3.6 Uncertainties and Outlook
There are several uncertainties in monitoring/estimating the dry and wet deposition
of N across China. For dry deposition, large uncertainty remains in the deposition
velocities of various N r species, which can vary from near zero to 0.02 m s
À1 under
different land use types. Meanwhile, the bidirectional flux of NH 3 has not been
considered in NH 3 dry deposition estimation (Shen et al. 2009; Pleim et al. 2013;
Sutton and Bleeker 2013). For wet deposition, the uncertainty mainly comes
from different concepts, sampling procedures and analysis methods used in
different monitoring networks, which makes it difficult to ensure the comparability
of data. Organic N deposition, as an important component of the atmospheric
N cycle (Cornell et al. 2003), is normally not considered in both wet and dry
deposition in most monitoring networks worldwide (e.g. NADP, EANET, EMEP,
NNDMN). For satellite observation, there are also some uncertainties in the transformation of tropospheric column concentrations of NH 3 and NO 2 into surface
Nr concentrations and deposition. These uncertainties may limit the accurate understanding of the wet and dry deposition processes and their potential eco-environmental
impacts.
Therefore, future work should concentrate on (1) establishing a systematic and
long-term N deposition monitoring network (including all N r species, especially
organic N, in both wet and dry deposition), based on a combination of existing
observation stations administered by different departments in China; (2) improving
the spatial distribution of the N monitoring networks, with more representative
observation sites established in regions with little data (e.g. in northwest China
and Tibetan regions); (3) adopting uniform procedures for sampling, storage and
analysis; (4) strengthening biomonitoring and satellite monitoring for atmospheric
N r concentration and deposition; (5) decreasing the uncertainty in quantifying dry
deposition velocity of N r species, in particular NH 3 via a better understanding of the
deposition process; and (6) strengthening national and international collaboration on
N deposition measurements and the evaluation of its eco-environmental impacts.
Besides its academic benefits, the improved N deposition monitoring network can
serve as a mechanism for involving national stakeholders in the formulation of N
regulations and mitigation strategies (Liu et al. 2017e; Yu et al. 2019). A large
number of documents indicate that long-term monitoring benefits from research
programmes that support in-depth analysis of data in combination with models
and other tools. However, due to the need to invest a lot of human, material and
financial resources, China is facing huge challenges in maintaining and developing
the N deposition monitoring network(s). In the context that the Chinese government
is increasingly ambitious with regard to N r (e.g. NH 3 )-induced air quality issues, the
authors of this chapter call for an intensive cooperation mechanism between the
government and research institutes to establish long-term integrated national N
deposition monitoring network and open-access atmospheric N r concentration and
deposition datasets.
60
X. Liu et al.
There are several uncertainties in monitoring/estimating the dry and wet deposition
of N across China. For dry deposition, large uncertainty remains in the deposition
velocities of various N r species, which can vary from near zero to 0.02 m s
À1 under
different land use types. Meanwhile, the bidirectional flux of NH 3 has not been
considered in NH 3 dry deposition estimation (Shen et al. 2009; Pleim et al. 2013;
Sutton and Bleeker 2013). For wet deposition, the uncertainty mainly comes
from different concepts, sampling procedures and analysis methods used in
different monitoring networks, which makes it difficult to ensure the comparability
of data. Organic N deposition, as an important component of the atmospheric
N cycle (Cornell et al. 2003), is normally not considered in both wet and dry
deposition in most monitoring networks worldwide (e.g. NADP, EANET, EMEP,
NNDMN). For satellite observation, there are also some uncertainties in the transformation of tropospheric column concentrations of NH 3 and NO 2 into surface
Nr concentrations and deposition. These uncertainties may limit the accurate understanding of the wet and dry deposition processes and their potential eco-environmental
impacts.
Therefore, future work should concentrate on (1) establishing a systematic and
long-term N deposition monitoring network (including all N r species, especially
organic N, in both wet and dry deposition), based on a combination of existing
observation stations administered by different departments in China; (2) improving
the spatial distribution of the N monitoring networks, with more representative
observation sites established in regions with little data (e.g. in northwest China
and Tibetan regions); (3) adopting uniform procedures for sampling, storage and
analysis; (4) strengthening biomonitoring and satellite monitoring for atmospheric
N r concentration and deposition; (5) decreasing the uncertainty in quantifying dry
deposition velocity of N r species, in particular NH 3 via a better understanding of the
deposition process; and (6) strengthening national and international collaboration on
N deposition measurements and the evaluation of its eco-environmental impacts.
Besides its academic benefits, the improved N deposition monitoring network can
serve as a mechanism for involving national stakeholders in the formulation of N
regulations and mitigation strategies (Liu et al. 2017e; Yu et al. 2019). A large
number of documents indicate that long-term monitoring benefits from research
programmes that support in-depth analysis of data in combination with models
and other tools. However, due to the need to invest a lot of human, material and
financial resources, China is facing huge challenges in maintaining and developing
the N deposition monitoring network(s). In the context that the Chinese government
is increasingly ambitious with regard to N r (e.g. NH 3 )-induced air quality issues, the
authors of this chapter call for an intensive cooperation mechanism between the
government and research institutes to establish long-term integrated national N
deposition monitoring network and open-access atmospheric N r concentration and
deposition datasets.
60
X. Liu et al.