Climate impacts of atmospheric low volatility organic compounds

There exist huge gaps in the knowledge of how cloud-aerosol interaction affects climate. Consequently global models estimating the radiative forcing by anthropogenic aerosols show considerable discrepancy. Especially challenging to quantify is the role of volatile organic compounds in forming aerosol particles which can act as cloud condensation nuclei. Volatile organic compounds are emitted into the atmosphere in large quantities by biogenic and anthropogenic sources. In the atmosphere they undergo chemical oxidation reactions and typically produce products that are highly oxygenated and have lower volatility. Volatility of these highly oxygenated molecules span a wide range and determine the ease with which they transfer to the aerosol phase - either via participation in new particle formation or by contributing to the growth of bigger particles. The extent to which the highly oxygenated molecules contribute to new particle formation or their subsequent growth impacts the number concentration of cloud condensation nuclei in the atmosphere. Hence to accurately estimate cloud condensation nuclei, global models need to take into account the role of highly oxygenated molecules of varying volatility in modulating the atmospheric aerosol size distribution. In this thesis a new nucleation parameterisation based solely on highly oxygenated species of extremely low volatility is added to the model and its impact on the estimated cloud albedo effect is assessed. The nucleation mechanism is based on the findings of the CLOUD Experiment at CERN. The implementation of this new parameterisation reduces previous model estimates of cloud albedo forcing through its impact on the pre-industrial atmosphere. The thesis then goes on to introduce a new secondary aerosol formation scheme from highly oxygenated organic molecules based on the understanding of recent scientific advancements and assesses the effect of implementing the scheme on the estimated cloud albedo effect. Results show highly oxygenated molecules of semi-volatile nature play a significant role in determining the number concentration of cloud relevant particles. Although their higher volatility renders them incapable of new particle formation, their atmospheric abundance and contribution to the growth of particles which are relatively larger, provide an efficient pathway for producing cloud condensation nuclei in the atmosphere.
Further, an ensemble of simulations are produced and analysed to explore a 6-D parameter space based on pre-defined uncertainty ranges of these highly oxygenated molecules. The work identifies plausible and implausible regions within the 6-D space, based on model-observation comparison against three model outputs - number concentration of all particles, number concentration of CCN-relevant sized particles and organic aerosol concentration. The work provides a top-down estimate of yields of highly oxygenated molecules (that contribute to SOA formation) based on model skill score against ground-based observations. Such yields are typically based on laboratory experiments and is broadly considered to be an important reason behind the failure of global models to estimate realistic mass of secondary organic aerosols produced in the atmosphere. The work particularly highlights the importance of simulating cluster growth from low-volatility organic compounds to account for atmospheric cloud droplets.