Comprehensive Isotopic Composition of Atmospheric Nitrate during CalNex Inferring Sinks and Sources of NO X from Nitrate Stable Isotope Ratios William C. Vicars, Samuel Morin, and Joël Savarino Laboratoire de Glaciologie et Géophysique de l’Environnement Université Joseph Fourier, CNRS, Grenoble, France

Stable Isotope Basics Oxygen: 16 O, 17 O, 18 O Nitrogen: 14 N, 15 N Atmospheric nitrate (HNO 3 / NO 3 - ) isotope ratios ( 17 O/ 16 O, 18 O/ 16 O, and 15 N/ 14 N) serve as interpretive tools to quantify NO X sources and removal mechanisms Delta (δ and Δ) values in units of “per mil” (‰) δ 17 O sample = ( 17 O/ 16 O) sample / ( 17 O/ 16 O) standard - 1

Stable Isotope Basics Oxygen: 16 O, 17 O, 18 O Nitrogen: 14 N, 15 N Atmospheric nitrate (HNO 3 / NO 3 - ) isotope ratios ( 17 O/ 16 O, 18 O/ 16 O, and 15 N/ 14 N) serve as interpretive tools to quantify NO X sources and removal mechanisms Delta (δ and Δ) values in units of “per mil” (‰) δ 18 O sample = ( 18 O/ 16 O) sample / ( 18 O/ 16 O) standard - 1

Stable Isotope Basics Oxygen: 16 O, 17 O, 18 O Nitrogen: 14 N, 15 N Atmospheric nitrate (HNO 3 / NO 3 - ) isotope ratios ( 17 O/ 16 O, 18 O/ 16 O, and 15 N/ 14 N) serve as interpretive tools to quantify NO X sources and removal mechanisms Delta (δ and Δ) values in units of “per mil” (‰) δ 15 N sample = ( 15 N/ 14 N) sample / ( 15 N/ 14 N) standard - 1

Stable Isotope Basics Oxygen: 16 O, 17 O, 18 O Nitrogen: 14 N, 15 N Atmospheric nitrate (HNO 3 / NO 3 - ) isotope ratios ( 17 O/ 16 O, 18 O/ 16 O, and 15 N/ 14 N) serve as interpretive tools to quantify NO X sources and removal mechanisms Delta (δ and Δ) values in units of “per mil” (‰) Δ 17 O = δ 17 O *δ 18 O

The Oxygen Isotope “Anomaly” (Δ 17 O) of Ozone Thiemens, 2006, Annu. Rev. Earth Planet Sci. O 3 tropo. δ 17 O = 0.52*δ 18 O Δ 17 O = 0 Δ 17 O = ‰

Isotope Transfer During NO X Oxidation NO O3O3 X XO NO 2 RO 2 XO XONO 2 NO 3 N2O5N2O5 O3O3 NO 2 RH OH Nighttime Daytime HNO 3 Lower Δ 17 O Transfer Higher Δ 17 O Transfer time scale: minutes hours to days heterogeneous

Δ 17 O of Atmospheric Nitrate as a Useful Tool in Atmospheric Chemistry Morin et al., 2008, Science Seasonal minimum values concurrent with low O 3 (polar day, daytime chemistry) Δ 17 O (NO 3 - ) values higher in winter due to increased interaction with O 3 (polar night) Seasonal maxima reflect interaction of NO x with BrO and are not predicted by O 3 chemistry alone

CalNex 2010 High Volume PM sampling 63 sets of samples – coarse and fine fractions (1 μm cut-point) 26 sets from S. Cal. are 12 hour (day and night) collections (May ) – daytime vs. nighttime chemistry 34 subsequent sets have lower sampling durations (May 29 - June 7) LA San Francisco San Diego

Atmospheric Nitrate Concentration During CalNex LA San Francisco San Diego Ave.Max.Min. Std. Dev. LA SF

Air Mass Origins during CalNex

O3O3 NONO 2 O 3 and NO X during CalNex

Particle Size Distribution of Nitrate LA San Francisco San Diego Ave.Max.Min. Std. Dev. LA- C LA-F SF-C SF-F

Oxygen Isotope Anomaly (Δ 17 O) of Atmospheric Nitrate LA San Francisco San Diego Ave.Max.Min. Std. Dev. LA SF

Δ 17 O (NO 3 - ) - Diurnal Variations

Lifetime Effect of Atmospheric Nitrate Typical atmospheric residence time (τ) of 1 day Function of NO 3 - particle size distribution (i.e., deposition velocity) CalNex May samples were collected from approx. 9am - 7pm (“day”) and 7pm - 9am (“night”) Mixing of “daytime” and “nighttime” Δ 17 O signal Morin et al., 2010, EGU Annual Meeting

Δ 17 O (NO 3 - ) - Diurnal Variations

P n and P d A simplified Δ 17 O (NO 3 - ) interpretive framework Based on isotope mass balance, Δ 17 O (NO 3 - ) is expected to be proportional to the the ratio of nighttime to daytime nitrate production P n NO 2 + O 3  NO 3 + O 2 P n α k [NO 2 ] [O 3 ] P d NO 2 + OH  HNO 3 P d α k [NO 2 ] [OH] Δ 17 O (NO 3 - ) α P n /(P n + P d ) j(O 3 )

P n / (P n + P d ) and Δ 17 O (NO 3 - ) Co-evolve

Towards a Better Understanding of the CalNex Δ 17 O (NO 3 - ) Data P1P1 P2P2 P3P3 NO 2 + OH + M  HNO 3 + M NO 2 + O 3  NO 3 + O 2 and NO 3 + RH (DMS)  HNO 3 + products NO 2 + NO 3  N 2 O 5 and N 2 O 5 + H 2 O (surface)  2HNO 3(aq) Michalski et al., 2003, Geophys. Res. Lett. Air Mass Chemical Histories 3 days preceding sample collection along air-mass trajectory NO 3 - concentration and residence time (split into size fractions) NO, NO 2, NO 3, N 2 O 5, O 3, HO 2, OH, H 2 O 2 T and P Relative Humidity Agreement between modeled and measured Δ 17 O

δ 15 N and NO X Source Apportionment δ 15 N of atmospheric NO 3 can help elucidate sources of NO X to the atmosphere, but the data is often difficult to interpret – fuel combustion: 6-13 ‰ – soil (biological): < -20 ‰ – lightning: ≈ 0 ‰ Differences in δ 15 N between coarse and fine particles may be useful diagnostic tools δ 15 N sample = ( 15 N/ 14 N) sample / ( 15 N/ 14 N) standard - 1

δ 15 N of Atmospheric Nitrate LA San Francisco San Diego Ave.Max.Min. Std. Dev. LA SF

Average Difference = 2.3 (± 1.5) δ 15 N (NO 3 - ): Coarse vs. Fine Particles 13% Average Difference = 2.7 (± 1.8) 27%

Summary Variations in Δ 17 O (NO 3 - ) during CalNex are a function of the relative importance of “daytime” (P 1 ) and “nighttime” (P 2 and P 3 ) reaction mechanisms Offset in data due to residence time effect of NO 3 - A more thorough interpretation of the atmospheric Δ 17 O (NO 3 - ) signal will require detailed modeling of the dominant reaction mechanisms Potential for collaboration, joint publications, etc.

Summary Small differences in δ 15 N between coarse and fine particles suggest a common source of NO X in both size fractions Different reaction mechanisms (P 1, P 2, P 3 ) leading to NO 3 - formation are sensitive to particle size Higher δ 15 N in S. Cal. May reflect greater contribution of NO X from fuel combustion The higher Δ 17 O (NO 3 - ) for the samples collected near Los Angeles may be due to the greater abundance of coarse particle mass Greater HNO 3 production via N 2 O 5 hydrolysis in coarse particles?

Thank You!

δ 18 O and δ 17 O Δ 17 O = δ 17 O *δ 18 O = 0 slope = 0.52 Terrestrial Fractionation Line For most oxygen-bearing compounds: δ 17 O = 0.52*δ 18 O

The Oxygen Isotope “Anomaly” (Δ 17 O) of Ozone For Ozone (O 3 ): δ 17 O ≠ 0.52*δ 18 O Δ 17 O = δ 17 O *δ 18 O = ‰ Thiemens, 2006, Annu. Rev. Earth Planet Sci.

Isotope Transfer During NO X Oxidation NO 2 + O 3 NO 3 + O 2 N 16 O 18 O + 16 O 18 O 17 O N 16 O 18 O 17 O + 16 O 18 O For Atmospherically-Derived Nitrate (NO 3 - ): Δ 17 O = δ 17 O *δ 18 O = ‰ (tropospheric O 3 ), ‰ (statospheric O 3 ) Important nighttime NO 2 removal mechanism

δ 15 N (NO 3 - ) - Diurnal Variations