Theoretical calculation of oxygen equilibrium isotope fractionation factors involving various NOy molecules, OH, and H2O and its implications for isotope variations in atmospheric nitrate. (15th October 2016)

• Record Type: Journal Article
• Authors: Walters, Wendell W.; Michalski, Greg
• Is Part Of: Geochimica et cosmochimica acta. Volume 191(2016:Oct. 15)
• Page Start: 89
• Page End: 101
• Publication Date: 2016-10-15
• Subjects: NOx -- Nitrate -- Isotopes -- Isotope Exchange -- Equilibrium -- Fractionation; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2016.06.039
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: The oxygen stable isotope composition (δ 18 O) of nitrogen oxides [NO x = nitric oxide (NO) + nitrogen dioxide (NO2 )] and their oxidation products (NO y = NO x + nitric acid (HNO3 ) + particulate nitrate (p-NO3 − ) + nitrate radical (NO3 ) + dinitrogen pentoxide (N2 O5 ) + nitrous acid (HONO) + …) have been shown to be a useful tool for inferring the proportion of NO x that is oxidized by ozone (O3 ). However, isotopic fractionation processes may have an influence on δ 18 O of various NO y molecules and other atmospheric O-bearing molecules pertinent to NO x oxidation chemistry. Here we have evaluated the impacts of O isotopic exchange involving NO y molecules, the hydroxyl radical ( OH), and water (H2 O) using reduced partition function ratios ( x β) calculated by hybrid density functional theory. Assuming atmospheric isotopic equilibrium is achieved between NO and NO2 during the daytime, and NO2, NO3, and N2 O5 during the nighttime, δ 18 O–δ 15 N compositions were predicted for the major atmospheric nitrate formation pathways using our calculated exchange fractionation factors and isotopic mass-balance. Our equilibrium model predicts that various atmospheric nitrate formation pathways, including NO2 + OH → HNO3, N2 O5 + H2 O + surface → 2HNO3, and NO3 + R → HNO3 + R will yield distinctive δ 18 O–δ 15 N compositions. Our calculated δ 18 O–δ 15 N compositions match well with previous atmospheric nitrate measurements, and will potentially help better understand the role oxidation chemistry plays on the N and O isotopic composition of atmospheric nitrate.