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*Department of Atmospheric Sciences University of Washington

Quantifying nitrate formation pathways based on a global model of the oxygen isotopic composition ( D 17 O) of atmospheric nitrate. Becky Alexander*, Meredith G. Hastings, Daniel J. Allman, Jordi Dachs, Joel A. Thornton, and Shelley A. Kunasek. *Department of Atmospheric Sciences

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*Department of Atmospheric Sciences University of Washington

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  1. Quantifying nitrate formation pathways based on a global model of the oxygen isotopic composition (D17O) of atmospheric nitrate Becky Alexander*, Meredith G. Hastings, Daniel J. Allman, Jordi Dachs, Joel A. Thornton, and Shelley A. Kunasek *Department of Atmospheric Sciences University of Washington

  2. D17O(nitrate) D17O(nitrate) Nitrate (HNO3 + NO3-) formation and D17O D17O = d17O – 0.5 x d18O D17O(O3) ≈35‰ D17O(HO2,RO2,OH) ≈ 0‰ Observations of D17O(nitrate) ≈20-40‰

  3. Don’t consider in calculations of D17O of inorganic nitrate Organic versus inorganic nitrate

  4. Calculated D17O(nitrate) and comparison with observations* *Daily, monthly and annual mean observations from Alert, Canada; Summit, Greenland; Princeton, NJ; La Jolla, CA; Bermuda; Subtropical N. Atlantic cruise; Atacama desert, Chile; PNF, Ecuador; DDU, Antarctica; South Pole ‰

  5. Fractional importance of each nitrate formation pathway at the surface (global, annual-mean troposphere) (76%) (18%) (4%) (2%)

  6. Model discrepancies • Lack of BrO chemistry in model leads to 1-10‰ underestimate in spring and summer (maximum in spring) in polar regions Simpson et al. [2007] BrO + NO  NO2 + Br BrO + NO2 + M  BrONO2 + M BrONO2 + H2O(aq)  HNO3 + BrOH D17O(nitrate) > 40‰ organic nitrate/total nitrate • Some observations of D17O(nitrate) (depending on method used) may include measurements of organic nitrates, leading to overestimates of up to 10‰. NO + RO2 RONO2 Originates from isoprene oxidation products 2‰ < D17O(nitrate) < 10‰

  7. Extra slides

  8. NO O O O O O O NO D17O(O3) and isotopic transfer mechanism 1. Mechanism of NO oxidation by O3 NO reacts preferentially with the terminal O atom of O3.Savarino et al. [2008] NO2 + O2 NO2 + O2 2. The bulk D17O value of tropospheric O3 Range of observations: D17O(O3) = 25 – 35‰ Model calculations: D17O(O3) = 35‰ Lyons [2001] Johnston et al. [1997]; Krankowsky et al. [1995] Other studies have assumed either an equal probability of all 3 O-atoms of O3 participating in oxidation of NO [Michalski et al., 2003], or D17O(O3) = 25‰ [Morin et al., 2008].

  9. Comparison with observations* *Daily, monthly and annual mean observations from Alert, Canada; Summit, Greenland; Princeton, NJ; La Jolla, CA; Bermuda; Subtropical N. Atlantic cruise; Atacama desert, Chile; PNF, Ecuador; DDU, Antarctica; South Pole D17O(O3)=25‰ (terminal) D17O(O3)=35‰ (statistical) D17O(O3)=35‰ (terminal) D17O(O3)=35‰ (terminalx2)

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