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Accueil du site → Doctorat → États-Unis → 2010 → Transpacific transport of mineral dust : Its impact in the United States and on sulfate, nitrate, and ozone in Asian pollution plumes

HARVARD UNIVERSITY (2010)

Transpacific transport of mineral dust : Its impact in the United States and on sulfate, nitrate, and ozone in Asian pollution plumes

Fairlie, Thomas Duncan

Titre : Transpacific transport of mineral dust : Its impact in the United States and on sulfate, nitrate, and ozone in Asian pollution plumes

Auteur : Fairlie, Thomas Duncan

Université de soutenance  : HARVARD UNIVERSITY

Grade : Doctor of Philosophy (PhD) 2010

Résumé
This thesis examines the transpacific transport of mineral dust from Asia, its impact on aerosol concentrations in the United States, and on nitrate, sulfate, and ozone in Asian pollution plumes. We use observations from ground stations, aircraft, and satellite platforms, interpreted using a global three-dimensional chemical transport model (GEOS-Chem) in which we have represented dust mobilization, transport, and deposition. We find that the best simulation of North American surface observations is achieved when we restrict dust sources to year-round arid areas, but include a significant wind threshold for mobilization. The model captures the seasonal cycle in surface dust concentrations over the northern Pacific, the outflow of dust from Asia in the free troposphere, and the timing and distribution of Asian dust outbreaks in the United States in spring 2001. We find that Asian dust persists in surface air in western states beyond these sudden spring outbreaks, and accounts for 40% of the worst visibility days due to dust in the West in 2001. Thus, state governments need to account for transpacific dust in setting attainable visibility goals. We have also represented the uptake of acid gases SO2, H 2SO4, and HNO3 on dust in the model, and used it to interpret aircraft observations of nitrate and sulfate partitioning in transpacific dust plumes during April-May 2006. The observations show that particulate nitrate was primarily associated with the dust, sulfate was primarily associated with ammonium, and that Asian dust remained alkaline across the Pacific. To reproduce this in the model requires that uptake of HNO3 and SO2 on dust is much weaker than assumed in previous model studies. The model overestimates gas-phase HNO3 by a factor of 2-3, typical of other models ; we demonstrate that this cannot be corrected by uptake on dust. Dust remains alkaline in the model because the uptake of acid gases is slow relative to the lifetime of dust against deposition. This argues against the hypothesis that iron in dust could become bio-available upon deposition to the oceans. The aircraft observations show no detectable ozone depletion in Asian dust plumes. The uptake of HNO3 suppressing its recycling to NOx only weakly impacts ozone, but enough to reduce the Asian pollution influence on US surface ozone by 10-15%.

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Page publiée le 11 novembre 2013, mise à jour le 4 décembre 2018