The goal of the CAINA (Cloud-Aerosol Interactions in a Nitrogen-dominated Atmosphere) project is to investigate multiple aspects of aerosol-cloud interactions under high concentrations of reactive nitrogen. This chemical regime is starting to emerge in many regions following the strong reduction of SO2 emissions, but is already firmly established at our study location in the Netherlands. CAINA is a consortium project that aims to combine in-situ and remote sensing observations of aerosols and clouds with chamber experiments and high-resolution modelling to study the formation of CCN, cloud chemistry, and aerosol effects on clouds.

This talk will present first highlights of the CAINA project focussing on the cloud chamber
experiments and the field campaign conducted in March/April 2025.

Extensive studies in the AIDA cloud chamber have shown that substantially more secondary organic aerosol is formed under high humidity (80-90%) than at dry conditions, when liquid seed particles are present. This is accompanied with distinct differences in the chemical composition of the formed SOA. We can show considerable formation of secondary organic aerosol in the aqueous phase and that the presence of ammonium nitrate in the particles causes the formation of organic nitrogen species and other higher-order reaction products.

First results from the field campaign at a coastal and a regional background site in the Netherlands highlight the high ammonium nitrate contributions to the aerosol mass concentration and especially high gas-phase NH3 concentrations (up to 50 mg m-3) during the field campaign, indicating a chemical regime dominated by reactive nitrogen and relatively high aerosol pH. Further highlights include strong new particle formation events, as well as distinct differences in particle chemical composition between the ground and at 250 m height, particularly when clouds were overhead. A potential effect of nitrogen pollution on cloud properties will be investigated, combining ground-based data, remote sensing by cloud profilers, and in-situ cloud measurements using the helicopter-borne cloud probe ACTOS.

Aerosol-cloud interactions (ACI) are a significant source of uncertainty in climate projections. Nitrogen-dominated aerosol episodes are emerging over the Netherlands, strongly influencing local air quality and climate, but our understanding of aerosol-cloud interactions under these nitrogen-dominated conditions is still not well quantified. Ground-based remote-sensing instruments like cloud radars can provide us high temporal and spatial resolution data for cloud microphysics, like cloud droplet number concentration, and aerosol properties can be obtained using lidar measurements. In this study, we quantify how these aerosol particles in nitrogen-polluted episodes affect low-level clouds by combining remote-sensing observations with aerosol speciation measurements at the Ruisdael Observatory in the Netherlands.

Generally, column aerosol optical depth (AOD) from sun photometers and vertically resolved attenuated backscatter (ATB) from ceilometers are used as aerosol proxies. A key difference is that AOD represents extinction integrated over the full atmospheric column, whereas ATB is a vertically resolved backscatter profile, and ATB must therefore be vertically integrated for a meaningful comparison. However, both respond differently to meteorological parameters, aerosol loadings, and the instrument’s configurations. Therefore, understanding the variation of ATB and AOD in response to meteorology is essential. Overall, our framework will provide consistent conditions under which ceilometer ATB can be used as an aerosol proxy along with the column AOD during nitrogen-dominated episodes.

Here, we use a Mie model framework to investigate how ATB and AOD behave under different aerosol compositions, loadings, and meteorological conditions. Further, using a long-term observation from Cabauw (the Netherlands) as a case site, we focus on periods when nitrate clearly dominates the aerosol composition. Surface data from aerosol mass spectrometry and size-distribution measurements are combined with ceilometer profiles, sun-photometer retrievals, and meteorological data. Together, these measurements allow nitrogen-dominated episodes to be grouped by composition, relative humidity, and boundary-layer conditions, providing a consistent way to quantify aerosol-cloud interactions.

Our initial results indicate that, during nitrate-dominated episodes, hygroscopic aerosol particles build up in the boundary layer and strongly enhance light extinction. Extinction, backscatter, and other related aerosol optical properties respond strongly to RH-driven particle growth, making the growth factor a key control on the observed signals. We will investigate these relationships in more detail using measurements from both the RITA-2021 and the CAINA-2025 campaign datasets. These nitrate-rich aerosols act as cloud condensation nuclei (CCN), and they are expected to increase cloud droplet number concentration with more but smaller cloud droplets, which can be detected by ground-based cloud radar observations.

The resulting framework provides insight into how nitrogen-rich aerosol pollution affects clouds' microphysical properties and strengthens the understanding of aerosol-cloud interactions in nitrate-dominated environments.

Black carbon (BC) aerosol is a short-lived climate forcer. BC aerosols are known to perturb the earth’s energy balance by interacting with radiation, modifying the cloud properties, and thereby changing the atmospheric heating pattern associated with various phenomena including the South Asian Summer Monsoon (SASM) system. However, many facets of BC in modulating the SASM have been debated. In this study, we investigate the relative impacts of an increases in local versus remote and global BC emissions on the SASM precipitation. We further the investigate the fast and slow responses of the SASM system to increases in BC emissions. We use a variant of the state-of-the-art Community Earth System Model (CESM1.2) and conduct a series of numerical experiments to understand the BC effects on the SASM system. Our results show that increased global BC emissions results in significant surface cooling across the south Asian region (especially over north India) and surface warming over the Tibetian Plateau, northern China and Mongolia region. Increased BC emissions also result in upper atmospheric warming in mid latitudes and a northward shift of the subtropical westerly jet stream. South-westerly winds over the Arabian Sea gets stronger with increases in both local and remote BC emissions. Increases in global BC emissions increases the seasonal mean SASM precipitation almost across South Asia except over the eastern Himalayan region and central peninsular India. While decreases in precipitation over peninsular India are found to be largely due to increases local BC emissions but increases in precipitation over northwest India and changes in precipitation in all other parts of India are mostly due to increases in remote BC emissions. Our results further show that unlike the combined effect of increases in all aerosols, SASM system response to increases in BC aerosols alone is dominated by the fast responses as compared to the slow responses induced through SST change.