7 Long-Term Trends in Sulfur and Reactive Nitrogen …
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QUESTIONER: Talat Odman
QUESTION: The N-critical load in the northeast has decreased while it remained
the same (or increased) in eastern North Carolina. What is this telling us in terms of
sources of N?
ANSWER: The trends in total-N deposition are influenced by individual trends in
reduced and oxidized-N deposition. In the northeast U.S., the 1990–2010 trends in Ndeposition are dominated by trends in oxidized-N deposition while those in eastern
North Carolina are dominated by trends in reduced-N deposition. Consequently,
reductions in NO x emissions across the northeastern U.S. have resulted in decreasing
trends in N-deposition in the region. In contrast, increasing NH 3 emissions in eastern
North Carolina resulted in increase in reduced-N and total-N deposition in that region.
QUESTIONER: Camilla Geels
QUESTION: Did you consider changes in land-use—could be important for dry
deposition. If not, do you have any idea how significant they are?
ANSWER: To our knowledge no single dataset exists that describes in a consistent
manner the changes in land-use for the multi-decadal period analyzed here. To minimize uncertainties associated with using different land-use data sets for different
periods during 1990–2010, we decided to use the same land-use characterization
for the entire period. While we acknowledge the likely impact of changing land use
on simulated atmospheric dynamics and dry deposition amounts, we do not have a
detailed assessment of the magnitude of the impact for this period. However, one
could speculate that since both NH 3 and HNO 3 dry deposit efficiently to most surfaces, and since changes in land-use have been gradual, it is likely that the assumption
of time invariant land-use does not have a significant impact on our estimated trends
and results. Nevertheless, sensitivity simulations would need to be conducted to
confirm this in a more quantitative manner.
Disclaimer The views expressed in this paper are those of the authors and do not necessarily reflect
the views and policies of U.S. Environmental Protection Agency.
References
1. R. Mathur, J. Xing, R. Gilliam, G. Sarwar, C. Hogrefe, J. Pleim, G. Pouliot, S. Roselle, T.L. Spero,
D.C. Wong, J. Young, Extending the Community Multiscale Air Quality (CMAQ) modeling
system to hemispheric scales: overview of process considerations and initial applications. Atmos.
Chem. Phys. 17, 12449–12474 (2017). https://doi.org/10.5194/acp-17-12449-2017
2. J. Xing, R. Mathur, J. Pleim, C. Hogrefe, C.-M. Gan, D.C. Wong, C. Wei, R. Gilliam, G.
Pouliot, Observations and modeling of air quality trends over 1990–2010 across the Northern
Hemisphere: China, the United States and Europe. Atmos. Chem. Phys. 15, 2723–2747 (2015).
https://doi.org/10.5194/acp-15-2723-2015
3. J. Xing, J. Pleim, R. Mathur, G. Pouliot, C. Hogrefe, C.-M. Gan, C. Wei, Historical gaseous
and primary aerosol emissions in the United States from 1990 to 2010. Atmos. Chem. Phys. 13,
7531–7549 (2013). https://doi.org/10.5194/acp-13-7531-2013
45
QUESTIONER: Talat Odman
QUESTION: The N-critical load in the northeast has decreased while it remained
the same (or increased) in eastern North Carolina. What is this telling us in terms of
sources of N?
ANSWER: The trends in total-N deposition are influenced by individual trends in
reduced and oxidized-N deposition. In the northeast U.S., the 1990–2010 trends in Ndeposition are dominated by trends in oxidized-N deposition while those in eastern
North Carolina are dominated by trends in reduced-N deposition. Consequently,
reductions in NO x emissions across the northeastern U.S. have resulted in decreasing
trends in N-deposition in the region. In contrast, increasing NH 3 emissions in eastern
North Carolina resulted in increase in reduced-N and total-N deposition in that region.
QUESTIONER: Camilla Geels
QUESTION: Did you consider changes in land-use—could be important for dry
deposition. If not, do you have any idea how significant they are?
ANSWER: To our knowledge no single dataset exists that describes in a consistent
manner the changes in land-use for the multi-decadal period analyzed here. To minimize uncertainties associated with using different land-use data sets for different
periods during 1990–2010, we decided to use the same land-use characterization
for the entire period. While we acknowledge the likely impact of changing land use
on simulated atmospheric dynamics and dry deposition amounts, we do not have a
detailed assessment of the magnitude of the impact for this period. However, one
could speculate that since both NH 3 and HNO 3 dry deposit efficiently to most surfaces, and since changes in land-use have been gradual, it is likely that the assumption
of time invariant land-use does not have a significant impact on our estimated trends
and results. Nevertheless, sensitivity simulations would need to be conducted to
confirm this in a more quantitative manner.
Disclaimer The views expressed in this paper are those of the authors and do not necessarily reflect
the views and policies of U.S. Environmental Protection Agency.
References
1. R. Mathur, J. Xing, R. Gilliam, G. Sarwar, C. Hogrefe, J. Pleim, G. Pouliot, S. Roselle, T.L. Spero,
D.C. Wong, J. Young, Extending the Community Multiscale Air Quality (CMAQ) modeling
system to hemispheric scales: overview of process considerations and initial applications. Atmos.
Chem. Phys. 17, 12449–12474 (2017). https://doi.org/10.5194/acp-17-12449-2017
2. J. Xing, R. Mathur, J. Pleim, C. Hogrefe, C.-M. Gan, D.C. Wong, C. Wei, R. Gilliam, G.
Pouliot, Observations and modeling of air quality trends over 1990–2010 across the Northern
Hemisphere: China, the United States and Europe. Atmos. Chem. Phys. 15, 2723–2747 (2015).
https://doi.org/10.5194/acp-15-2723-2015
3. J. Xing, J. Pleim, R. Mathur, G. Pouliot, C. Hogrefe, C.-M. Gan, C. Wei, Historical gaseous
and primary aerosol emissions in the United States from 1990 to 2010. Atmos. Chem. Phys. 13,
7531–7549 (2013). https://doi.org/10.5194/acp-13-7531-2013