Novel characterization of pollen fragments and secondary aerosol in the Midwestern United States
Creators: name: Hughes, Dagen D
Publisher: name: University of Iowa
Year: 2024
Subjects: Bioaerosols
Midwest
Organosulfates
Particulate Matter
PMF
Pure sciences
public abstract
Environmental Science

description: Atmospheric particulate matter (PM) is a complex mixture of solid and liquid particles that range in diameter from a few nanometers to 100 micrometers. PM adversely affects human health and influences Earth’s climate by direct and indirect radiative forcing. Through field-based research in the Midwestern United States, this thesis provides new insight into the composition and sources of atmospheric PM in this region with three primary objectives. First, the concentrations and properties of atmospheric pollen particles are evaluated during rain events and severe weather, including sub-pollen particles (SPP) that form when intact pollen grain rupture. Second, the impact anthropogenic emissions of sulfur dioxide have on secondary organic aerosol (SOA) formation is examined through qualitative and quantitative measurements of organosulfates. Third, the sources of atmospheric PM in the Midwestern United States are assessed during air quality episodes along the coast of Lake Michigan. Together, these studies demonstrate that air quality in the Midwestern United States is dynamic, with diverse and temporally variable influences from natural and anthropogenic sources.
description: Presented herein are the first online measurements of SPP in the atmosphere. One such example occurred during a thunderstorm in the spring tree pollen season (May 18, 2019) when the submicron fluorescent particle concentration increased from 0.1 cm-3 to 1.3 cm-3. At the peak, 34% of submicron particles fluoresced compared to only 5% prior to the storm, indicating an enrichment in fluorescent particles. Fluorescent particles were confirmed as SPP using collocated measurements of carbohydrates associated with pollen and microscopy images of intact and ruptured pollen grains. Across 26 rain events, SPP varied in size, ranging from the lower detection limit of the instrument (0.5 µm) to 13.2 µm in diameter. The maximum abundance was observed for 0.5-1.1 µm particles that accounted for an average of 40% of the total fluorescent signal. In general, elevated concentrations of SPP were associated with thunderstorms that had strong downdrafts, rainfall rates greater than 20 mm hr-1, elevated wind speeds, and lighting strikes. Due to their small size, SPP persist in the atmosphere for longer periods of time and penetrate deeper into the human respiratory system compared to intact pollen grains. This work confirms prior laboratory studies that show pollen grains can rupture in the presence of water or high humidity. Moreover, this work highlights the concentration, properties, and occurrence of SPP in the atmosphere using online, single-particle measurements, which provide new insights regarding their health risks and potential exposures.
description: Air quality is also impacted by the formation of PM in the atmosphere through secondary processes. Organosulfates are a unique class of SOA that form through reactions of biogenic gases released from plants with bisulfate that originates from fossil fuel combustion. Through the chemical characterization of PM2.5 (particles with a diameter less than 2.5 µm) collected in Iowa City, Iowa, my research provides the first empirical evidence that organosulfates have a sizable contribution to ambient PM2.5 organic carbon (OC) in the Midwestern United States (4.4%). Importantly, organosulfates were determined to be stable on substrates stored frozen at least one-year post-collection, supporting their chemical measurements by offline filter sampling. Of the 22 organosulfate species identified, 11 were derived from isoprene, 7 from monoterpenes, 3 from anthropogenic volatile organic compounds, and one of unknown origin. This study provides new insights into the abundance and identity of organosulfates in the Midwestern United States and demonstrates that the organosulfates contribution to OC is significant. The observed concentrations were within a factor of 2-3 of those observed in the Southeastern United States. Following that organosulfates are effective at tracking anthropogenically influenced biogenic SOA, this study highlights a portion of PM2.5 that can be controlled by the reduction of fossil fuel use.
description: Organic molecules were also used to identify the sources of PM2.5 OC during the 2017 Lake Michigan Ozone Study (LMOS 2017), which included three periods of elevated ozone. PM2.5 mass concentrations ranged from 1.5 – 13 µg m-3 with an average of 5.2 µg m-3. The most significant contributor to PM2.5 mass was organic matter (OM; calculated as 1.7 x OC; contributing 59%), followed by sulfate (17%), ammonium (6%), nitrate (4%), and elemental carbon (EC; 3%). One of three ozone episodes in LMOS 2017 was largely influenced by biogenic SOA with chemical signatures of organosulfates, leading to relatively high PM levels. During this episode, the highest concentrations of sulfate and aerosol acidity were measured and organosulfates accounted for 15% OM, which compared to 4% OM contribution on average, demonstrating the importance of anthropogenic sulfate emissions and aerosol acidity on SOA formation.
description: Using a combination of inorganic and organic tracers in positive matrix factorization, nine sources of ambient PM2.5 in Zion, Illinois were identified. These included crustal material (accounting for 11% of PM2.5 mass on average), biomass burning (6%), vehicle emissions (5%), secondary sulfate (24%), secondary nitrate (11%), acid-catalyzed biogenic SOA (10%), isoprene SOA (7%), and two mixed primary and secondary sources having chemical signatures of fossil fuel combustion and aromatic SOA that peaked in either daytime (17%) or nighttime (9%). In total, these nine sources accounted for 96% of the measured PM2.5 mass on average. For the three periods of elevated ozone, period A (2–3 June) was strongly influenced by anthropogenic sources, period B (10–13 June) was largely impacted by secondary sources of PM with the greatest contribution to PM observed for acid-catalyzed biogenic SOA, and period C (13–17 June) was heavily influenced by anthropogenic sources with smaller contributions from biogenic sources. Together, these results highlight the major sources impacting PM throughout LMOS 2017. These results also demonstrate that anthropogenic sources influence primary and secondary aerosol across all three ozone periods.
description: Overall, the work presented in this dissertation advances the knowledge of the sources and chemical composition of PM in the Midwestern United States. This work provides insight into the spatial and temporal variation in biological and anthropogenic PM, while demonstrating temporal variability driven by meteorology and emission sources. Through the study of bioaerosols, it was found that air quality in the Midwestern United States was worse during rainfall and can remain degraded for hours after the rain subsides. Through the study of SOA, it was found that anthropogenic sources consistently impact air quality in the Midwestern United States by introducing PM directly as well as enhancing the formation of secondary PM. Ultimately, this advancement in the understanding the sources of PM can aid in air resources management in the Midwestern United States and globally.