sub> values (Supp. Mat., Fig. A.2C) were observed at the highest from April (median 1.94) to August (median 1.82). SAE<sub>450/635</sub> values rapidly decreased in September (median 1.57) and continued to decrease towards the minimum value in January (median 1.45), where the largest particles occurred.

The patterns of σ<sub>sp</sub> and σ<sub>bsp</sub> were similar for all seasons, with the most prominent peaks at the peak of morning and evening rush hours in winter and fall (Figs. 3B and E). Daily maximum σ<sub>sp</sub> and σ<sub>bsp</sub> values were found between 10 PM and 7 AM UTC (27.31 to 29.82 Mm<sup>-1</sup> and 3.81 to 3.91 Mm<sup>-1</sup>, respectively). Both variables found their minimum values between 1 PM and 4 PM UTC (18.43 to 18.79 Mm<sup>-1</sup> and 2.55 to 2.64 Mm<sup>-1</sup>, respectively) (Fig. 3A). This diurnal variation is driven mainly by the increased atmospheric stability and the decrease in the height of the planetary boundary layer during the night and early morning hours. Neither b, SAE450/635, nor g showed significant daily variations.

Temperature, RH, and wind speed were analyzed, and no significant trend change appeared between 2018 and 2023. However, Table A.7 in Supp. Mat. shows that temperature correlated moderately negatively with both σ<sub>sp</sub> and σ<sub>bsp</sub> (525 nm) in winter and weakly negatively in fall, suggesting an increase in aerosol light scattering due to local heating emissions. Moreover, lower temperatures often lead to a shallower planetary boundary layer (PBL) due to weaker surface heating, preventing the dispersion of particles. Moreover, the aerosols trapped within PBL decrease the amount of radiation reaching Earth's surface (aerosol light scattering), therefore reducing heat transfer and suppressing dry air transfer from the free troposphere into PBL. All this could result in an increase of relative humidity (RH) and subsequent condensation on presenting particles, leading to aerosol hygroscopic growth and support secondary aerosol formation (Nair et al., 2023). Interestingly, a high concentration of aerosols within PBL enhances its stability by creating thermal inversion and reducing turbulent mixing, leading to an even larger decrease of PBL. Consequently, it creates a positive feedback by promoting a larger increase in aerosol particle concentration and accumulation (Li et al., 2017; Petäjä et al., 2016). A moderate negative correlation was observed between wind speed and the cold seasons, with a negligible influence during spring and summer. RH exhibits a weak positive correlation with both coefficients without significant seasonal variation, implying the subtle link between size growth from water condensation on particles and increased light scattering.

## 3.2. Physical properties of aerosols: Particle number concentration

Measured particle number concentrations (PNC) were divided into five size intervals: N<sub>20-30</sub> nm, N<sub>30-60</sub> nm, N<sub>60-300</sub> nm, N<sub>300-800</sub> nm, and N<sub>20-800</sub> nm to cover the full-size spectrum. Data availability is 75 %, with the highest availability in summer and the lowest in fall (87% and 58%, respectively). The median concentration of total particle number (N<sub>20-800</sub> nm) was 5780 # cm<sup>-3</sup>, with median values of N<sub>20-30</sub> nm = 1212 # cm<sup>-3</sup>, N<sub>30-60</sub> nm = 1999 # cm<sup>-3</sup>, N<sub>60-300</sub> nm = 2075 # cm<sup>-3</sup>, and N<sub>300-800</sub> nm = 43 # cm<sup>-3</sup> (Supp. Mat, Table A.4). To compare, N<sub>10-800</sub> nm in Trechera et al. (2023) varied between 3500 and 11,200 # cm<sup>-3</sup> at urban sites. These authors also show that mean N<sub>10-800</sub> nm concentration in a suburban site near Paris of 4465 # cm<sup>-3</sup> is significantly lower than that of suburban ATOLL site in Lille.

The Mann-Kendall seasonal test detected a statistically significant shift towards higher particle concentrations within N<sub>20-30</sub> nm, N<sub>30-60</sub> nm, and N<sub>20-800</sub> nm (Supp. Mat, Table A.5). On the contrary, N<sub>60-300</sub> nm and N<sub>300-800</sub> nm did not show significant changes over time, either annually or seasonally. The annual and seasonal slopes for size fractions (exhibiting any significant change over the observed period) are shown in Table 2, while the annual and seasonal variations are illustrated in Fig. 4 and Fig. 5, respectively.

Analysis of PNC evolution revealed noticeable differences between annual and seasonal trends. The annual trend represents an aggregation of the changes occurring in all seasons, smoothing out the stronger seasonal variations. In contrast, individual seasons may display more pronounced trends due to distinct processes, such as enhanced photochemical nucleation in spring or increased traffic emissions during autumn. These include the commencement of residential heating, a shallower planetary boundary layer in comparison to the summer months and increased traffic density due to the return of French residents from their summer holidays. Analysis of the number of personal vehicles (Supp. Mat., Fig. A.6), including cars and trucks traversing the metropolitan area of Lille, revealed that the highest number of vehicles were observed in February and March for personal cars (4530 and 4158 cars per day, respectively) and for trucks (128 and 118 trucks per day, respectively). Conversely, the lowest traffic density was recorded in August and November for both vehicle types (1719 and 2235 personal cars per day and 43 and 38 trucks per day, respectively, Cerema, 2024). Table 2 illustrates a significant annual increase in PNC, driven mainly by particles in the N<sub>20-30</sub> nm size range.

The N<sub>20-30</sub> nm and N<sub>30-60</sub> nm fractions are typically related through nucleation and subsequent growth processes. Despite the correlation in PNC between these two size ranges (Fig. 4), strong seasonal variations, meteorological conditions, nonlinear growth dynamics, or additional sources of N<sub>30-60</sub> nm particles may complicate their more complex relationship. The most substantial increase was observed in spring in all fractions, with the most significant change in N<sub>20-30</sub> nm showing a 37 % per year increase. This result suggests a potential increase in frequency of NPF (increase of radiation, more favorable meteorological conditions for NPF, changed dynamics of VOC emissions). Interestingly, N<sub>20-30</sub> nm reached a maximum concentration in the fall (median = 1366 # cm<sup>-3</sup>) and a minimum in the spring (median = 1051 # cm<sup>-3</sup>). This outcome could likely be attributed to the traffic-related nucleation in fall, which becomes more prominent due to lower volatility of precursors, more dense traffic compared to summer and increased atmospheric stability. Spring favored a higher concentration of N<sub>30-60</sub> nm, N<sub>60-300</sub> nm, and N<sub>20-800</sub> nm, likely due to photochemical activity promoting nucleation and growth.

Freshly nucleated particles in spring may undergo rapid size growth in the N<sub>30-60</sub> nm range and beyond, which is justified by the fact that the significant increase in spring ruled the annual increase in this size range (Table 2). This phenomenon could explain the relatively lower concentration of N<sub>20-30</sub> nm particles during this season despite active nucleation. The highest concentration of N<sub>300-800</sub> nm was observed

<caption>**Table 2**</caption>

Annual and seasonal slopes (# cm<sup>-3</sup> per year) with corresponding percentual changes per year of selected particle size modes during 2018–2023. The significance of seasonal slope trends was tested with the Mann-Kendall test, with an asterisk (*) indicating a statistically significant change (α = 0.05, p < 0.05). The percentage of change is calculated as the ratio of the slope to the median value over all years or seasons.

<table><thead><tr><th>Modes (nm)</th><th>Annual</th><th>Winter</th><th>Spring</th><th>Summer</th><th>Fall</th></tr></thead><tbody><tr><td>N<sub>20-30</sub></td><td>121/10<br>%*</td><td>172/15<br>%*</td><td>393/37 %*</td><td>126/10<br>%*</td><td>229/16<br>%*</td></tr><tr><td>N<sub>30-60</sub></td><td>54/3 %*</td><td>131/7 %</td><td>553/26 %*</td><td>64/3 %</td><td>115/8 %</td></tr><tr><td>N<sub>20-800</sub></td><td>207/3 %*</td><td>427/8 %*</td><td>1609/27<br>%*</td><td>165/3 %</td><td>516/10<br>%*</td></tr></tbody></table>
<figure><img src="image_9.png" /><figcaption>**Fig. 4.** Box plots representing the interannual variation of PNC A) N<sub>20-30 nm</sub>, B) N<sub>30-60 nm</sub>, C) N<sub>60-300 nm</sub>, D) N<sub>300-800 nm</sub>, and E) N<sub>20-800 nm</sub> from 2018 to 2023. Violet areas represent the interquartile range. The whiskers extend to the minimum and maximum values within 1.5 times the interquartile range. Orange lines indicate the direction of the trend. The width of the box plots corresponds to the availability of the data. An asterisk (*) indicates a statistically significant change.</figcaption></figure>

during the winter period, when atmospheric stability and the influence of various sources play a crucial role in the concentration of this size range (Supp. Mat. Table A.6). The only non-significant increase of N<sub>20-800 nm</sub> was observed in summer.

Given the location of the ATOLL station in the Hauts-de-France region, which was previously a hub for heavy industry, the region now comprises over 68 % of utilized agricultural land and has become highly agriculturally productive in recent decades (The European union, 2024). The ATOLL station is situated in vicinity of agricultural regions to the south and northeast, indicating a potential influence of agricultural activities on aerosol characteristics and concentrations. In a study conducted by Arslan et al. (2010), personal exposure to particulate matter (PM) in agricultural operations was examined. The findings revealed that while PM<sub>10</sub> emissions are prevalent in most mechanical processes, PM<sub>1</sub> emissions are also produced during spring activities such as soil or soil tilt packing. However, these emissions are significantly more prevalent during wheat harvest or hay making, which occurs from summer to early autumn. However, Arslan et al. limited their analysis to the mass concentration of PM, thus precluding any insight into PNC. Furthermore, the application of lime, pesticides, fertilizers, and soil erosion also contribute to the considerable source of PM (Phillips et al., 2002). Another potentially significant source of fine and ultrafine particles from agriculture is the use of agricultural vehicles and the related combustion of fossil fuels.

Regarding monthly variations, the highest median values of N<sub>20-30 nm</sub> were found in October (1554 # cm<sup>-3</sup>), followed by November and July (Supp. Mat. Fig. A.3). N<sub>30-60 nm</sub> and N<sub>20-800 nm</sub> peaked in July (2606 and 6560 # cm<sup>-3</sup>, respectively; Supp. Mat., Fig. A.3). July, one of the hottest

months with the highest solar radiation at ATOLL (median temperature 19.21 °C, maximum temperature 40.25 °C), favors NPF (N<sub>20-30 nm</sub>) and subsequent particle growth (N<sub>30-60 nm</sub> and N<sub>60-300 nm</sub>; Crumeyrolle et al., 2023). However, Garcia-Marlès et al. (2024) found that photonucleation contributed only 10.8 % to the ATOLL annual particle number concentration (N<sub>20-496 nm</sub>). Their analysis identified a substantial contribution (nearly half of all sources) of mixed traffic (43–55 nm, a highly traffic-influenced urban background) and a source of traffic nucleation (11–20 nm, NPF from emitted semi-volatile organic compounds), indicating the substantial role of traffic in the PNC at the site. The highest concentration of N<sub>20-30 nm</sub> in October at the ATOLL site may be a result of several factors: mild temperatures, stable atmospheric layers, and less wind could favor the formation of ultrafine and fine particles from traffic emissions; relatively low coagulation rate and condensation sink; traffic-related emissions dominate in October over photochemical processes. Similarly to the trends observed in σ<sub>sp</sub> and σ<sub>bsp</sub>, particle concentrations in the sizes ranges of N<sub>60-300 nm</sub> and N<sub>300-800 nm</sub> peaked in March, measuring 2571 and 76 # cm<sup>-3</sup>, respectively. Although nucleation events might not directly contribute to the enhancement of the smallest particle size ranges during this month, the increased presence of larger particles suggests the influence of different sources and atmospheric processes at this time of year. These processes may include transformation from winter to springtime, changes in atmospheric dynamics, enhanced atmospheric mixing, the synergistic effects of winter and spring particle sources, and potential contributions from agricultural activities. Although the highest concentrations of particles in different size ranges occurred in different months, the steepest positive monthly trends per year, across all size ranges, were observed in March and April,
<figure><img src="image_10.png" /><figcaption>**Fig. 5.** Box plots representing the seasonal variation of PNC within the fractions A) N<sub>20-30 nm</sub>, B) N<sub>30-60 nm</sub>, C) N<sub>60-300 nm</sub>, D) N<sub>300-800 nm</sub>, and E) N<sub>20-800 nm</sub> from 2018 to 2023. The colored areas represent the interquartile range. The whiskers extend to the minimum and maximum values within 1.5 times the interquartile range. The width of the box plots corresponds to the availability of the data.</figcaption></figure>

with slopes of 405 and 552 for N<sub>20-30 nm</sub>, 660 and 624 for N<sub>30-60 nm</sub>, 1015 (March only) for N<sub>60-300 nm</sub>, 45 (March only) for N<sub>300-800 nm</sub> and 2115 and 1479 for N<sub>20-800 nm</sub> particles cm<sup>-3</sup> per year, respectively. This evolution is thus consistent with the one observed for σ<sub>sp</sub> and σ<sub>bsp</sub>. Flemming et al. (2017) observed irregularities in atmospheric behavior in the northern hemisphere in spring in the CAMS reanalysis of aerosol optical depth; therefore, further investigation is warranted into the increasing σ<sub>sp</sub> and σ<sub>bsp</sub> and PNC in March not only at the ATOLL site but also in broader regional contexts. Interestingly, negative monthly slopes were observed for N<sub>60-300 nm</sub> in May, July, and November (–225, –188, and –117 # cm<sup>-3</sup> per year), for N<sub>300-800 nm</sub> in May, summer months and November (ranging from –7 to –9 # cm<sup>-3</sup> per year) and for N<sub>20-800 nm</sub> in July (–184 # cm<sup>-3</sup> per year). These observations highlight the dominant increase in PNC in the smallest size ranges, mainly in the spring

season.

In recent years, wildfires have become a lasting issue the combination of heat waves and reduced precipitation, not only in southern Europe but also in France (European Commission, 2024). The European Forest Fire Information System which is part of the EU Copernicus program, has been monitoring the conditions that precede fires and assessing the damage caused by fires in 43 countries in Europe, the Middle East, and North Africa. In France, the annual extent of burned areas increased from 2 581 ha in 2018 to 66,337 ha in 2022 (European Commission, 2024). While the Hauts-de-France region, where the ATOLL station is situated, was not significantly impacted, the prevailing southwest wind direction at the station could have transported particles originating from wildfires in other French regions. The majority of wildfires occurred within two distinct timeframes: February and March,
which could contribute to a positive annual monthly trend in March and summer period, July and August. This could potentially lead to increased levels of certain size ranges of PNC in summer.

A more detailed examination of the diurnal variation across different seasons revealed that, except for summer, the patterns of morning and evening peaks remained consistent (Fig. 6).

A clear dominance of traffic over photonucleation was identified in the diurnal variation of N<sub>20-30 nm</sub>, N<sub>30-60 nm</sub>, and N<sub>20-800 nm</sub> with two distinct maxima (5–7 AM and 6–9 PM UTC) occurring during traffic hours and a minimum between 11 AM and 3 PM (N<sub>20-30 nm</sub> first minimum between 12 AM and 4 PM UTC). The evening rush hour peaks were more prominent than the morning peaks for all PNC size ranges.

The ATOLL is a site where the temporal variation of PNC is driven by traffic sources (morning and evening rush hours) with weak midday photonucleation peaks, called the ‘Intermediate type site’ (Trechera et al., 2023). When photonucleation is the dominant source in summer, a noticeable midday maximum peak of N<sub>20-30 nm</sub> (1518–1749 # cm<sup>-3</sup>, 12 to 4 PM UTC) occurred. Following this peak, concentrations of N<sub>30-60 nm</sub> particles increased slowly correspondingly, indicating a sequential growth process from ultrafine to the larger size range.

Crumeyrolle et al. (2023) studied NPF at the ATOLL station (2017–2020) and identified that NPF occurred in 11 % of the measurements annually, with the highest occurrence in summer (19 %) and spring (15 %). The observed maximum peak of N<sub>20-30 nm</sub> is from 9 AM to 2 PM, where most of the NPF occurred.

When it comes to the effect of meteorological conditions on PNC in different size ranges, no significant correlation was observed between N<sub>20-30 nm</sub> and temperature, irrespective of season (Supp. Mat., Table A.7). This finding suggests the presence of additional, significant sources of this size range beyond those attributable to photonucleation, even in summer, with a significant fraction from traffic sources. However, a medium negative correlation (from −0.34 for N<sub>30-60 nm</sub> to −0.55 for N<sub>300-800 nm</sub>) with temperature was observed for all particles of larger sizes in winter, similar to ASP (−0.53 and −0.54 for σ<sub>sp</sub> and σ<sub>bsp</sub>, respectively probably due to higher residential emission coupled with higher atmospheric stability in winter leading to particle growth from

coagulation and condensation. Therefore, larger particles are observed which are responsible for the majority of aerosol light scattering in the atmosphere. Interestingly, a medium positive correlation of N<sub>60-300 nm</sub> with temperature was observed in summer, suggesting the rapid growth of particles from photonucleation (N<sub>20-30 nm</sub> and N<sub>30-60 nm</sub>) consistently with larger growth rate observed when temperature reach 20 °C in Crumeyrolle et al. (2023). Wind speed was negatively correlated with size ranges for all seasons. The strongest correlation is observed in winter and autumn, where stronger winds could lead to the dispersion of pollutants, such as preexisting particles and precursor gases, accumulated in the stable atmosphere below lower planetary boundary layer (Supp. Mat., Table A.7, and Fig. A.1). The influence of relative humidity was most pronounced for N<sub>20-30 nm</sub> in spring (R = −0.24) when RH can negatively affect NPF consistently with previous observations (Crumeyrolle et al., 2023). High RH could lead to higher cloud cover, thus decreasing the oxidative potential of the atmosphere; increase the condensational sink or RH can directly influence the formation of precursors or cluster formation (Li et al., 2019). The effect changes from negative to positive when particle size (up to R = 0.25 for N<sub>300-800 nm</sub>). In this case, increased RH can initiate faster condensation of water vapors on the particle, causing particle growth. No strong seasonal variation was observed between RH and ASP.

Our findings on the increasing particle number concentrations (PNC) in specific size ranges at the ATOLL site diverge from observations from other urban/suburban environments published thus far (see Introduction). This research project concentrated on the period between 2018 and 2023. In contrast, the data in the literature under review originated from a time point prior to 2019 in Europe and 2020 in the United States. It is essential to acknowledge that the period of interest spanned the onset of the global pandemic caused by the SARS-CoV-2 virus and the following years, as well as the beginning of the armed conflict in Ukraine in February 2022. These major events are likely to influence the evolution of PNC over large part of Europe over the most recent years, as shown here over Northern France.

Additionally, the selection of different size ranges and the presence of site-specific local particle sources may also explain some of the

<figure><img src="image_11.png" /><figcaption>**Fig. 6.** Diurnal variation of N<sub>20-30 nm</sub> (pink), N<sub>30-60 nm</sub> (yellow), and N<sub>20-800 nm</sub> (purple) in A) winter, B) spring, C) summer, and D) fall. The colored areas represent the 25th and 75th percentiles and the horizontal curves represent the median values.</figcaption></figure>
differences in time evolutions. Moreover, the majority of the reviewed publications asserted that the decline in particle number concentration was predominantly attributable to SO₂ emissions from traffic and industry, coupled with enhanced traffic management strategies (distinct from sulfur-rich fuels and advanced technologies) (Asmi et al., 2013; Hopke et al., 2024; Lorelei de Jesus et al., 2020; Mikkonen et al., 2020). It is, therefore, essential to assess the contribution of fossil fuel combustion to particle size concentration, in conjunction with the chemical analysis of traffic tracers, to evaluate traffic's influence on particle size concentration at the site. In a study conducted by Carnerero et al. (2021), an increase in the concentration of nucleation-mode particles was observed at the regional background site in Spain. This increase was primarily attributed to a reduction in the condensation sink from accumulation-mode particles and an increased insolation trend. Given the distinct locations of the ATOLL site (suburban, Northern France) and the Spanish site, a study of NPF frequency and condensational sink is recommended to elucidate the role of NPF in the observed increase in concentration of these particle size ranges alongside primary sources (mainly from traffic) at the ATOLL station.

### 3.3. Relationship between aerosol light scattering properties and particle number concentration

The results obtained for ASP and PNC are of significant long-term importance as indicators of the potential direction of evolution. A further study of these variations is recommended to confirm the emerging trends. The observed decline in $\sigma_{sp}$ at 525 and 635 nm was not accompanied by a corresponding increase in the concentration of the particle size ranges N$_{20-30}$ nm and N$_{30-60}$ nm, resulting in an overall increase in PNC (N$_{20-800}$ nm).

The results indicate a reduction in the scattering component of the direct aerosol effect on climate, accompanied by an increase in the hemispheric backscattering ratio (b), suggesting a heightened cooling potential of aerosol at the site or an elevated concentration of carbonaceous aerosol particles at the site. However, the observed increase in PNC in the PM₁ fraction indicates a potential increase in the number concentration of CCN, which may amplify the indirect climate effect. In light of these findings, further research should priorities the indirect aerosol effect, while also focusing on the absorption component of the direct aerosol effect, as both may play a critical role in future climate dynamics.

The Russian invasion of Ukraine in February 2022 led an overall crisis in the supply of crude oil and natural gas in Europe. Additionally, France encountered an energy crisis due to the unavailability of numerous nuclear power plants, which were undergoing scheduled maintenance (Rüdinger, 2023); https://www.banque-france.fr/en/publications-and-statistics/publications/energy-balance-2; Accessed 2024-11-20. The nuclear power generation crisis occurred at the worst possible time, coinciding with a period of elevated energy consumption prices in France. In January 2023, energy prices in France were 37 % higher than in January 2020 (Creel et al., 2023). Consequently, the French government has taken the decision to extend the operational lifetime of two coal-fired power stations in order to ensure the country's energy security (https://www.power-technology.com/news/france-to-extend-life-of-coal-plants-2024/; Accessed 2024-11-20), which, in conjunction with the utilization of alternative fuels such as wood or coal for domestic heating in France and neighboring countries could have resulted in a notable increase in aerosol loading and particle number concentration in higher accumulation size mode at the ATOLL station in December 2022. Further investigation is required to assess the influence of this crisis on air quality and strive for zero-net emissions in Europe.

In light of the mounting interest in urban air quality, low-cost sensors have become a popular tool to assess concentration of PM$_{10}$, PM$_{2.5}$, SO$_{2}$, CO, O$_{3}$, NO$_{X}$ etc. all over the world (Filonchyk and Peterson, 2024; Giordano et al., 2021; Morawska et al., 2018; Zheng et al., 2018). The most significant benefit of this approach is the reduced cost in

comparison to traditional in-situ measurements. Additionally, the operation is straightforward and does not require the involvement of highly trained personnel. However, low-cost optical PM sensors often operate on the principle of scattering efficiency of the bulk aerosol population. This constrains their capacity to detect the lower end of the fine and ultrafine particle size spectrum, as well as the resolved particle number size distribution. Consequently, the information they provide is limited to that pertaining to mass (Huang et al., 2021; Ouimette et al., 2022). Therefore, the results obtained from the market-available low-cost sensors should be interpreted with caution and should be combined with other measurement techniques in order to obtain comprehensive information about ultrafine particle number concentration. The discrepancy between trends in aerosol scattering properties and their particle number concentration also highlights the need for more regulations regarding the monitoring of ultrafine PNCs across atmospheric monitoring sites worldwide.

The discrepancy between our findings on PNC and previous research may be attributed to several factors. These include the focus of our study on a more recent period, which encompasses the onset of the global pandemic caused by the SARS-CoV-2 virus and the subsequent armed conflict in Ukraine; the use of slightly different size range intervals; site-specific local emissions; the characteristics of the site itself and changes in particle sources or their condensational sinks. The overall discrepancy between the observed annual decrease in PM₁ aerosol light scattering and the increase in PNC highlights the need for further, more regulated, harmonized ultrafine PNC investigation. These annual changes hide specific seasonal evolutions that need to be better understood, as for the spring season characterized by the most substantial increase in N$_{20-30}$ nm concentration, and no statistically significant change in aerosol light scattering. Although this study covers slightly over five years, the results are of long-term interest, and increased attention should be paid to additional research to confirm the emerging trends.

There is a need to shift the focus from mass concentration alone to fine particle number concentration, coupled with black carbon concentration and chemical composition, in order to facilitate detailed source apportionment and to better understand the rising prevalence of fine and ultrafine particle concentrations in predominantly urban and suburban locations in Europe, with a particular focus on the potential health implications and air quality.

### 4. Summary and conclusions

This study characterized the detailed temporal variation of light scattering properties, including scattering and backscattering coefficients ($\sigma_{sp}$ and $\sigma_{bsp}$) and climate relevant variables within the PM$_{1}$ fraction as well as particle number concentration within different size ranges (N$_{20-30}$ nm; N$_{30-60}$ nm; N$_{60-300}$ nm; N$_{300-800}$ nm; and N$_{20-800}$ nm). The study was conducted at the ATOLL suburban station in Villeneuve-d'Ascq, Northern France, from January 2018 to February 2023 and represents one of the first discussions on light scattering properties in the PM$_{1}$ fraction in an urban/suburban European environment.

The $\sigma_{sp}$ decreased significantly at 525 and 635 nm (by 6 % and 8 % annually) suggesting a reduction in the aerosol scattering mass at the site. This decrease accompanied with no significant change in $\sigma_{bsp}$ could indicate a relative increase in the aerosol cooling potential. The most pronounced decrease in $\sigma_{sp}$ at 525 nm was observed during the summer season, with a reduction of 13 % per year.

The particle number concentration demonstrated a considerable rise in the N$_{20-30}$ nm and N$_{30-60}$ nm fractions, which led to an overall enhancement in the N$_{20-800}$ nm fraction. The N$_{20-30}$ nm fraction exhibited the most substantial increase by 37 % annually in spring, possibly due to the enhanced frequency of the new particle formation. However, traffic sources exerted a dominant influence over photonucleation sources of PNC in the diurnal variation, except for summer. Further research is recommended on traffic-related emissions, NPF frequency, condensational sink, and chemical composition of particles in order to gain a
complete understanding of the increasing trend of ultrafine PNC emerging at the ATOLL site.

Even though the scattering part of the direct effect of aerosol on climate decreased at the site, an increase in the fine particle number concentration arises an interest to study the indirect aerosol effect concerning cloud formation and modification of their properties. Furthermore, research should be carried out on particles that are capable of absorbing radiation in order to gain a more comprehensive understanding of the aerosol's effect on climate. It is crucial to conduct a more comprehensive investigation of the increasing prevalence of fine and ultrafine particle concentrations in predominantly urban/suburban locations in Europe, with a particular focus on the potential health implications and climate effects.

### CRediT authorship contribution statement

**Lenka Suchánková:** Writing – review & editing, Writing – original draft, Visualization, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Suzanne Crumeyrolle:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Eric Bourrianne:** Writing – review & editing, Validation, Investigation. **Roman Prokes:** Writing – review & editing, Project administration. **Ivan Holoubek:** Writing – review & editing, Supervision. **Vladimír Ždímal:** Writing – review & editing, Supervision, Project administration. **Isabelle Chiapello:** Writing – review & editing, Supervision, Project administration.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

This work was supported by the French National Research Agency (ANR) under the MABCaM (ANR-16-CE04-0009) contract; CaPPA project, funded by the French National Research Agency (ANR) through the PIA (Program d'Investissement d'Avenir) under the contract "ANR-11-LABX-0005-01", and by the Regional Council "Hauts-de-France"; the Ministère de l'Enseignement Supérieur et de la Recherche (CPER Clibibio), and the European Fund for Regional Economic Development; the Ministry of Education, Youth and Sports of CR within the Large Research Infrastructure ACTRIS Czech Republic (LM2023030), CzeCOS (LM2023048) and MOBLILEX (Université de Lille) scholarship. Authors acknowledge support from AdAgriF - Advanced methods of greenhouse gases emission reduction and sequestration in agriculture and forest landscape for climate change mitigation (CZ.02.01.01/00/22_008/0004635). Authors thank the RECETOX RI (No LM2023069) for supportive background. This work was supported from the European Union's Horizon 2020 research and innovation program under grant agreement No 857560 (CETOCOEN Excellence). This publication reflects only the author's view, and the European Commission is not responsible for any use that may be made of the information it contains. The authors acknowledge András Hoffer from the Hungarian Meteorological Service and Maria del Mar Sorribas Panero from the National Institute of Aerospace Technology, INTA, for providing data sets of PM₁ aerosol light scattering properties and Saliou Mbengue for valuable advices during the "responding to reviewers" phase.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.scitotenv.2024.178190.

### Data availability

Data used in this research paper can be found at https://ebas.nileno/.

### References

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