Browsing by Author "Cohan, Daniel S"

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    Atmospheric chemistry and aerosol associated with the coastal urban environment
    (2025-02-26) Chao, Chun-Ying; Cohan, Daniel S; Griffin, Robert J
    This study aims to understand the dynamics of atmospheric secondary organic aerosol (SOA) in Houston, Texas. It (1) explores the relationship between sea breeze and air quality, (2) investigates the spatiotemporal characteristics of aerosols and SOA formation rates, and (3) analyzes factors that impact new particle formation (NPF). Long-term monitoring demonstrated that PM2.5 (particles with diameters of 2.5 m or smaller) concentrations are ~30% larger on days with southerly wind or sea breeze recirculation compared to those under other wind patterns. On southerly wind days, 53% of PM2.5 was attributed to long-range transport of soil. In contrast, on sea breeze recirculation days, 60% of PM2.5 was attributed to anthropogenic sources and only 15% to soil sources. SOA also appeared to be important on sea breeze recirculation days. To investigate in-situ SOA mechanisms and NPF in Houston, aerosol composition and size distribution were measured during the TRACER field campaign from July to September 2022. During this campaign, OA contributed the most significant fraction of aerosol. Assuming two OA factors - less-oxidized oxygenated OA (LO-OOA), and more-oxidized oxygenated OA (MO-OOA) - represent SOA, a mass balance model was used to estimate the SOA formation rate. For MO-OOA, the largest formation rates aligned with intense photo-oxidation processes. For LO-OOA, the most significant formation rates occurred at night, likely associated with nitrate radical chemistry. Substantial differences in particle compositions and concentrations were observed on NPF and non-NPF days. PM1 mass concentrations were 7% to 76% lower on NPF days compared to non-NPF days across all sites. However, the fractional increase in the contributions of sulfate and MO-OOA on NPF days allows enhanced uptake of gases. These results suggest that, given similar condensation sinks, other atmospheric conditions play a critical role in determining the occurrence of NPF. This study provides a comprehensive investigation of SOA formation, NPF, and impacts of meteorological conditions in Houston. These findings contribute to efforts to improve air quality in coastal urban areas in order to meet the National Ambient Air Quality Standards.
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    Integrated Modeling of Agricultural Reactive Nitrogen in the United States: Emissions, Impacts, and Mitigation Strategies
    (2023-08-10) Luo, Lina; Cohan, Daniel S
    Fertilizer-intensive agriculture is a leading source of reactive nitrogen (Nr) emissions, including nitric oxide (NO), nitrous acid (HONO), and ammonia (NH3), that damage air quality, climate, and human health. As air pollutants, NO and NH3 contribute to the formation of ozone (O3) and particulate matter (PM), with impacts that depend on spatiotemporally variable atmospheric conditions. However, N2O, as a potent greenhouse gas, has climate impacts that are independent of when and where it is emitted. Additionally, the impacts of control strategies on Nr emissions vary widely and may introduce trade-off effects. Therefore, there is a strong need for a comprehensive assessment that considers the emissions, impacts, and mitigation strategies for Nr. I established an integrated assessment framework that connects a process-based agroecosystem model with reduced-complexity air quality and health models, along with the social cost of greenhouse gas. This framework enables us to consistently estimate Nr emissions, spatially track their contributions to air pollution, jointly quantify the associated damages using monetized damages as the metric, and assess the efficacy of control strategies. I applied this framework to investigate Nr emissions from U.S. croplands, their associated adverse impacts, and their responses to two mitigation strategies: carbon amendments and nitrification inhibitors (NIs). This study revealed that the Nr emissions are highest in regions with intensive fertilizer use. NH3 is the most damaging, especially in densely populated regions. Impacts of carbon amendments on Nr emissions would vary widely, with net effects depending mostly on how nitrification is affected. Biochar amendments are most likely to mitigate emissions if applied at high rates in acidic soils with low organic carbon and inorganic nitrogen content. Adding NIs to fertilizers can mitigate N2O and NO emissions but inadvertently stimulate NH3 emissions. Impacts of NIs are largest in regions with moderate soil temperatures. Net impacts of NIs would be negative across most regions as the health harms from NH3 stimulation outweigh the benefits of N2O and NO mitigation. This study provides valuable insights into Nr emissions mitigation. However, the current agroecosystem model, enhanced with carbon amendments and NIs algorithms, needs more field measurements for further evaluation and refinement.