1 Submitted to ACPD: Paper acp-2011-970 1 2 Nitrogen deposition to the United States: distribution, sources, and processes 3 4 Lin Zhang 1,2 , Daniel J. Jacob 1,2 , Eladio M. Knipping 3 , Naresh Kumar 4 , J. William 5 Munger 1,2 , Claire C. Carouge 2 , Aaron van Donkelaar 5 , Yuxuan Wang 6 , Dan Chen 7 6 7 [1] {Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, 8 USA} 9 [2] {School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, 10 USA} 11 [3] {Electric Power Research Institute, Washington, DC, USA} 12 [4] {Electric Power Research Institute, Palo Alto, CA, USA} 13 [5] {Department of Physics and Atmospheric Science, Dalhousie University, Halifax, 14 Canada} 15 [6] {Ministry of Education Key Laboratory for Earth System Modeling, Center for Earth 16 System Science, Institute for Global Change Studies, Tsinghua University, Beijing, 17 China} 18 [7] {Department of Atmospheric and Oceanic Sciences, University of California, Los 19 Angeles, CA, USA} 20 21 Correspondence to: Lin Zhang ([email protected]) 22 23 24 25 26 27 28 29 30 31
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Submitted to ACPD: Paper acp-2011-970 1 2
Nitrogen deposition to the United States: distribution, sources, and processes 3
4
Lin Zhang1,2, Daniel J. Jacob1,2, Eladio M. Knipping3, Naresh Kumar4, J. William 5
Munger1,2, Claire C. Carouge2, Aaron van Donkelaar5, Yuxuan Wang6, Dan Chen7 6
7
[1] {Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, 8
USA} 9
[2] {School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, 10
USA} 11
[3] {Electric Power Research Institute, Washington, DC, USA} 12
[4] {Electric Power Research Institute, Palo Alto, CA, USA} 13
[5] {Department of Physics and Atmospheric Science, Dalhousie University, Halifax, 14
Canada} 15
[6] {Ministry of Education Key Laboratory for Earth System Modeling, Center for Earth 16
System Science, Institute for Global Change Studies, Tsinghua University, Beijing, 17
China} 18
[7] {Department of Atmospheric and Oceanic Sciences, University of California, Los 19
compares the monthly mean measurements at Harvard Forest for 1999-2002 to model 411
results for 2006-2008. Measured NOy fluxes peak in summer and are minimum in winter. 412
The model has a weaker seasonality; it captures the summertime fluxes but is too high in 413
fall and winter. The mean measured annual NOy deposition flux is 5.4 kg N ha-1 a-1, and 414
the model is 33% higher (7.2 kg N ha-1 a-1). The measured flux was particularly high in 415
2000 (8.1 kg N ha-1 a-1), and had little variation for the other three years (4.2-4.4 kg N ha-416 1 a-1). Model results show little inter-annual variation for 2006-2008 (6.9-7.5 kg N ha-1 a-417 1). The model overestimate of NOy dry deposition in fall and winter may reflect in large 418
part an excessive N2O5 hydrolysis in aerosols, as discussed above. 419
420
Eddy covariance flux measurements of PAN have been reported at Duke Forest, North 421
Carolina (Turnipseed et al., 2006), and at Blodgett Forest, California (Wolfe et al., 2009). 422
Turnipseed et al. (2006) found that PAN deposition accounted for 20% of the daytime 423
NOy deposition at Duke Forest in July 2003, but Wolfe et al. (2009) found only a 4% 424
contribution at Blodgett Forest in August-October 2007 after correcting for the PAN 425
15
thermal decomposition between the altitude of measurement and the surface. We find in 426
GEOS-Chem that PAN contributes respectively 5% and 4% of NOy dry deposition at the 427
two sites in summer. 428
429
We find in the model that 4.2 Tg N of NOy and 2.3 Tg N of NHx are deposited annually 430
over the contiguous US. Comparison to US emissions in Table 2 indicates an annual net 431
export of 2.5 Tg N as NOy (38% of NOx emissions) and 0.60 Tg N as NHx (21% of NH3 432
emissions). Our results are consistent with Dentener et al. (2006), who found by 433
averaging results from 23 chemical transport models that net export of NOy from the US 434
amounts to 37% of US NOx emissions. 435
436
5. Domestic, foreign, and natural contributions to nitrogen deposition 437
438
Figure 9 shows the simulated spatial distribution of annual total (wet and dry) nitrogen 439
deposition over the US. Nitrogen deposition is generally > 8 kg N ha-1 a-1 in the eastern 440
US and 1-4 kg N ha-1 a-1 in remote areas of the west. It is highest in the industrial 441
Midwest with regional values in excess of 15 kg N ha-1 a-1. Bobbink et al. (1998) and 442
Bouwman et al. (2002) estimate a “critical load” threshold of 10 kg N ha-1 a-1 for 443
sensitive ecosystems above which disturbance could be significant. In our simulation, 444
35% of the US land receives nitrogen deposition exceeding this load. 445
446
We separated the contributions to nitrogen deposition from domestic anthropogenic, 447
foreign anthropogenic, and natural sources by conducting sensitivity simulations for 2006 448
with (1) US domestic NH3 and NOx anthropogenic emissions shut off, (2) global 449
anthropogenic emissions shut off. Table 4 summarizes the budgets for the contiguous US. 450
Domestic anthropogenic emissions account respectively for 81% and 71% of NOy and 451
NHx deposition to the US (78% of total nitrogen deposition). Foreign anthropogenic 452
emissions contribute 6% of NOy deposition, 8% of NHx deposition, and 6% of the total 453
deposition. Natural sources account for the rest: 13% of NOy deposition, 21% of NHx 454
deposition, and 16% of total nitrogen deposition. 455
456
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Figure 10 shows how these deposition enhancements and relative contributions vary by 457
receptor region. The domestic anthropogenic contribution generally exceeds 70% in the 458
east and in populated areas of the west, falling off to 50-70% in remote areas of the west. 459
Foreign anthropogenic contributions are generally less than 10% except near the 460
Canadian/Mexican border areas (up to 30%). The rising emissions of NOx and NH3 from 461
oil production and agriculture in western Canada (Schindler et al., 2006) could affect 462
Montana and North Dakota. Natural source contributions are less than 10% in the eastern 463
US and the West Coast, and about 20-30% in the intermountain West, with maximum 464
contributions of 40% over the southwest US due to lightning emissions and over Idaho 465
due to wildfires. 466
467
6. Conclusions 468
469
We have presented a simulation of nitrogen deposition over the United States in 2006-470
2008 using a nested-grid version of the GEOS-Chem global chemical transport model 471
with 1/2° × 2/3° horizontal resolution over North America and adjacent oceans (140°-472
40°W, 10°-70°N), and 2° × 2.5° horizontal resolution for the rest of the world. The model 473
includes a detailed representation of oxidant-aerosol chemistry. Our focus was to quantify 474
the processes and species contributing to nitrogen deposition over the contiguous US as 475
well as the relative contributions of domestic anthropogenic, foreign anthropogenic, and 476
natural sources. 477
478
Total NOx and NH3 emissions in the contiguous US in the model are 6.7 and 2.9 Tg N a-1 479
respectively. Natural sources account for about 20% annually for both (up to 39% for 480
NOx in summer). Previous studies (Gilliland et al., 2003, 2006; Pinder et al., 2006) 481
identified large seasonal biases in US emission inventories for NH3. Our model imposes a 482
seasonality of NH3 emissions fitted to surface NHx measurements from the Midwest RPO 483
and SEARCH networks, such that emissions in winter are about a third those in summer. 484
Successful simulation of observations for NHx concentrations and ammonium wet 485
deposition fluxes lends support to the NH3 emissions used in the model, except in the 486
upper Midwest where emissions appear to be too low. 487
17
488
We evaluated the model with an ensemble of relevant data sets for deposition fluxes and 489
concentrations. The model reproduces the wet deposition fluxes of sulfate, nitrate and 490
ammonium measured at the NADP sites in the US and the CAPMoN sites in Canada with 491
high correlations and no significant bias. Comparison to observed HNO3 concentrations 492
at CASTNet sites shows a mean positive model bias of 69%, but we show that this 493
largely reflects the expected concentration gradient between the CASTNet measurement 494
altitude (10 m) and the midpoint of the lowest model layer (70 m). Correcting for this 495
gradient reduces the mean model bias over the US to 18% and localizes it to the industrial 496
Midwest in winter (88%). Comparisons with aerosol measurements of sulfate, 497
ammonium and nitrate at CASTNet and EPA-AQS networks show no significant biases 498
for sulfate, but positive biases of 17-34% for ammonium and 40-81% for nitrate. The 499
model reproduces closely the spatial pattern of satellite NO2 tropospheric column 500
measurements from OMI; it is on average 23% too low but this could reflect biases in the 501
satellite retrieval. Comparison to multi-year eddy correlation measurements of NOy dry 502
deposition fluxes at Harvard Forest, Massachusetts shows good agreement in summer but 503
a factor of 2 high bias in winter. 504
505
The main model flaw identified through comparison to the ensemble of observations is 506
excessive HNO3 production in winter. This production in the model is mainly from N2O5 507
hydrolysis in aerosols, with a mean reactive uptake coefficient γN2O5 = 0.003 (Evans and 508
Jacob, 2005; Macintyre and Evans, 2010) that is not inconsistent with values inferred 509
from field observations in summer (Brown et al., 2009). However, the model does not 510
account for inhibition of hydrolysis by aerosol nitrate (Davis et al., 2008; Bertram and 511
Thornton, 2009), which would be important in winter when nitrate is a major constituent 512
of the aerosol. It also does not account for reaction of N2O5 with chloride aerosol 513
(Roberts et al., 2009; Thornton et al., 2010), which would decrease the HNO3 yield. 514
These effects should be included in future versions of the model. 515
516
We analyzed model results for 2006-2008 to quantify the processes contributing to 517
nitrogen deposition. We find that 6.5 Tg N a-1 is deposited over the contiguous US: 4.2 518
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Tg N as NOy and 2.3 Tg N as NHx. Dry deposition accounts for 70% of total deposition 519
for NOy and 43% for NHx. NHx dry deposition is mainly through NH3 gas (82%). Dry 520
deposition of NOy is partitioned as 55% HNO3, 22% NO2, 9% isoprene nitrates, 3.0% 521
PAN, 2.3% nitrate aerosol, and 8.7% other species. The US is a net annual exporter of 2.5 522
Tg N as NOy (38% of domestic NOx emissions) and 0.60 Tg N as NHx (21% of domestic 523
NH3 emissions). Domestic anthropogenic emissions contribute respectively 81% and 524
71% of NOy and NHx deposition over the contiguous US, foreign anthropogenic 525
emissions contribute 6% and 8%, and natural emissions 13% and 21%. The contribution 526
from domestic anthropogenic sources to total nitrogen deposition generally exceeds 70% 527
in the east and populated areas of the west, and is typically 50-70% in remote areas of the 528
west. 35% of the land surface in the contiguous US receives nitrogen deposition in excess 529
of 10 kg N ha-1 a-1. A follow-up study will provide a more detailed source attribution of 530
nitrogen deposition in the US. 531
532
Acknowledgments. This work was supported by the Electric Power Research Institute 533
(EPRI). The authors acknowledge the work of many individuals who have made the 534
Midwest RPO, SEARCH, NADP, CAPMoN, CASTNet, EPA-AQS and OMI 535
measurements. 536
537
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Roberts, J. M., Osthoff, H. D., Brown, S. S., Ravishankara, A. R., Coffman, D., Quinn, 781 P., and Bates, T.: Laboratory studies of products of N2O5uptake on Cl−containing 782 substrates, Geophysical Research Letters, 36, 10.1029/2009gl040448, 2009. 783 784 Sanderson, M. G., Dentener, F. J., Fiore, A. M., et al.: A multi-model study of the 785 hemispheric transport and deposition of oxidised nitrogen, Geophysical Research Letters, 786 35, 2008. 787 788 Sauvage, B., Martin, R. V., van Donkelaar, A., Liu, X., Chance, K., Jaeglé, L., Palmer, P. 789 I., Wu, S., Fu, T.-M.: Remote sensed and in situ constraints on processes affecting 790 tropical tropospheric ozone. Atmospheric Chemistry and Physics 7, 815– 838, 2007. 791 792 Schindler, D. W., Dillon, P. J., and Schreier, H.: A review of anthropogenic sources of 793 nitrogen and their effects on Canadian aquatic ecosystems, Biogeochemistry, 79, 25-44, 794 10.1007/s10533-006-9001-2, 2006. 795 796 Sievering, H., Enders, G., Kins, L., et al.: Nitric acid, particulate nitrate and ammonium 797 profiles at the Bayerischer Wald: evidence for large deposition rates of total nitrate. 798 Atmospheric Environment, 28 (2), 311-315, 1994. 799 800 Sievering, H., Kelly, T., McConville, G., Seibold, C., Turnipseed, A.: Nitric acid dry 801 deposition to conifer forests: Niwot Ridge spruce–fir–pine study. Atmos. Environ. 35, 802 3851–3859, 2001. 803 804 Smith, S. N., and Mueller, S. F.: Modeling natural emissions in the Community 805 Multiscale Air Quality (CMAQ) Model-I: building an emissions data base, Atmospheric 806 Chemistry and Physics, 10, 4931-4952, 2010. 807 808 Stevens, C. J., Dise, N. B., Mountford, J. O., Gowing, D. J.: Impact of nitrogen 809 deposition on the species richness of grasslands, Science, 303, 1876-1879, 2004. 810 811 Sutton, M. A., Burkhardt, J. K., Guerin, D., Nemitz, E., and Fowler, D.: Development of 812 resistance models to describe measurements of bi-directional ammonia surface-813 atmosphere exchange, Atmospheric Environment, 32, 473-480, 1998. 814 815 Thornton, J. A., Kercher, J. P., Riedel, T. P., Wagner, N. L., Cozic, J., Holloway, J. S., 816 Dubé, W. P., Wolfe, G. M., Quinn, P. K., Middlebrook, A. M., Alexander, B., and 817 Brown, S. S.: A large atomic chlorine source inferred from mid-continental reactive 818 nitrogen chemistry, Nature, 464, 271-274, 10.1038/nature08905, 2010. 819 820 Turnipseed, A. A., Huey, L. G., Nemitz, E., Stickel, R., Higgs, J., Tanner, D. J., Slusher, 821 D. L., Sparks, J. P., Flocke, F., and Guenther, A.: Eddy covariance fluxes of peroxyacetyl 822 nitrates (PANs) and NOy to a coniferous forest, Journal of Geophysical Research, 111, 823 10.1029/2005jd006631, 2006. 824 825
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Figures and Tables 883 884 Figures 885 886
887 888 Figure 1. NOx and NH3 emissions over the contiguous US. The left panels show annual 889 total emissions at the 1/2° × 2/3° resolution of GEOS-Chem. The right panels show 890 seasonal variations for each source type. Annual totals by source type are given in Table 891 2. 892 893 894
895
28
896
897 Figure 2. Atmospheric concentrations of total reduced nitrogen (NHx ≡ NH3 + NH4
+) at 898 the Midwest-RPO and SEARCH networks. Site locations are shown in the left panel. 899 Monthly mean concentrations averaged across all sites of each network are shown in the 900 central and right panels. Observations (black) are compared to model results using the 901 NEI NH3 anthropogenic emissions with no seasonal variation (blue line in the central 902 panel) and with seasonal variation fitted to the Midwest-RPO data (red lines). The 903 Midwest-RPO and SEARCH data are for 2004-2005 and 2006, respectively, and model 904 results are for 2006. Vertical bars represent standard deviations in the observed monthly 905 means for individual sites and years. 906 907 908
29
909 910 Figure 3a. Annual and seasonal mean sulfate wet deposition fluxes measured at NADP 911 and CAPMoN sites (left panels) and simulated by GEOS-Chem (central panels) in 2006. 912 The right panels show scatter-plots of simulated versus observed values at individual 913 sites. Correlation coefficients (r), normalized mean biases (NMB), and mean normalized 914 biases (MNB) are shown inset. Reduced-major-axis regression lines (solid) and the 1:1 915 lines (dash) are also shown. 916 917
30
918 919 Figure 3b. Same as Figure 3a but for nitrate (NO3
-) wet deposition. 920 921
922
31
923 924 Figure 3c. Same as Figure 3a but for ammonium (NH4
+) wet deposition. 925 926 927 928 929
32
930 931 Figure 4. Annual mean HNO3 concentrations in near-surface air in 2006. Measurements 932 from the CASTNet sites at 10-m altitude (left panel) are compared to GEOS-Chem model 933 values in the lowest model layer (70m; middle panel). The right panel shows GEOS-934 Chem HNO3 concentrations at 10 m inferred from aerodynamic resistances to dry 935 deposition. The correlation coefficients (r), normalized mean biases (NMB), and mean 936 normalized biases (MNB) are shown inset. 937 938 939
940
33
941
942 943 Figure 5. Annual mean concentrations of sulfate (left), ammonium (middle), and nitrate 944 (right) aerosol in surface air in 2006. Results from the GEOS-Chem model (top) are 945 compared to observations from CASTNet (middle), and EPA AQS (bottom). Statistics 946 for model comparisons to observations are shown inset as correlation coefficients (r), 947 normalized mean biases (NMB), and mean normalized biases (MNB). 948 949
950
34
951
952 953 Figure 6. Mean tropospheric NO2 columns in March-November 2006. OMI satellite 954 observations mapped on the 1/2° × 2/3° GEOS-Chem grid (left) are compared to GEOS-955 Chem results (center). The GEOS-Chem minus OMI difference is shown in the right 956 panel. 957 958 959
960
35
961 962 Figure 7. Simulated annual total fluxes of NO3
- wet deposition, NOy dry deposition, 963 NH4
+ wet deposition, and NHx dry deposition. Values are 3-year means for 2006-2008. 964 Annual totals over the contiguous US from each process are shown inset in unit of Tg N 965 a-1. 966 967 968
969
36
970 971 Figure 8. Monthly NOy dry deposition fluxes at Harvard Forest, Massachusetts (42.53°N, 972 72.18°W). Eddy covariance flux measurements for 1999-2002 (black) are compared to 973 model results averaged for 2006-2008 (red). The vertical bars indicate the range of the 974 monthly mean values for the four years of measurements and three years of model 975 results. 976 977 978 979
980 Figure 9. Simulated annual total nitrogen deposition fluxes over the US. Values are 981 averages for 2006-2008. 982 983 984
37
985 986 Figure 10. Domestic anthropogenic, foreign anthropogenic, and natural contributions to 987 annual nitrogen deposition over the contiguous US. Values are from GEOS-Chem 988 sensitivity simulations for 2006 (see text) and are presented as both absolute and relative 989 contributions. 990 991
992
38
993 Table 1. Mean daytime dry deposition velocities over the contiguous US a 994 Species vd (cm s-1) NH3 0.65 ± 0.40 Aerosol NH4
- 0.15 ± 0.03 a Annual mean daytime (10-16 local time) values computed in GEOS-Chem for the ensemble of 995 1/2° × 2/3° grid squares covering the contiguous US and for the midpoint of the lowest grid level 996 (~70 m above the surface). Standard deviations describe the spatial variability of the annual 997 means. 998 b Peroxyacetyl nitrate (PAN) and higher peroxyacyl nitrates 999 1000 1001 1002 1003 1004 Table 2. NOx and NH3 emissions over the contiguous US a 1005
Source type Emission (Tg N a-1) NOx Total 6.7
Anthropogenic 5.3 Lightning 0.63 Soil 0.41 Aircraft 0.13 Fertilizer use 0.12 Open fires 0.055
NH3 Total 2.9 Anthropogenic 2.3 Natural 0.56
a Annual GEOS-Chem emissions for 2006-2008. 1006 1007 1008 1009
1010
39
1011 Table 3. Nitrogen deposition over the contiguous US a 1012
Deposition process Deposition (Tg N a-1) NOy Total 4.2
+ aerosol 0.20 a Annual total nitrogen deposition for 2006-2008 computed with the GEOS-Chem model. 1013 1014 1015 1016 1017 Table 4. Source contributions to nitrogen deposition over the contiguous US a 1018 Source NOy NHx Total Anthropogenic Domestic 3.4 1.6 5.0 Foreign 0.24 0.18 0.42 Natural 0.57 0.47 1.0
a Nitrogen deposition fluxes from different sources computed by the GEOS-Chem model 1019 as described in the text. Values are annual total fluxes in unit of Tg N a-1. 1020 1021