Cloud-Aerosol-Interactions in a Nitrogen-dominated Atmosphere (CAINA)

Reactive nitrogen compounds emitted from agriculture, traffic and industry are quickly becoming dominant air pollutants worldwide. This has severe consequences for air quality, ecosystems, and global climate. CAINA aims towards fully understanding the aerosol-cloud interactions under high reactive nitrogen concentrations. We focus on the Netherlands, where ammonia emissions from agriculture combine with NOx from industry and traffic, resulting in high concentrations of ammonium nitrate. These regularly exceed concentrations of sulfates, the main air pollutants in most other regions, by a factor of 5 or more. This makes the selected study region ideal for understanding the chemistry of the future global atmosphere.

Clouds are an important component in the life-cycle of reactive nitrogen. First, they are crucial for wet deposition of nitrogen, which has detrimental effects on vulnerable ecosystems. In addition, via chemical reactions in cloud droplets, clouds contribute to the production of nitrogen-containing inorganic (e.g., ammonium nitrate) and organic aerosol particles, with important consequences for air quality and human health. However, it is uncertain how these aqueous-phase reactions proceed under high concentrations of reactive nitrogen and at the relatively high pH values associated with nitrogen-dominated aerosols.

Clouds themselves can also be changed by nitrogen pollutants. Increases of cloud condensation nuclei (CCN) and nitric acid concentrations lead to clouds with more, but smaller droplets. This potentially results in more reflective clouds, with possible cooling effects on global climate. However, we do not yet fully understand the role of reactive nitrogen in CCN production and cloud droplet activation. Therefore, field and laboratory experiments combined with model studies are urgently needed to better understand how cloud properties are modified by reactive nitrogen pollutants.

Our consortium combines strong expertise in aerosol/cloud physics with expertise in atmospheric chemistry to answer the following research questions:

i. How are inorganic and organic pollutants produced in clouds under high reactive nitrogen concentrations?

ii. How important is this pathway for the overall air pollution in regions with high nitrogen emissions?

iii. What is the concurrent effect on cloud microphysics and cloud reflectivity?

A central role in the CAINA project will be played by the Ruisdael Observatory, equipped with extensive observational tools for pollutants, wind fields, and clouds. This creates an ideal outdoor laboratory to study pollutant-cloud interactions under varying nitrogen regimes. We will combine long-term observations and intensive field experiments to study processes that govern CCN concentrations, effects of nitrogen pollutants on cloud microphysics, and cloud processing of aerosols. The field measurements will be interpreted with the help of controlled cloud chamber experiments in the dynamic AIDA cloud chamber, and with the use of high-resolution, cloud-resolving modelling. The observational strategies will optimally constrain a unique Large Eddy Simulation (LES) model that can simulate both non-equilibrium chemistry of nitrogen pollutants and cloud-aerosol interactions. The constrained LES model will quantify both the relevance of cloud processes for nitrogen-containing pollutant levels as well as the pollutant effects on cloud properties. This knowledge is of vital importance to better predict the impact of the anticipated global shift to nitrogen-dominated pollution regimes.