Quantifying nitrate formation pathways based on a global model of the oxygen isotopic composition (  17 O) of atmospheric nitrate Becky Alexander*, Meredith.

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Presentation transcript:

Quantifying nitrate formation pathways based on a global model of the oxygen isotopic composition (  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 University of Washington

Nitrate (HNO 3 + NO 3 - ) formation and  17 O  17 O(O 3 ) ≈ 35‰  17 O(HO 2,RO 2,OH) ≈ 0‰ Observations of  17 O(nitrate) ≈ 20-40‰  17 O(nitrate)  17 O =  17 O – 0.5 x  18 O

Organic versus inorganic nitrate Don’t consider in calculations of  17 O of inorganic nitrate

Calculated  17 O(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 ‰

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

Model discrepancies Lack of BrO chemistry in model leads to 1-10‰ underestimate in spring and summer (maximum in spring) in polar regions BrO + NO  NO 2 + Br BrO + NO 2 + M  BrONO 2 + M BrONO 2 + H 2 O (aq)  HNO 3 + BrOH Simpson et al. [2007] Some observations of  17 O(nitrate) (depending on method used) may include measurements of organic nitrates, leading to overestimates of up to 10‰.  17 O(nitrate) > 40‰ organic nitrate/total nitrate NO + RO 2  RONO 2 Originates from isoprene oxidation products 2‰ <  17 O(nitrate) < 10‰

Extra slides

 17 O(O 3 ) and isotopic transfer mechanism NO reacts preferentially with the terminal O atom of O 3. Savarino et al. [2008] O O ONO O O O 1. Mechanism of NO oxidation by O 3 NO 2 + O 2 2. The bulk  17 O value of tropospheric O 3 Range of observations:  17 O(O 3 ) = 25 – 35‰ Model calculations:  17 O(O 3 ) = 35‰ Other studies have assumed either an equal probability of all 3 O-atoms of O 3 participating in oxidation of NO [Michalski et al., 2003], or  17 O(O 3 ) = 25‰ [Morin et al., 2008]. Lyons [2001] Johnston et al. [1997]; Krankowsky et al. [1995]

Comparison with observations*  17 O(O 3 )=35‰ (statistical)  17 O(O 3 )=25‰ (terminal)  17 O(O 3 )=35‰ (terminal)  17 O(O 3 )=35‰ (terminalx2) *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