Universität zu Köln

Milousis, Alexandros ORCID: 0000-0002-8975-9210 (2025). Advances in Understanding Nitrate Aerosol Formation and the Implications for Atmospheric Radiative Balance. PhD thesis, Universität zu Köln.

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Abstract

Recent decades have highlighted the profound consequences of air pollution on air quality and human health, resulting in millions of deaths worldwide and contributing to the intensification of the Earth's climate due to ongoing anthropogenic emissions. These emissions, originating from densely populated and industrialized regions, lead to the release of gaseous pollutants that undergo chemical transformations in the atmosphere, producing secondary particulate pollutants. Atmospheric modeling is a valuable tool that facilitates a more profound comprehension of physicochemical processes, thereby providing guidelines for mitigating air pollution and enhancing our understanding of climatic feedback mechanisms. Recent policies aimed at reducing emissions from anthropogenic activities have predominantly focused on specific species, including carbon dioxide (CO2), methane (CH4), sulfur dioxide (SO2), and nitrogen oxides (NOx). This is expected to cause a change in the landscape of secondary aerosol population characteristics as the abundancy of their precursors will also change. For example, the observed historical increase in ammonia (NH3) emissions is expected to enhance the importance of certain inorganic aerosol species at the expense of others. A substantial body of research conducted in the most heavily polluted regions of the Northern Hemisphere has already demonstrated that the average concentration of aerosol nitrate is comparable to, if not greater than, the respective concentration of aerosol sulfate. Sulfate is currently recognized as the most substantial contributor to the total inorganic aerosol mass on a global scale. Consequently, the estimation of aerosol nitrate by atmospheric models has become increasingly crucial, and the number of models that include this species in their calculations is steadily rising, despite not being the norm in the past. This thesis aims to address a key assumption that can influence the estimation of nitrate aerosols (NO3-) by models. This assumption is their physical state (i.e., solid or liquid). Aerosols typically crystallize and form solids when exposed to decreasing ambient relative humidity, though this process is often complex due to various aerosol compositions and the hysteresis effect .In thermodynamics, particles that form solids are considered to be in a stable state; however, aerosol water can exist even in very low humidity values, leaving particles in a supersaturated aqueous state called metastable. Utilizing a state-of-the-art chemistry and climate model (EMAC) and a recently developed version of a thermodynamic equilibrium model (ISORROPIA-lite), the study explores the hypothesis that the state assumption significantly impacts inorganic aerosol estimations. Additionally, it examines the impact of the aerosol physical state on the estimated particle acidity, as this is another quantity that influences the aerosol partitioning process. Furthermore, the thesis investigates a number of factors that are known to influence the model's ability to accurately estimate NO3- concentrations in regions of high anthropogenic activity, with a particular focus on the polluted North Hemisphere (East Asia, India, Europe, and North America). The objective is to ascertain the most significant factors that contribute to the best replication of observations of NO3- in sizes less than 1 μm and 2.5 μm in diameter (PM1 and PM2.5, respectively). The analysis is further expanded to encompass the recognition of any seasonal patterns as well as measurement location patterns. Finally, the study examines the interactions between nitrate aerosol and mineral dust, thereby investigating the climatic impact of NO3- with respect to its radiative effect from both aerosol-radiation interactions (direct effect) and aerosol-cloud interactions (indirect effect).The importance of considering dust-nitrate interactions when examining such metrics is also quantified. The study found that the physical state assumption has a minimal impact on the global budget of key inorganic aerosol species, including NO3-, SO42-, and NH4+, as well as non-volatile cations, with overall differences being less than 10%. Consequently, for the purposes of climatic or air quality simulations that cover a long time period and consider a global scale, that choice is not expected to have a significant impact. However, the metastable assumption has been shown to yield faster simulation times, with an average increase of approximately 4-5%.In regions characterized by consistently low relative humidity values and/or mid-range temperatures, the assumption of considering only liquid particles has been found to result in lower concentration estimates, with NO3- concentrations being reduced by up to 40%, and slightly more acidic particles by up to 1 unit. Consequently, for analyses that consider specific regions, the aerosol physical state assumption assumes greater importance. Concerning the factors influencing the accuracy of NO3- estimations, it was ascertained that, on average, a high model grid resolution and a low dinitrogen pentoxide (N2O5) hydrolysis coefficient tend to yield better agreement with observations in both sizes. The employment of disparate anthropogenic emissions databases emerged as a significant factor influencing model estimations, particularly in replicating PM1 NO3- concentrations across diverse regions. In general, there is no 'perfect' model setup capable of best capturing both PM1 and PM2.5 NO3- concentrations across all regions simultaneously. Depending on the area of interest, different parameterizations yield superior rates of agreement. Furthermore, it was determined that nitrate aerosols induce a net cooling direct radiative effect of -0.11 W/m2, primarily attributable to the scattering of SW radiation by smaller size modes, accounting for 85% of this estimate. Conversely, nitrate aerosols have been observed to induce a net warming indirect radiative effect of +0.17 W/m2, which is attributed to the depletion of smaller-sized particles (i.e., anthropogenic pollution) through coagulation with larger particles (i.e., dust). This depletion results in the formation of less low-level warm clouds, which decreases the amount of SW radiation that is reflected back to space. The efficacy of this mechanism is further augmented by nitrate-dust interactions, which augment the size of dust particles through adsorption and coating processes. The incorporation of dust chemistry is of paramount importance when compared to assumptions for dust composition or dust loading, as its omission engenders an underestimation of the aforementioned estimates by up to 45%.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Milousis, Alexandros a.milousis@fz-juelich.de orcid.org/0000-0002-8975-9210 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-780673
Date:	7 May 2025
Publisher:	Zentralbibliothek
Place of Publication:	Forschungszentrum Juelich
ISBN:	978-3-95806-823-0
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Außeruniversitäre Forschungseinrichtungen > Forschungszentrum Jülich
Subjects:	Natural sciences and mathematics Physics Earth sciences
Uncontrolled Keywords:	Keywords Language Nitrate English Aerosols English Radiative English Model English Bias English Acidity English Dust English Thermodynamic English
Date of oral exam:	24 March 2025
Referee:	Name Academic Title Fuchs, Hendrik Prof. Dr. Pozzer, Andrea Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/78067