Understanding the Removal of Atmospheric Aerosol in a Tropical Marine Environment
Publisher
The University of Arizona.Rights
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction, presentation (such as public display or performance) of protected items is prohibited except with permission of the author.Abstract
Aerosols and their interactions with clouds remain the largest sources of uncertainty in our understanding of the atmosphere and climate. A major factor in this uncertainty is the wet scavenging (removal) of aerosols in global models, which negatively impacts model capabilities to capture aerosol lifetimes and, consequently, aerosol impacts on climate and air quality. This dissertation focuses on scavenging over the tropical West Pacific region and consists of three distinct approaches: (1) a ground-based study investigating factors contributing to the inter-seasonal persistence of aerosol concentrations in a tropical coastal megacity despite higher precipitation during the wet season, (2) a multi-tool study using aircraft data that determines meteorological variables relevant for scavenging during long-range transport, and (3) an aircraft-based study calculating in-cloud scavenging efficiencies of multiple aerosol species and sizes in tropical convection. In the first part of the dissertation, we analyzed size-resolved aerosol composition, aerosol optical depth, and meteorology to understand why Metro Manila, Philippines exhibits similar aerosol concentrations across seasons despite large differences in seasonal rainfall. We identified two major factors: (1) opposing seasonality of black carbon and water-soluble aerosol, and (2) inefficient scavenging by short rain events (< 1 h). We demonstrated that the presence of rain does not imply efficient wet scavenging and it is important to consider rain characteristics like duration. In a changing climate with increasing urbanization, these factors are expected to become more critical for air quality policymaking and sustainable urban development. This work was published in Environmental Science: Atmospheres (Hilario et al., 2022). In the second part, we identified meteorological variables relevant for estimating wet scavenging using trajectory modeling and a combination of aircraft, satellite, and reanalysis data. We found that the accumulated precipitation along trajectories – often interpreted as a wet scavenging indicator in the literature – does poorly when used to predict aerosol scavenging and was outperformed by the following variables: (1) upper percentiles of relative humidity (RH) along trajectories, (2) the fraction of hours along trajectories exceeding a threshold value for RH or water vapor mixing ratio, and (3) precipitation intensity along trajectories. This work was published in Atmospheric Measurement Techniques (Hilario et al., 2024). The final part of this dissertation quantified in-cloud scavenging efficiencies (SE) in tropical convection. In-cloud scavenging is the primary removal pathway for accumulation mode aerosols, but SEs have not been calculated for shallow to moderate convection. We used aircraft data to calculate SEs for three cases. Efficient scavenging was observed for sulfate (>86%) and black carbon (70 – 80%); moderate scavenging for organic aerosol (53 – 60%) and nitrate (62%); and a wide range of SEs for ammonium (53 – 87%). We also found that accumulation and coarse mode aerosol volume concentrations were nearly totally scavenged in-cloud (>92%), suggesting a preferential activation of large aerosols. Comparisons of differing cloud tops showed that SEs did not vary significantly on an aerosol mass basis with cloud top height. These results demonstrate that aerosol size and composition are more important for in-cloud SEs. This work was published in the Journal of the Atmospheric Sciences (Hilario et al., 2025). This dissertation provides an explanation for how aerosol loadings can be sustained during the wet/rainy season, which should be investigated in other developing cities due to the health risks associated with pollutant accumulation. The dissertation also provides suggestions for meteorological variables that could be considered in model scavenging parameterizations. The presented method can be repeated in different environments to identify regional differences in factors that influence scavenging. Finally, the calculated in-cloud SEs motivate improvements in chemical transport models through future observation-model comparisons.Type
textElectronic Dissertation
Degree Name
Ph.D.Degree Level
doctoralDegree Program
Graduate CollegeAtmospheric Sciences