Atmospheric Composition Changes During Droughts in the Continental U.S.

The abnormal meteorological conditions under droughts can impose large changes in atmospheric compositions. In this dissertation, we quantified these changes using long-term atmospheric composition observations over the continental U.S. during summertime. Specifically, we revealed the spatial east-west variation in ozone (O3) response to drought: higher O3 enhancement in the southeast U.S. (SEUS) and no significant change or even a decrease in the west. We attributed this spatial discrepancy to O3 chemistry caused by the opposite response of isoprene: a 37% decrease in isoprene under exceptional drought in California in contrast to a 41% increase in Georgia. The enhanced isoprene in the SEUS also contributes to the 24% higher value of organic aerosol (OA), which can be largely attributed to the increase of isoprene epoxydiols derived secondary organic aerosol (IEPOX SOA) with a high dependence on sulfate. The elevated OA in the Pacific Northwest under droughts is caused by increasingly higher wildfire emissions. We evaluated the chemical transport model GEOS-Chem regarding its capability in capturing the observed drought-air pollution relationships. The model under- and overestimates the drought-induced O3 and OA changes in the SEUS, respectively, which can be partly caused by the overprediction of biogenic isoprene emissions. A satellite-derived drought stress factor by minimizing the model-to-observed bias of formaldehyde column to temperature sensitivity was implemented in GEOS-Chem. The resulted reduction in isoprene emissions can lower the OA positive bias by 7%-12% and improve the O3 enhancement by 1-3 ppb over low-NOx regions. We also found a decrease of 11% in surface fine dust over the SEUS under droughts in contrast to the expected increase in other regions. Through the teleconnection to the negative North Atlantic Oscillation, a lower-than-normal and more northeastward displacement of the Bermuda High is present during SEUS droughts, resulting in less dust being transported into the SEUS. The enhanced precipitation in the Sahel associated with the northward shift of the Intertropical Convergence Zone also leads to lower dust emissions therein. The GEOS-Chem model can capture the weakened African dust transport and reproduce the reduced dust in the SEUS while misses the enhanced dust in the western areas.