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

Aerosol particles in the PM 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 PM 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 PM fraction at the atmospheric site ATOLL (The Atmospheric Observations in Lille), Northern France from January 2018 to February 2023. The total scattering coefficient σ 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) 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 and N which led to an increase in the total N size range. N 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.