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N emissions and the changing landscape of air quality
Rob PinderUS EPA
Office of Research and DevelopmentAtmospheric Modeling & Analysis Division
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Global nitrogen and carbon cycles:much of the anthropogenic impacts
start in the atmosphere
Source: Gruber and Galloway, An Earth-system perspective of the global nitrogen cycle, Nature, 2008
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Questions:
• What are the trends in reactive nitrogen atmospheric concentrations?
• How is the atmospheric reactive nitrogen chemical composition changing?
• What aspects of the atmospheric reactive nitrogen budget are poorly constrained?
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Sources of reactive nitrogen
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Rapid dry deposition | Slow Dry Deposition
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Less Water Soluble | More Water Soluble
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Atmospheric Modeling Tools
• Use the air quality model to understand fate and transport of reactive nitrogen
• CMAQ: Community Multi-scale Air Quality– Emissions– Advection / dispersion– Chemistry– Aerosol thermodynamics– Wet and dry deposition
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Outline
• Can models and observations constrain the– Trends in NO2 and NH3?
– Aerosol and gas phase reactive nitrogen?– Soluble reactive nitrogen in the free
troposphere?
• What additional information is needed?
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Trends in NO2 and NH3
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Trend in space-based observations of NO2
A. Richter et al., Increase in tropospheric nitrogen dioxide over China observed from space, Nature, 437 (2005)
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SCIAMACHY 2003 2004 2005
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SCIAMACHY 2003 2004 2005
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SCIAMACHY 2003 2004 2005
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OMI 2005 2006 2007 2008
molecules NO2 × 1015
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molecules NO2 × 1015
OMI 2005 2006 2007 2008
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molecules NO2 × 1015
OMI 2005 2006 2007 2008
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molecules NO2 × 1015
OMI 2005 2006 2007 2008
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OMI trend for summer NO2
Trend in polluted areas: 5-6% per year reduction in NO2 column density
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NH3 is not well
constrained, but deposition measurements
indicate anupward
trend
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Satellite retrieval of NH3
Source: Clarisse et al., Global NH3 distribution derived from infrared satellite observations, Nat. Geo., 2009
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TES NH3 Comparison Example : Transects over North Carolina USA
•February 2009 to February 2010
•CAMNet NH3 monitoring sites match-up with TES overpass
•Two week integrated samples
•Sited away from livestock operations to be representative of TES footprint
•Allows detection of spatial variability and seasonal trendsAcknowledge: John Walker, Karen Cady-Pereira, Mark Shephard, Daven Henze, Ming Lou
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AMoNNH3 surface measurements from Dec. 2007 - today
Low bias: overall amount of emissions is reasonable
High error: spatial and temporal distribution of emissions are uncertain
CMAQ compared with all siteserror: 60%bias: -6.4%
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Conclusions about NO2 and NH3
• Trend in NO2 is reasonably well constrained by satellite observations
• Appears to be consistent with emission changes
• Wet deposition NH4+
provides the best constraint on NH3 trend
• Surface monitoring and satellite NH3 retrievals are under development
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Rapid dry deposition | Slow Dry Deposition
Trends in Aerosol and Gas Phase
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CMAQ represents spatial distribution of nitrate decrease summer 2002 – 2005
Change in total nitrate, μg m-3
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Acknowledge: Wyat Appel
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Acknowledge: Wyat Appel
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Decrease in nitrate at CASTNet sites
CASTNet(2002-2009)
CMAQ (2002-2006) r2 p
May – Sept.
Total Nitrate
6% year-1
5% year-10.96 0.001
Oct. – April
Aerosol Nitrate
2% year-1
1.5% year-1
0.81
0.47
0.001
0.08
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Conclusions about aerosol and gas phase reactive nitrogen
• Significant decreases in total nitrate and aerosol nitrate
• CMAQ captures these trends well• Increase in the fraction in the aerosol phase:
subtle effect on spatial distribution
• Need co-located measurements of NH3 and NH4
+ to understand trend in reduced nitrogen
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Less Water Soluble | More Water Soluble
Trends in Long-lived Reactive Nitrogen
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Half of reactive nitrogen is in the free troposphere
Acknowledge: Ken Pickering, Dale Allen, Barron Henderson
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Important contribution from NO produced from lightning
Acknowledge: Ken Pickering, Dale Allen, Barron Henderson
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Chemical partitioning between soluble forms of oxidized nitrogen in the free troposphere has biases
Acknowledge: Ken Pickering, Dale Allen, Barron Henderson
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CMAQ simulation with lightning NO production successfully reproduces wet deposition flux
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Conclusions: long-lived reactive nitrogen
• Long-range transport is controlled by chemical state of oxidized nitrogen
• The vertical profile of soluble oxidized nitrogen is not well simulated by CMAQ
• CMAQ is able to simulate wet deposition in the eastern US
• Little is known about reduced nitrogen in the free troposphere
• CalNex study will be very helpful
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Conclusions• Trends in measurements, CMAQ simulations, and
emissions of oxidized nitrogen are consistent– Oxidized nitrogen is decreasing– Reduced nitrogen is increasing
• CMAQ simulations of the aerosol and chemical partitioning of oxidized nitrogen are sufficiently consistent with observations to – assess the regional budget (in the East)– estimate impacts of future emission scenarios
• Need more observational constraints of sources, transport and fate of reduced nitrogen