Aerosol Cloud Interactions in a Nitrogen-dominated Atmosphere (CAINA) – first highlights from AIDA cloud chamber studies

The goal of the CAINA project is to investigate multiple aspects of aerosol-cloud interactions under high concentrations of reactive nitrogen. The CAINA project is a consortium project (7 PhD students) that combines in-situ and remote sensing observations of aerosols and clouds with high-resolution modeling to study the formation of CCN, cloud chemistry, and aerosol effects on clouds under high reactive nitrogen concentrations. This is chemical regime is starting to emerge in many regions on the globe following the strong reduction of SO₂ emissions and consequently particulate sulfate concentrations. The presentation will give a short overview of the whole CAINA project and focus mainly on the results from 2 campaigns that were conducted at the AIDA cloud chamber to study the formation of aqueous SOA (AqSOA) under high reactive nitrogen concentrations.

The CAINA-AIDA campaigns are among the first experiments that investigate the influence of inorganic compounds on AqSOA formation under atmospherically relevant conditions, as opposed to more common bulk solution and flow-tube experiments. Seed aerosol consisting of inorganic salts (NaCl, NH₄NO₃, (NH₄)₂SO₄) were nebulized as aqueous solution into the 84.5 m³ AIDA chamber at 90% RH. Subsequently, secondary organic aerosol (SOA) was generated from various precursors (a-pinene, limonene, isoprene) to study aqSOA formation for several hours under dark and irradiated conditions, followed by a cloud activation of ~ 8 min. The chemical composition of the organic gas and particle phase were characterized by high-resolution mass spectrometry, using both Iodide-CIMS and PTR-MS based techniques.

First results show that SOA mass yields are strongly enhanced at 90% RH compared to dry conditions, e.g. for a factor of more than 3 for the a-pinene experiments. This coincides with changes in chemical mass spectra, which are drastic for isoprene and more moderate for a-pinene. In the case of a-pinene, considerably higher concentrations of dicarboxylic acids and water-soluble oxidation products, such as DTAA, can be observed at 90% compared to dry conditions. At 90% RH the chemical composition of the SOA depends more strongly on the type of inorganic seed particle than at dry conditions. Particularly, nitrogen containing compounds as well as oxalic and malonic acid concentrations are clearly enhanced in NH₄NO₃ containing solutions compared to NaCl. A control experiment using NaCl seeds, where NH₃ and NOx were added in the gas phase, gives a first indication that some of these compounds are preferentially formed in the liquid phase, but others in the gas phase with subsequent partitioning into the liquid phase. The effects of UV illumination and subsequent cloud activation on SOA composition will be also be presented.