Hygroscopic and optical properties of complex aerosol mixtures at the Rambouillet forest super-site of the ACROSS project

The ACROSS (Atmospheric ChemistRy Of the Suburban foreSt) project is an integrative and innovative large-scale initiative that seeks to definitively improve the understanding of the impacts of mixing urban and biogenic air masses in the largest Paris area (the Île-de-France region), where the Paris anthropogenic emissions and the biogenic emissions from the forested surroundings  give rise to the coexistence of several particulate and gaseous species. This scheme promotes the formation of complex mixtures and also influences the physico-chemical aging of aerosol plumes, with consequences on the optical and hygroscopic properties of aerosols, and ultimately on their impacts. Representing the physico-chemical properties, distribution and radiative effect of aerosols at the regional scale of Ile de France is a challenge, limiting our ability to provide reliable climate projections.

ACROSS is based on a multiple platform intensive measurement campaign which was held in June-July 2022. In this paper, we present the first results of the ground-based observations of the aerosol hygroscopic and optical properties at the ACROSS supersite within the downwind Rambouillet forest, located on the south-west of Paris. Here, the PEGASUS (PortablE Gaz and Aerosol Sampling UnitS) mobile facility was operated to deploy an HTDMA (humidified tandem differential mobility analyser) to measure the size-resolved aerosol hygroscopic properties, and a suite of optical analysers including a spectral aethalometer (Magee sci model AE33), a spectral nephelometer (TSI Inc model 3596), and three CAPS-type instruments (Aerodyne Inc, models PMEx and SSA) to measure the particle absorption, scattering and extinction coefficients at wavelengths ranging between 370 and 950 nm. The HDTMA was set to operate primarily at 85% relative humidity. . The observational period was characterised by variable conditions ranging from clean to very polluted depending on the weather.  The surface mass concentrations of submicron particles varied between 2 and 15 µg m-3 through the experimental campaign. The aerosol single scattering albedo at 450 nm varied between 0.88 and 1 during the campaign, the lowest values being observed during the heatwave period between 15 and 22 June when the aerosol optical depth reached 0.5 at 440 nm. The size growth factor (GF) for particles of 200 nm varied between 1 and 2 depending on air mass type and time of the day. The observations will be linked to the aerosol chemical composition and aerosol mixing state also observed during the campaign.