UC Irvine

ABSTRACT OF THE DISSERTATION

Global Observations of Oceanic Alkyl Nitrates and Dimethyl Sulfide during the NASA Atmospheric Tomography Mission (ATom)

By

Christopher Thomas Woods

Doctor of Philosophy in Chemistry

University of California, Irvine, 2020

Professor Donald R. Blake, Chair

Oceanic trace gases were measured onboard the NASA DC-8 flying laboratory during the NASA Atmospheric Tomography field campaign with the intent to examine the spatial and temporal fluctuations to the global distribution of these species, as well as establish a dataset of global background levels for future use in the validation of satellites and the constraint of chemical transport models. Airborne in situ measurements of VOCs were obtained via the UC Irvine whole air sampler (WAS) system and analyzed with gas chromatography/mass spectrometry.

The ATom mission was comprised of 4 deployments, each corresponding to the seasons. Each deployment, the NASA DC-8 flew a near circumnavigation of the earth, from 82 °N to 86 °S, while performing vertical soundings from 0.5-12.5 km in altitude. Flight paths over the Pacific, Atlantic and Southern Oceans allowed for the investigation of seasonal effects on anthropogenic and biogenic emissions in the remote atmosphere.

Alkyl nitrates (RONO2) are an important source of reactive nitrogen to the remote atmosphere, where they can constitute the majority of the NOy budget. In the Northern Hemisphere, C2-C5 nitrates are mostly generated photochemically. However, smaller carbon number alkyl nitrates (C1-C4) have a significant ocean source with spatial and temporal variability that are not fully understood. These ‘light’ alkyl nitrates were found to contribute up to 88% of measured NOy in the Southern Hemisphere marine boundary layer. Global measurements of RONO2 showed enhancements at the equator and in the Southern Ocean, with seasonal maximums coinciding with conditions of increased marine biological productivity. MeONO2 was the most abundant, with enhancements > 100 ppt at the equator, contributing a up to 77% of measured NOy mixing ratios alone. Observations of 2-Butyl nitrate in the southern hemisphere marine boundary layer were shown to correlate with MeONO2, suggesting an oceanic source unconsidered in recent literature.

Dimethyl Sulfide (DMS), another oceanic trace gas, is a by-product of marine biological production with impacts to short-wave forcing through incorporation into aerosol, causing direct and indirect radiative effects. In remote atmospheres, DMS may condense onto existing particles or participate in new particle formation: becoming a significant contributor to cloud condensation nuclei (CCN). Global observations of DMS in the boundary layer showed enhancements in the tropics (> 150 ppt), the Southern Ocean (>100 ppt) and the northern midlatitudes in the Atlantic (>100 ppt). Although, these increases in DMS production were not found to follow a consistent latitudinal gradient.