Alkyl nitrate measurements using Whole Air Sampling (WAS) during ATom

The Atmospheric Tomography (ATom) field mission provides a unique opportunity to study the composition of the remote atmosphere on a global scale. Our interest is on the spatial distribution, seasonal variability and source influence of alkyl nitrates (RONO2). Alkyl nitrates are important volatile organic compounds (VOCs) directly emitted from marine sources (C1-C2) or formed in the atmosphere during hydrocarbon oxidation. Because of their relatively long lifetime 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.

We present results from the whole air samples (WAS) collected by the UC Irvine group during three of the four ATom deployments: July-August 2016 for ATom-1; January-February 2017 for ATom-2; and September-October 2017 for ATom-3. A total of about 5,000 samples were collected during the entire campaign and analyzed using gas chromatography (coupled with flame ionization detector, electron capture detector and quadrupole mass spectrometer detection). Over 50 VOCs were quantified including eight alkyl nitrates: 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 analysis focuses on areas like the equatorial Pacific region, where methyl nitrate reached 88 pptv during ATom-2, and Southern Hemisphere latitudes (< 60°S). We also present results from the Arctic region where we observed elevated mixing ratios of alkyl nitrates ≥C3 during the winter deployment. During ATom-1, alkyl nitrates ≥C3 together with their parent hydrocarbons showed maximum levels (up to 53 pptv for 2-butyl nitrate) over the oil and gas regions of the continental USA, consistent with a predominant photochemical source. The contribution of the measured alkyl nitrates to the NO_y budget shows that summed RONO₂ are an important portion of the NO_y in the south Pacific as well as other remote regions. These and other results will be discussed including an investigation of alkyl nitrate sources in the remote environment using the NCAR CAM-chem model.