1 Submitted to ACPD: Paper acp-2011-970 1 2 Nitrogen deposition to the United States

39 Pages·2011·3.56 MB·English

by  Lin Zhang

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  Submitted to ACPD: Paper acp-2011-970
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  Nitrogen deposition to the United States: distribution, sources, and processes
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  Lin Zhang1,2, Daniel J. Jacob1,2, Eladio M. Knipping3, Naresh Kumar4, J. William
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  Munger1,2, Claire C. Carouge2, Aaron van Donkelaar5, Yuxuan Wang6, Dan Chen7
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  [1] {Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA,
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  USA}
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  [2] {School of Engineering and Applied Sciences, Harvard University, Cambridge, MA,
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  USA}
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  [3] {Electric Power Research Institute, Washington, DC, USA}
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  [4] {Electric Power Research Institute, Palo Alto, CA, USA}
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  [5] {Department of Physics and Atmospheric Science, Dalhousie University, Halifax,
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  Canada}
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  [6] {Ministry of Education Key Laboratory for Earth System Modeling, Center for Earth
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  System Science, Institute for Global Change Studies, Tsinghua University, Beijing,
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  China}
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  [7] {Department of Atmospheric and Oceanic Sciences, University of California, Los
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  Angeles, CA, USA}
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  Correspondence to: Lin Zhang ([email protected])
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  Abstract
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  We simulate nitrogen deposition over the US in 2006-2008 by using the GEOS-Chem
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  global chemical transport model with 1/2° × 2/3° horizontal resolution over North
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  America and adjacent oceans. US emissions of NO and NH in the model are 6.7 and 2.9
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  Tg N a-1 respectively, including a 20% natural contribution for each. Ammonia emissions
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  are a factor of 3 lower in winter than summer, providing a good match to US network
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  observations of NH (≡ NH gas + ammonium aerosol) and ammonium wet deposition
x 3
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  fluxes. Model comparisons to observed deposition fluxes and surface air concentrations
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  of oxidized nitrogen species (NO ) show overall good agreement but excessive
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  wintertime HNO production over the US Midwest and Northeast. This suggests a model
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  overestimate N O hydrolysis in aerosols, and a possible factor is inhibition by aerosol
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  nitrate. Model results indicate a total nitrogen deposition flux of 6.5 Tg N a-1 over the
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  contiguous US, including 4.2 as NO and 2.3 as NH . Domestic anthropogenic, foreign
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  anthropogenic, and natural sources contribute respectively 78%, 6%, and 16% of total
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  nitrogen deposition over the contiguous US in the model. The domestic anthropogenic
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  contribution generally exceeds 70% in the east and in populated areas of the west, and is
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  typically 50-70% in remote areas of the west. Total nitrogen deposition in the model
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  exceeds 10 kg N ha-1 a-1 over 35% of the contiguous US.
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  1. Introduction
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  Atmospheric inputs of reactive nitrogen (fixed nitrogen) to ecosystems have increased by
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  more than a factor of 3 globally due to human activity, significantly perturbing the global
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  nitrogen cycle (Vitousek et al., 1997; Galloway et al., 2004). Adverse effects may include
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  soil acidification (Bowman et al., 2008), eutrophication (Bouwman et al. 2002), and a
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  reduction in plant biodiversity (Stevens et al., 2004). Increased nitrogen deposition may
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  enhance CO uptake by the land and ocean, though the climate benefit would be offset by
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  associated N O emission (Reay et al., 2008). The US Environmental Protection Agency
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  (EPA) is presently developing secondary air quality standards for protection of
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  ecosystems against the detrimental effects of nitrogen deposition (US EPA, 2008). This
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  requires a better understanding of nitrogen deposition over the US in its various forms
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  and including contributions from sources both natural and anthropogenic, foreign and
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  domestic. We use here a nested version of the global GEOS-Chem chemical transport
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  model (CTM) to address these issues.
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  The anthropogenic contribution to nitrogen deposition is mainly driven by emissions of
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  fixed nitrogen including nitrogen oxide radicals (NO ≡ NO + NO ) and ammonia (NH ).
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  These species also have natural sources. NO is emitted to the atmosphere by
x
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  combustion, microbial activity in soils, and lightning. In the atmosphere, NO is oxidized
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  to nitric acid (HNO ) and organic nitrates on a time scale of less than a day. These
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  different forms can be deposited to ecosystems by direct uptake (dry deposition). In
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  addition, HNO is highly soluble in water and is scavenged efficiently by precipitation
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  (wet deposition). NH is a major component of nitrogen cycling through the biosphere. It
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  is emitted to the atmosphere by agriculture (mostly animal husbandry and fertilizer use),
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  natural terrestrial and marine ecosystems, and fires. NH in the atmosphere can combine
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  with H SO (from SO oxidation) and HNO to produce ammonium sulfate and nitrate
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  particles. Dry deposition is fast for gaseous NH but slow for ammonium particles, while
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  wet deposition is efficient for both.
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  The lifetime of fixed nitrogen in the atmosphere is sufficiently short that most of the
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  nitrogen deposition for a large country such as the US is expected to be of domestic
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  origin. However, transboundary transport including on intercontinental scales can also be
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  significant (Asman et al., 1998; Dentener et al., 2006; Sanderson et al. 2008). A number
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  of studies have estimated an export efficiency of 20-30% for nitrogen oxides (NO ≡ NO
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  and its oxidation products) emitted in the US (Jacob et al., 1993; Kasibhatla et al., 1993;
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  Liang et al., 1998; Li et al., 2004). No analysis has been conducted so far on the relative
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  contributions from domestic, foreign, and natural sources to the different forms of
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  nitrogen deposition over the US.
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  Here we use a nested continental scale version of the GEOS-Chem global CTM (Y.
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  Wang et al., 2004; Chen et al., 2009) with horizontal resolution of 1/2° × 2/3° over North
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  America and 2° × 2.5° for the rest of the world. Three-year GEOS-Chem simulations for
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  2006-2008 are conducted to quantify the sources and processes for nitrogen deposition to
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  the US. We present an extensive evaluation for 2006 with surface measurements of wet
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  deposition fluxes, HNO and aerosol concentrations, and satellite observations of NO .
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  We quantify the contributions to nitrogen deposition from wet vs. dry processes and from
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  individual nitrogen species. We also separate the contributions from domestic
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  anthropogenic, foreign anthropogenic, and natural sources.
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  2. The GEOS-Chem nested-grid model
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  2.1 General description
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  The GEOS-Chem 3-D global model of atmospheric composition (v8-02-03; http://geos-
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  chem.org) was originally described by Bey et al. (2001) and Park et al. (2004). The
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  model here is driven by GEOS-5 assimilated meteorological data for 2006-2008 from the
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  NASA Global Modeling and Assimilation Office (GMAO). The data are available with a
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  temporal resolution of 6 hours (3 hours for surface variables and mixing depths), a
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  horizontal resolution of 1/2° latitude by 2/3° longitude, and 72 vertical layers from the
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  surface to 0.01 hPa. The lowest 5 layers are centered at 70, 200, 330, 470, and 600 m for
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  a column based at sea level. We use a nested version of GEOS-Chem (Y. Wang et al.,

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  2004; Chen et al., 2009) with the native 1/2° × 2/3° horizontal resolution over North
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  America and adjacent oceans (140°-40°W, 10°-70°N), and 2° × 2.5° horizontal resolution
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  for the rest of the world. Zhang et al. (2011) previously used the exact same model to
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  estimate policy-relevant background ozone in surface air over the US.
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  The model includes a detailed simulation of tropospheric ozone-NO -hydrocarbon-
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  aerosol chemistry, as recently described for example by Mao et al. (2010). Formation of
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  organic nitrates from the oxidation of biogenic isoprene emitted by vegetation can be a
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  significant sink for NO in the model (Horowitz et al., 1998). We assume that these
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  isoprene nitrates are removed by wet and dry deposition at the same deposition velocity
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  as HNO and do not regenerate NO . Earlier versions of GEOS-Chem did not explicitly
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  describe isoprene nitrates, treating them instead as HNO (Bey et al., 2001). Here we
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  describe them explicitly in order to compare simulated HNO with observations and to
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  quantify the contribution of isoprene nitrates to dry deposition.
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  Aerosol and gas-phase chemistry in GEOS-Chem are coupled through gas-aerosol
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  partitioning of semi-volatile species including NH and HNO , heterogeneous aerosol
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  chemistry parameterized as reactive uptake coefficients (Jacob, 2000), and aerosol effects
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  on photolysis rates (Martin et al., 2003). Partitioning of total NH and HNO between the
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  gas and aerosol phases is calculated using the RPMARES thermodynamic equilibrium
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  model (Binkowski and Roselle, 2003). The reactive uptake coefficients γ for N O in
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  aerosols are from Evans and Jacob (2005), reduced by a factor of 10 as discussed in
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  Macintyre and Evans (2010). The resulting annual mean value of γ in surface air over
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  the contiguous US is 0.003, comparable to measured values in the range of 0.0005-0.006
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  (Brown et al., 2009; Bertram et al., 2009).
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  We conduct three-year GEOS-Chem simulations for 2006-2008. We first conduct the
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  global GEOS-Chem simulation at 2° × 2.5° resolution, and then use the output archived
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  at 3-hour temporal resolution as dynamic boundary conditions for the nested model.
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  Output from the nested model does not affect the global simulation (one-way nesting).
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  2.2. Deposition
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  The wet deposition scheme for aerosols is described by Liu et al. (2001), and its
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  adaptation to soluble gases follows Mari et al. (2000). It includes scavenging in
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  convective updrafts as well as in-cloud and below-cloud scavenging from large-scale
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  precipitation. In warm (liquid) clouds with T > 268 K, aerosols are assumed to be 100%
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  incorporated in cloud droplets and gases are partitioned following Henry’s law. In mixed
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  (liquid/ice) clouds (248 < T < 268 K), where precipitation takes place by riming, aerosols
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  are retained in the rime ice while gases are retained with varying efficiencies (unity for
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  HNO but 0.05 for NH ; J. Wang et al. (2008)). In cold (ice) clouds (T < 248 K), both
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  aerosols and HNO are scavenged with 100% efficiency (HNO is taken up as a
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  monolayer; Abbatt (1997)), while NH is not scavenged.
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  Dry deposition of gases and aerosols is simulated with a standard big-leaf resistance-in-
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  series model (Wesely, 1989). The dry deposition flux F out of the lowest model layer
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  (midpoint z ≈ 70 m above the surface) is calculated as:
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  F = n C(z )v (z ) (1)
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  where n (molecules cm-3) is the number density of air, C(z )is the mixing ratio of the
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  depositing species at height z , and v is its deposition velocity (cm s-1) at that height. The
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  deposition velocity is a function of the local meteorological condition and surface type, as
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  given by:
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  v (z )=(R (z ,z )+R +R )−1 (2)
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  HereR (z ,z ) is the aerodynamic resistance to turbulent transfer from z to the
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  roughness height z close to the surface where turbulence vanishes, R is the boundary
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  layer resistance to molecular diffusion from z to the actual surface, and R is the canopy
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  or surface uptake resistance.
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  Table 1 lists the annual mean daytime (10-16 local time) dry deposition velocities for
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  different species computed in the model over the contiguous US. Values average 2.7 ±
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  1.5 cm s-1 for HNO , N O , and isoprene nitrates; 0.65 ± 0.40 cm s-1 for NH ; and 0.15-
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  0.36 cm s-1 for aerosols, NO , peroxyacetyl nitrate (PAN), and other organic nitrates.
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  Other nitrogen species are not significantly removed by dry deposition. Model values are
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  consistent with experimental studies, which report daytime dry deposition velocities to
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  land in the 2-10 cm s-1 range for HNO (Sievering et al., 2001; Horii et al., 2005), and in
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  the 0.1-1.0 cm s-1 range for PAN (Doskey et al., 2004; Turnipseed et al. 2006; Wolfe et
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  al., 2009). Biosphere-atmosphere exchange of NO and NH is bi-directional (Sutton et
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  al., 1998; Lerdau et al., 2000; Ellis et al., 2011), but is treated here as uncoupled emission
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  and deposition processes.
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  2.3. Emissions
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  US anthropogenic emissions are from the EPA National Emission Inventory for 2005
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  (NEI 05) with modifications for NH described below. Anthropogenic emissions of NO ,
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  CO, volatile organic compounds (VOCs), and SO outside the US are from the Emission
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  Database for Global Atmospheric Research (EDGAR) inventory (Olivier and Berdowski,
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  2001). Anthropogenic emissions of NH outside the US are from the Global Emission
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  Inventory Activity (GEIA) (Bouwman et al., 1997). These global inventories are
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  superseded by regional emission inventories from Q. Zhang et al. (2009) for Asia in
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  2006, the European Monitoring and Evaluation Program (EMEP) for Europe (Vestreng
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  and Klein, 2002), the Criteria Air Contaminants (CAC) emission inventory for Canada
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  (http://www.ec.gc.ca/pdb/cac/cac_home_e.cfm), and the Big Bend Regional Aerosol and
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  Visibility Observational (BRAVO) emission inventory for Mexico (Kuhns et al., 2005).
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  The EDGAR, EMEP, CAC, and BRAVO emissions are scaled on the basis of energy
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  statistics to 2006 as described by van Donkelaar et al. (2008). Global anthropogenic NO
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  emissions also include fertilizer use from Yienger and Levy (1995) and aircraft from
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  Baughcum et al. (1996).
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  Natural NO emissions include open fires, lightning, and soil. We use monthly biomass
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  burning emissions from the Global Fire Emission Database version 2 (GFED-v2) (van
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  der Werf et al., 2006). Lightning NO emissions are linked to deep convection following
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  the parameterization of Price and Rind (1992) with vertical profiles from Pickering et al.

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  (1998). The global spatial distribution of lightning flashes is rescaled to match the 10-
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  year climatology of OTD/LIS satellite observations (Sauvage et al., 2007) with higher
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  NO yield per flash at northern mid-latitudes than in the tropics (Hudman et al., 2007).
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  The global lightning source is imposed to be 6 Tg N a-1 (Martin et al., 2007). Soil NO
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  emissions are computed using a modified version of the Yienger and Levy (1995)
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  algorithm with canopy reduction factors described in Wang et al. (1998). Biogenic VOC
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  emissions (important for the conversion of NO to organic nitrates) are from the Model of
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  Emissions of Gases and Aerosols from Nature (MEGAN) (Guenther et al., 2006). Natural
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  NH emissions from soils, vegetation, and the oceans are from the GEIA inventory
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  Figure 1 shows the spatial and seasonal distribution of US NO emissions and Table 2
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  gives annual totals from each source over the contiguous US. Anthropogenic sources (5.6
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  Tg N a-1 including fertilizer use and aircraft) account for 84% of the total NO emissions.
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  Natural sources from lightning, soil, and open fires account for 9.5%, 6.2%, and 0.7%,
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  respectively. Natural contributions peak in summer, accounting for 39% of US NO
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  emissions in July.
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  Gilliland et al. (2003, 2006) and Pinder et al. (2006) previously found large seasonally
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  varying errors in the US NEI emission inventory for NH by model comparison with
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  observed wet deposition fluxes of ammonium (NH +) and atmospheric concentrations of
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  total reduced nitrogen (NH ≡ NH gas + ammonium aerosol). Here we use NH
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  measurements from two networks (Figure 2, left panel) to constrain the seasonality of
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  NH emissions: the Midwest Ammonia Monitoring Project managed by the Midwest
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  Regional Planning Organization (RPO) for 2004-2005 (http://www.ladco.org), and the
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  Southeastern Aerosol Research and Characterization (SEARCH) for 2006 (Edgerton et
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  al., 2006). Figure 2 (central panel) compares observations to model results in a simulation
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  with the August NEI-2005 NH emission applied to the whole year (aseasonal source).
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  Results agree well with measurements in summer but are far too high in winter, reflecting
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  at least in part a temperature dependence of NH emission (Aneja et al., 2000). We fit
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  monthly scaling factors as observed/simulated concentration ratios to the NEI emissions

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  to correct the discrepancy shown in the central panel of Figure 2 and apply them
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  nationwide. These scaling factors range from 0.9-1 in summer to 0.2-0.4 in winter.
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  Independent comparison of the resulting model to the SEARCH data for 2006 (Figure 2,
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  right panel) shows good agreement and thus supports these seasonal scaling factors.
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  Figure 1 shows the spatial and seasonal distribution of US NH emissions with the above
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  scaling factors applied, and Table 2 gives annual totals for each source. Emissions show a
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  broad May-September maximum. The highest emissions are in areas of major livestock
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  operations. Anthropogenic emissions (2.3 Tg N a-1, 81%) dominate over natural
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  emissions (0.56 Tg N a-1, 19%).
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  Our NO and NH emission estimates can be compared with those of Smith et al. (2010),
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  who implemented natural emission inventories into the CMAQ regional model for July
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  2002. For a model domain (130°-70°W, 23°-56°N) covering the contiguous US and large
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  fractions of Canada and Mexico, they found natural emissions to contribute 44% of NO
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  emissions and 28% of NH emissions. Our results for July 2006 over the same CMAQ
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  domain are comparable, with natural emission contributions of 40% for NO and 24% for
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  NH . The largest difference is for open fire NO emissions, which are a factor of 5 higher
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  in Smith et al. (2010). The GFED-v2 fire emissions used in GEOS-Chem indicate that
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  dry mass burned over the CMAQ domain was a factor of 4 higher in July 2002 than in
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  July 2006, mostly due to large fires in eastern Canada in 2002.
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  3. Deposition patterns and surface concentrations
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  Figure 3a-c compares simulated and observed sulfate, ammonium, and nitrate wet
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  deposition fluxes over the US and Canada for 2006. The observations are from the 251
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  sites of the National Atmospheric Deposition Program/National Trends Network
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  (NADP/NTN; data available at https://nadp.isws.illinois.edu/) for the US, and 26 sites of
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  the Canadian Air and Precipitation Monitoring Network (CAPMoN; data available at
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  http://www.on.ec.gc.ca/natchem/index.aspx) for Canada. We use sulfate as a check on the
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  wet deposition processes in the model since the SO source from coal combustion is well
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  constrained by stack measurements. We summarize the comparison between the model
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  (M) and observations (O) using the correlation coefficient, the normalized mean bias
N
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  (NMB) computed as NMB= i=1 i i for the N CASTNet sites, and the mean
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  normalized bias MNB= ∑ (M −O )/O . The NMB estimates the mean offset
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  between the model and observations, while MNB provides a sensitive evaluation of the
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  model performance for observed low values.
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  For all three species the model shows strong correlations with observations, no
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  significant annual biases, and little seasonal bias. MNB and NMB generally agree within a
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  few percent, except for some seasonal cases where MNB is ~20% higher due to model
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  overestimates of very low observations. Wet deposition generally peaks in summer for all
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  three species, in the case of sulfate and nitrate because of higher SO and NO oxidant
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  concentrations, and in the case of ammonium because of higher ammonia emissions. The
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  summer peak of nitrate wet deposition is particularly pronounced around the Gulf of
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  Mexico where it reflects the seasonal maxima in both lightning emissions and
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  precipitation. Simulated nitrate wet deposition also peaks in Michigan and southeastern
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  Canada in winter, caused by transport of HNO and nitrate aerosols produced mostly
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  from N O hydrolysis. This winter maximum is less pronounced in the observations,
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  suggesting that N O hydrolysis in the model may be too fast as discussed further below.
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  The model does not capture the observed high values of ammonium wet deposition in the
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  upper Midwest, as previously noted by Fisher et al. (2011), likely because of regional
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  underestimate of emissions.
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  No routine direct measurements of dry deposition fluxes are available at US sites.
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  However, the Clean Air Status and Trends Network (CASTNet) makes weekly integrated
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  measurements of gas-phase HNO concentrations from which dry deposition fluxes can
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  be estimated using modeled dry deposition velocities (Clarke et al., 1997). Figure 4
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  compares annual mean HNO concentrations from CASTNet with GEOS-Chem results in
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  2006. The model has a mean positive bias of 69%, which is due in part to the vertical

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