Chemical Transport Modeling of Nitrogen Deposition in the...
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Chemical Transport Modeling of Nitrogen Deposition in the Western U.S.: A National
Park Perspective
Mike Barna1 Marco Rodriguez2
Kristi Gebhart1
Bret Schichtel1
Bill Malm3
1 NPS, Fort Collins, CO 2 AECOM, Fort Collins, CO
3 CIRA, Fort Collins, CO NADP Annual Meeting
Portland, ME October 2-5, 2012
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Characterizing N deposition
• NADP and CASTNet are invaluable resources for investigating trends and patterns in nitrogen deposition
• Unfortunately, it’s very hard to measure deposition for • All potential nitrogen species of interest, • For all places, • At all times
• Chemical transport models can help to
• Estimate deposition in unmonitored areas • Round-out the ‘total N deposition’ budget 2
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Unknowns: reduced gas-phase N
• Ammonia • Sparse observations, but can play a large role
• Reduced organic nitrogen gases
• e.g., urea, amino acids, methylated amines
• Emissions, not chemistry, is important
• Associated with fires?
• Important?
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Unknowns: gas-phase organic nitrates
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• Historically, only considered PAN-like compounds, but now chemistry mechanisms treat a wider variety of ON, e.g., isoprene nitrate
• Many of the organic components are ‘lumped’, resulting in average estimates for dry deposition velocity and wet scavenging
• Could be important, especially in more polluted environments, or where lots of isoprene exists
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Unknowns: particle organic nitrates
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• “There is growing evidence that organo-nitrogen compounds may constitute a significant fraction of the aerosol nitrogen (N) budget. However, very little is known about the abundance and origin of this aerosol fraction.” (Lin et al., 2010)
• Important to overall N dep budget?
• No attempt to model this yet
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Simulated HNO3 and NH3 dry dep
HNO3 NH3
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Simulated ‘total’ N deposition
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“total N” = wet N + dry N =
Nitric acid + Ammonia + PM nitrate +
PM ammonium + Organic nitrates +
‘nighttime N’ + NOx
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N deposition at Rocky Mountain NP
• Nitrogen deposition and ecosystem change has been extensively studied at RMNP • NADP and CASTNet sites • RoMANS (2006) • RoMANS2 (2009)
• ‘resource management goal’ of 1.5 kg/ha/yr
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Simulated ‘total’ N dry dep at ROMO
0.00
0.05
0.10
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0.20
0.25
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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Nitr
ogen
Dry
Dep
ositi
on [k
g/ha
]
N-PNO3
N-PNH4
N-NOx
N-PAN
N-NxOy
N-Org N
N-NH3
N-HONO
N-HNO3
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Colorado Romans2 NH3 emissions
Colorado Totals NH3
(tons/yr) Area 76 Onroad 4,484 Nonroad 49 Point 526 Fires 2,152 Livestock 54,078 Fertilizer 14,527 Wild Animals 5,626 Domestic 2,099 Oil & Gas 350 Biogenic 0 Windblown Dust 0 Total Colorado 83,967
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Romans2 CAFO location improvements
Livestock Numbers
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Use a ‘top down’ approach for NH3 EI?
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• Clarisse et al., 2009, Nature Geoscience • IASI – Infrared Atmospheric Sounding Interferometer • “good qualitative agreement” • “emissions significantly underestimated in northern
hemisphere”
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WestJumpAQS modeling for ‘08, ‘11
4 km Intermountain
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Carbon Bond 6 (CB6)
• Latest version of ‘carbon bond’ mechanism
• Used in WRF-Chem, CMAQ, CAMx
• Updates germane to nitrogen
• More detailed treatment of org-N
• Surface hydrolysis of N2O5 to make HNO3
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CB6 vs. CB05 chemical mechanisms:
Gas-phase reactions: 218 vs. 156 Photolysis reactions: 28 vs. 23 Gas-phase species: 77 vs. 51
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Summary
• What is the role of unmonitored N?
• Ammonia
• Gas-phase reduced organic N
• Gas-phase organic nitrates
• Particle organic nitrates
• Ammonia is important, but do other species make a significant contribution to N deposition budget?
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Summary (cont’d)
• Models can be useful to address gaps:
• NH3 (yes)
• PAN and homologues (yes)
• NOx (yes)
• Nighttime radicals (maybe)
• Other gas-phase organic nitrates (maybe)
• Org N on particles (not yet)
• Reduced org N gases (not yet)
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Summary (cont’d)
• Accounting for ‘missing’ nitrogen can almost double the estimated dry deposition at RMNP (1.2 vs 2.2 kg/ha/yr).
Species N-flux [kg/ha yr] contribution
HNO3 1.16 53%
NH3 0.60 28%
NxOy 0.22 10%
PAN + Org N 0.11 5%
Other N species 0.12 6% 17