ARTICLE pubs.acs.org/est
Abiotic Mechanism for the Formation of Atmospheric Nitrous Oxide from Ammonium Nitrate Gayan Rubasinghege,† Scott N. Spak,‡ Charles O. Stanier,‡,§ Gregory R. Carmichael,‡,§ and Vicki H. Grassian†,§,* †
Department of Chemistry, ‡Center for Global and Regional Environmental Research and §Department of Chemical and Biochemical Engineering, University of Iowa, Iowa City Iowa 52242, United States
bS Supporting Information ABSTRACT: Nitrous oxide (N2O) is an important greenhouse gas and a primary cause of stratospheric ozone destruction. Despite its importance, there remain missing sources in the N2O budget. Here we report the formation of atmospheric nitrous oxide from the decomposition of ammonium nitrate via an abiotic mechanism that is favorable in the presence of light, relative humidity and a surface. This source of N2O is not currently accounted for in the global N2O budget. Annual production of N2O from atmospheric aerosols and surface fertilizer application over the continental United States from this abiotic pathway is estimated from results of an annual chemical transport simulation with the Community Multiscale Air Quality model (CMAQ). This pathway is projected to produce 9.3þ0.7/-5.3 Gg N2O annually over North America. N2O production by this mechanism is expected globally from both megacities and agricultural areas and may become more important under future projected changes in anthropogenic emissions.
’ INTRODUCTION Nitrous oxide (N2O), presently at 320 parts per billion by volume in the troposphere,1,2 is a potent greenhouse gas (GHG) with a long residence time in the atmosphere, ∼ 120 yrs. N2O has a global warming potential approximately 300 times larger than CO2 of equal mass,3 and is the fourth largest contributor to radiative forcing over the past 250 years.4 In addition, N2O serves as a major source of stratospheric NOx and thus contributes to catalytic ozone destruction. Ravishankara et al. found that N2O is currently the single most important ozone depleting substance, and is anticipated to remain the largest throughout the 21st century.5 Unlike most other GHGs, the quantification of global nitrous oxide sources is incomplete, with approximately 30% of unknown sources and unidentified mechanisms.6 Soils are estimated to account for ∼50-60% of global N2O emissions,7 with high N2O production rates reported from agricultural soils,8 linked with soil moisture9 and O2 availability.10 Most studies to date have focused on biological processes of nitrification, denitrification and nitrate ammonification as the predominant processes for nitrous oxide production in soil.11 As 41.8 Tg of solid ammonium nitrate and 13.7 Tg of calcium ammonium nitrate fertilizer are applied to agricultural fields as a N amendment each year (International Fertilizer Association, http://www.fertilizer.org/Home-Page/STATISTICS), it is important to determine if there are other processes leading to N2O r 2011 American Chemical Society
production from agricultural soils including abiotic processes (vide infra). Furthermore, ammonium nitrate is also a common component of ambient particles, which can be transported across the globe.12,13 It is formed as a secondary species in pure form as a product from gas phase reactions involving nitrogen oxides and ammonia. Nitrate is also formed via reactions of nitrogen oxides on mineral dust and sea salt surfaces, which lead to the formation of adsorbed nitrate, nitrate coatings, and deliquesced nitrate layers on the surface of the particles.14,15 Gaseous ammonia neutralizes adsorbed nitric acid to yield ammonium nitrate on these particle surfaces.16 On agricultural soil, the formation of secondary ammonium nitrate coatings can take place via a similar mechanism as on suspended soil particle surfaces. These reservoirs (agricultural soils and atmospheric aerosols) then represent a potential source of nitrous oxide.
’ EXPERIMENTAL SECTION Laboratory Studies. In laboratory studies, ammonium nitrate coatings on model mineral surfaces were prepared by first Received: September 29, 2010 Accepted: January 31, 2011 Revised: January 28, 2011 Published: March 03, 2011 2691
dx.doi.org/10.1021/es103295v | Environ. Sci. Technol. 2011, 45, 2691–2697
Environmental Science & Technology reacting particle surfaces with nitric acid to produce a surface layer of adsorbed nitrate. The preparation of saturated surfaces of adsorbed nitrate have been previously described in detail.17,18 Briefly, alumina (γ-Al2O3 - Degussa) was used as a model for mineral surfaces and mineral dust aerosol. Approximately 12 mg of alumina powder was pressed onto half of a tungsten grid and evacuated for 12 h in the FTIR cell to remove adsorbed water from the surface (see text and Supplemental Figure 1 diagram in Supporting Information (SI)). The alumina surface was then exposed to nitric acid vapor for 30 min at a pressure of approximately 1 Torr at 298 K. From previous studies, this process is known to produce a saturated surface coverage of adsorbed nitrate on alumina.18 The calculated nitrate coverage, when normalized to the BET surface area, is determined to be 5 ( 1 1014 molecules cm-2. The alumina surface saturated with nitrate is then exposed to gas phase ammonia for 30 min at a pressure of approximately 1 Torr. This process forms a stable ammonium nitrate coating on the particle surface. Experiments were also done with pure ammonium nitrate and for these experiments powders of the pure substance (Alfa Aesar, ACS, 98.0% NH4NO3) was pressed on to one-half of the tungsten grid and evacuated overnight. The experimental setup that is used in these studies allows for FTIR measurements of both the solid and gas phases as well as any changes that occur upon irradiation. These spectroscopic measurements can be done by simply probing each side of the sample grid with the infrared beam. This is done with the use of a linear translator upon which the entire FTIR cell sits inside the spectrometer. Experiments were carried out using different isotopes of ammonia 14NH3 (>99.5% 14N) and 15NH3 (>98% 15 N), to better understand the mechanism of formation of N2O. All FTIR spectra were recorded at 298 K. Additional details of the experimental apparatus are provided in the SI. Ammonium nitrate, either the pure from or coated on alumina, were exposed to light (300 < λ < 700 nm) using a broadband Hg light source and a broadband filter.17,19 Additionally, a water filter was used to minimize heating of the sample (
