N emissions and the changing landscape of air quality

Rob PinderUS EPA

Office of Research and DevelopmentAtmospheric Modeling & Analysis Division

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

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?

Sources of reactive nitrogen

Rapid dry deposition | Slow Dry Deposition

Less Water Soluble | More Water Soluble

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

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?

Trends in NO2 and NH3

Trend in space-based observations of NO2

A. Richter et al., Increase in tropospheric nitrogen dioxide over China observed from space, Nature, 437 (2005)

SCIAMACHY 2003 2004 2005

SCIAMACHY 2003 2004 2005

SCIAMACHY 2003 2004 2005

OMI 2005 2006 2007 2008

molecules NO2 × 1015

molecules NO2 × 1015

OMI 2005 2006 2007 2008

molecules NO2 × 1015

OMI 2005 2006 2007 2008

molecules NO2 × 1015

OMI 2005 2006 2007 2008

OMI trend for summer NO2

Trend in polluted areas: 5-6% per year reduction in NO2 column density

NH3 is not well

constrained, but deposition measurements

indicate anupward

trend

Satellite retrieval of NH3

Source: Clarisse et al., Global NH3 distribution derived from infrared satellite observations, Nat. Geo., 2009

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

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%

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

Rapid dry deposition | Slow Dry Deposition

Trends in Aerosol and Gas Phase

CMAQ represents spatial distribution of nitrate decrease summer 2002 – 2005

Change in total nitrate, μg m-3

Acknowledge: Wyat Appel

Acknowledge: Wyat Appel

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

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

Less Water Soluble | More Water Soluble

Trends in Long-lived Reactive Nitrogen

Half of reactive nitrogen is in the free troposphere

Acknowledge: Ken Pickering, Dale Allen, Barron Henderson

Important contribution from NO produced from lightning

Acknowledge: Ken Pickering, Dale Allen, Barron Henderson

Chemical partitioning between soluble forms of oxidized nitrogen in the free troposphere has biases

Acknowledge: Ken Pickering, Dale Allen, Barron Henderson

CMAQ simulation with lightning NO production successfully reproduces wet deposition flux

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

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