Periodic Reporting for period 1 - ExploreFNP (Exploring the Molecular Properties of Atmospheric Freshly Nucleated Particles)
Período documentado: 2022-04-01 hasta 2024-09-30
Atmospheric airborne nanoparticles, better known as aerosols, have a critical impact on the global climate and human health. The largest source of aerosol particles is from gas-to-particle conversion in the atmosphere, resulting in the burst of freshly nucleated particles (FNPs) with sizes around 1-2 nm. The formation mechanisms of FNPs are believed to start with the formation of stable hydrogen-bonded atmospheric molecular clusters, which then grow to larger sizes over time via collisions with other vapours. However, currently, little is known about aerosols in this 1-2 nm size range, as they are extremely difficult to measure experimentally, and contain too many molecules to be handled with currently applied quantum chemical methods. Hence, it remains unknown at what point a cluster of molecules actually transition to become an FNP.
The exploreFNP project aims to understand the formation and properties of 1-2 nm FNPs and determine the cluster-to-particle transition point. The overarching hypothesis is that the intrinsic properties of FNPs are decisive in determining their early growth behaviour and the potential of the FNPs to grow to sizes where they can act as seeds for cloud droplet formation. ExploreFNP addresses the following objectives:
Objective 1: Determining the chemical composition of FNPs and how it affect the onset of FNPs.
Objective 2: Studying how FNPs evolve over time via the exchange of vapours with the surrounding environment.
Objective 3: Investigate how FNPs transform over time as a consequence of chemical reactions taking place inside or at the surface of the particle.
We will use a palette of methods consisting of quantum chemical methods, molecular dynamics and machine learning algorithms to obtain these goals. Reaching these objectives will allow us to validate this hypothesis and provide much-needed thermochemical and kinetic parameters that can be directly used as inputs for atmospheric process models.
The exploreFNP project aims to understand the formation and properties of 1-2 nm FNPs and determine the cluster-to-particle transition point. The overarching hypothesis is that the intrinsic properties of FNPs are decisive in determining their early growth behaviour and the potential of the FNPs to grow to sizes where they can act as seeds for cloud droplet formation. ExploreFNP addresses the following objectives:
Objective 1: Determining the chemical composition of FNPs and how it affect the onset of FNPs.
Objective 2: Studying how FNPs evolve over time via the exchange of vapours with the surrounding environment.
Objective 3: Investigate how FNPs transform over time as a consequence of chemical reactions taking place inside or at the surface of the particle.
We will use a palette of methods consisting of quantum chemical methods, molecular dynamics and machine learning algorithms to obtain these goals. Reaching these objectives will allow us to validate this hypothesis and provide much-needed thermochemical and kinetic parameters that can be directly used as inputs for atmospheric process models.
The initial work has revolved around understanding the chemical composition of FNPs. State-of-the-art quantum chemical methods can be applied to study clusters up to roughly 8 molecules. Hence, to study FNPs containing up to potentially 50 molecules we need to lower the applied level of theory, while not introducing a large error in our calculations. In addition, the more molecules we need to handle, the more complicated it is to identify the molecular assembly, which is lowest in free energy and thereby most stable.
We have developed a new configurational sampling procedure that accurately can sample the complex configurational space of FNPs. This implies that we much more reliably can identify the lowest free energy cluster structures. Unfortunately, we found that none of the existing semi-empirical methods had adequate accuracy for our target purpose. So, we have re-parameterized a new semi-empirical quantum chemical method, that can be applied to study atmospheric molecular clusters (AMC-xTB) and freshly nucleated particles (FNP-xTB). Hence, now we for the first time have an accurate methodology that can directly follow the formation of FNPs all the way from single molecules to 2 nm sizes.
We have applied the newly identified sampling protocols and methods to study different sulfuric acid (SA) – base compositions of FNPs, with the bases being ammonia (AM), methylamine (MA), dimethylamine (DMA) and trimethylamine (TMA). We found that for small clusters (up to 4 acid-base pairs) the base molecule is very important and determines the nucleation properties. Hence, the formation follows the basicity of the base. For larger clusters (above 10 acid-base pairs) we found that the free energy per acid-base pair begins to level out, indicating that we are reaching a regime where the clusters behave as bulk. Here,
the basicity of the clustering base is less pronounced and hydrogen bond capacity of the base begins to contribute substantially. Overall, our results show that based on the properties of the clusters, we can disentangle the nucleation regime from the growth regime and have been able to determine the actual cluster-to-particle transition point in these systems. This cluster-to-particle transition point is found to coincide with the emergence of the first solvated ions in the cluster, i.e. the first fully coordinated molecule. We propose to define the onset of FNPs as this exact cluster-to-particle transition point.
We have developed a new configurational sampling procedure that accurately can sample the complex configurational space of FNPs. This implies that we much more reliably can identify the lowest free energy cluster structures. Unfortunately, we found that none of the existing semi-empirical methods had adequate accuracy for our target purpose. So, we have re-parameterized a new semi-empirical quantum chemical method, that can be applied to study atmospheric molecular clusters (AMC-xTB) and freshly nucleated particles (FNP-xTB). Hence, now we for the first time have an accurate methodology that can directly follow the formation of FNPs all the way from single molecules to 2 nm sizes.
We have applied the newly identified sampling protocols and methods to study different sulfuric acid (SA) – base compositions of FNPs, with the bases being ammonia (AM), methylamine (MA), dimethylamine (DMA) and trimethylamine (TMA). We found that for small clusters (up to 4 acid-base pairs) the base molecule is very important and determines the nucleation properties. Hence, the formation follows the basicity of the base. For larger clusters (above 10 acid-base pairs) we found that the free energy per acid-base pair begins to level out, indicating that we are reaching a regime where the clusters behave as bulk. Here,
the basicity of the clustering base is less pronounced and hydrogen bond capacity of the base begins to contribute substantially. Overall, our results show that based on the properties of the clusters, we can disentangle the nucleation regime from the growth regime and have been able to determine the actual cluster-to-particle transition point in these systems. This cluster-to-particle transition point is found to coincide with the emergence of the first solvated ions in the cluster, i.e. the first fully coordinated molecule. We propose to define the onset of FNPs as this exact cluster-to-particle transition point.
Identifying when a given assembly of molecules is characterized as a cluster and when it can be regarded as a particle has been a long-standing debate in the aerosol research community. We believe to have the first property-based evidence on how to define this point. This finding is certainly beyond the current state-of-the-art and open for further studies to understand how different vapours influence this cluster-to-particle transition point. We are eager to further study this phenomenon.