Alkyl Nitrate Measurements Using Whole Air Sampling (WAS) during ATom-1 and ATom-2

Alkyl nitrates (RONO₂) are important volatile organic compounds (VOCs) directly emitted from marine sources (for C1-C2 alkyl nitrates) or formed in the atmosphere during hydrocarbon oxidation (i.e. photochemical formation). Because they are fairly long-lived (days for pentyl nitrates, up to one month for methyl nitrate) alkyl nitrates can act as temporary reservoirs for nitrogen oxides (NO_x), and their long-range transport can lead to ozone (O₃) formation in remote regions.

The Atmospheric Tomography (ATom) field missions provide a unique opportunity to study remote regions relatively understudied by the global community, particularly with respect alkyl nitrate distribution and source influences. We present results from the 1579 whole air samples (WAS) collected by the UC Irvine group during ATom-1 (July-August 2016) and the 1643 samples collected during ATom-2 (January-February 2017). The samples were analyzed using gas chromatography (coupled with different detectors: flame ionization detector, electron capture detector and quadrupole mass spectrometer) for over 50 VOCs including eight alkyl nitrates, namely: methyl nitrate, ethyl nitrate, i-propyl nitrate, n-propyl nitrate, 2-butyl nitrate, 3-methyl-2-butyl nitrate, 2-pentyl nitrate and 3-pentyl nitrate. The alkyl nitrate detection limit is 0.005-0.010 pptv, the precision is 3-8% and the accuracy is 10%.

Our results confirm that methyl nitrate has a characteristic south-north gradient, and we observed higher mixing ratios over the equatorial Pacific than the equatorial Atlantic. In addition to the equatorial region, methyl nitrate was particularly elevated in the Southern Hemisphere at latitudes < -60°. During ATom-1, alkyl nitrates ≥C3 together with their parent hydrocarbons showed maximum levels over the oil and gas regions of the continental USA, consistent with a predominant photochemical source. During the winter deployment, elevated mixing ratios of alkyl nitrates ≥C3 were measured in the arctic region. We also calculated the contribution of the measured alkyl nitrates to the NO_y budget, which shows that summed RONO₂ are an important portion of the NO_y in remote regions such as the south Pacific. These and other results will be discussed.