Ambient Ammonium Contribution to total Nitrogen Deposition 2016 NADP Annual Meeting Santa Fe, New Mexico November 3, 2016
Rich Scheffe, Jason Lynch, Donna Schwede, James Kelly, Halil Cakir, Adam Reff U.S. Environmental Protection Agency
and for thinking – Jesse Bash, Gary Lear, Bret Schichtel, John Walker, Karen Wesson
Evolutional change in National Air Pollution Management
Initial CAA
1970
“Good bye EKMA” Biogenics Acid Deposition Regional science “1991 NAS Rethinking the ozone …” NAPAP
1990
Local/urban Regional Hemispheric
“Hello CTMs and PM networks” 8-hr ozone
PM2.5
(annual driver)
Regional Rules (Title 4, NOx SIP)
2000
“Western U.S. awakening” “Great Trades” New tighter Multiple PM, NO2 and pollutant SO2 primary Multiple media stnds, recognition emphasizing “2004 NAS local scales, AQM in U.S.” (O/G, fires, winter O3)
2010
“MDL dilemma” New lower O3 standard ”background O3” Climate-AQ Hemispherical Transport, HCHO, NH3
2050
Question: • What is the contribution of ambient particulate NH4 (pNH4)to total nitrogen deposition?
Challenge: • Contribution of ambient pNH4 (or any ambient species) to dry deposition is estimated through models and widely available. • Also available is wet (or precipitation) concentration of NH4 (wNH4) through measurements and models. • However, wNH4 is derived from transfer of both ambient NH3 and pNH4 to aqueous phase through cloud droplet formation (pNH4), mass transfer of NH3 to cloud/fog and eventual precipitation scavenging. Noting that virtually all NH3 transferred to wet phase is hydrated upon dissolution and then dissociates to form wNH4. • Consequently, wNH4 reflects the aggregate contribution from ambient NHx without a clear path to delineate separate contributions between pNH4 and NH3.
after Seinfeld and Pandis, 1998
Cloudwater/Fog NH4+1 NH3 + H2O
NH4OH
NH4+1 + OH-1
Ambient (dry) particulates
Ambient (dry) gases
(NH4)2SO4; NH4HSO4 ; NH4NO3
NH3
Rain/Snow
NH4+1
NH3 + H2O
Dry dep NH4
NH4OH
NH4+1 + OH-1
Wet Deposition NH4+1/NH4+1=?
Dry dep NH3
Why do we care? • Important ecological effects (e.g. eutrophication) are associated with total nitrogen deposition, to which pNH4 (as well as other pollutants) can be a significant contributor • pNH4, as well as pNO3 and pSO4, are components of total PM mass; in addition pNO3 and pSO4 are also transformation products of the criteria pollutants of NOx and SOx • Deposition driven ecosystem effects that have the potential to be adverse to public welfare are important to be considered in the current NAAQS reviews • Assessing the contribution of the various nitrogen species to the total nitrogen deposition in ecosystems allows us to better understand the emissions sources contributing to adverse ecosystem effects • Understanding the contribution of the various species to the total ecosystem deposition then helps inform decisions on the best and most appropriate policy option(s) for controlling sources and reducing associated impacts
What we know about ammonium (NH4) • Basically, all NH4 is derived from ammonia (NH3) • NH4 + NH3 = NHx, which nationally makes up nearly half of all nitrogen deposition
2011 Ratio of NHx to total N deposition
2011 total N deposition
Source: NADP TDEP
Challenge: how much N deposition is derived from ambient NH4?
Estimating pNH4 contribution to wet deposition • Assume mass transfer rates, regardless of mechanism, of pNH4 and NH3, from ambient to aqueous phase are identical; reasoning: • NH3 is highly soluble and enhanced by dissociation to NH4+ • pNH4 is efficiently removed through cloud droplet formation and scavenging
• Consequently, the relative rates of loss to the aqueous phase are given by ratios of ambient concentrations, leading to: • pNH4_wet = ([pNH4]/[NHx]) *wetdepNH4 where pNH4_wet = wet NH4 deposition attributed to pNH4
and, [ ] extracted from CMAQ; deposition from TDEP hybrid
Relative concentration ratios of pNH4 and NHx - Expect higher ratios in East given available NO3 and SO4 relative to West - Also expect higher ratios in North given temperature dependence on NH4 - NH3 thermodynamics - Spatial patterns mostly dominated by excess NH3, influenced by NH3, NOx and SOx emissions, sea salts, and thermodynamics
Contribution of pNH4 to wet NHx deposition Note dry dep
NHx wet deposition
NH3 cont to NHx wet deposition
Fraction of NHx wet deposition from pNH4
pNH4 cont to NHx wet deposition
Capacity differences between NOy and NOy plus particulate NH4, referenced to total N deposition.
NHx + NOy deposition
NOy deposition
NHx + NOy deposition – NH3 contributions
Critical Load exceedance example: Forest health in relation to N deposition components
Changes in ambient NH3 and pNH4 increasing NH3 trend, decreasing NH4
2002 CMAQ
NH3
pNH4 2011 CMAQ
Change in ambient pNH4/NHx Reflecting reductions in NOx and SOx emissions leading to more relative free NH3
2002 CMAQ
2011 CMAQ
ANH4/NH3_02 0.02 - 0.25 0.26 - 0.50 0.51 - 0.60 0.61 - 0.70 0.71 - 0.80 0.81 - 0.97
Emission changes
CAP (excluding CO) Emissions, Thousand Tons
30,000
NOx
25,000
20,000
More information on Trends can be found at: http/www.epa.gov/ttn/chief/trends/idex.html
CO
PM10 VOC
15,000
10,000
SO2
NH3 Biologically active normalized to NOx
PM2.5
5,000
NH3 0 2002
2003
2004
PM2.5
2005 PM10
2006
2007 SO2
2008 VOC
2009
2010 NOx
2011 NH3
2012
2013 CO
2020
Next Steps • Building a weight-of-evidence” argument • Examine quantifiable scavenging metrics in CTMs • e.g., GEOSchem estimates scavenging of NH3, but does not include aqueous phase chemistry
• Develop CMAQ process analysis results specific to NH3 production and loss, resources permitting • Explore other analyses, e.g. • Insights from atmospheric column profile data sets • Expected enhanced NH4 above surface level
• Observation sets before and after precipitation events • Temporally resolved analyses of modeled results • Are there significant differences in NH4/NH3 ratios before precipitation?
• Refine CL exceedance analyses • Monitoring Implications
