Quantifying nitrate formation Quantifying nitrate formation pathways based on a global model pathways based on a global model of the oxygen isotopic composition of the oxygen isotopic composition

((1717O) of atmospheric nitrateO) 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 (HNONitrate (HNO33 + NO + NO33--) formation and ) formation and 1717OO

17O(O3) ≈ 35‰

17O(HO2,RO2,OH) ≈ 0‰

Observations of 17O(nitrate) ≈ 20-40‰

17O(nitrate)

17O(nitrate)

17O = 17O – 0.5 x 18O

Organic versus inorganic nitrateOrganic versus inorganic nitrate

Don’t consider in calculations of 17O of inorganic nitrate

Calculated Calculated 1717O(nitrate) and O(nitrate) and comparison with observations*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 Fractional importance of each nitrate formation pathway at the surfaceformation pathway at the surface

(76%) (18%)

(4%) (2%)

(global, annual-mean troposphere)

Model discrepanciesModel discrepancies

• Lack of BrO chemistry in model leads to 1-10‰ underestimate in spring and summer (maximum in spring) in polar regions

BrO + NO NO2 + Br

BrO + NO2 + M BrONO2 + M

BrONO2 + H2O(aq) HNO3 + BrOH

Simpson et al. [2007]

• Some observations of 17O(nitrate) (depending on method used) may include measurements of organic nitrates, leading to overestimates of up to 10‰.

17O(nitrate) > 40‰ organic nitrate/total nitrate

NO + RO2 RONO2

Originates from isoprene oxidation products 2‰ < 17O(nitrate) < 10‰

Extra slides

17O(O3) and isotopic transfer mechanism

NO reacts preferentially with the terminal O atom of O3. Savarino et al. [2008]

OOO NO

NOOOO

1. Mechanism of NO oxidation by O3

NO2 + O2 NO2 + O2

2. The bulk 17O value of tropospheric O3

Range of observations:

17O(O3) = 25 – 35‰

Model calculations:

17O(O3) = 35‰

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 17O(O3) = 25‰ [Morin et al., 2008].

Lyons [2001]Johnston et al. [1997]; Krankowsky et al. [1995]

Comparison with observations*Comparison with observations*

17O(O3)=35‰ (statistical)

17O(O3)=25‰ (terminal)

17O(O3)=35‰ (terminal)

17O(O3)=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