2017b). The ranges of satellite-derived (0.16–7.99 μg N m
À3 ) and measured
(0.2–7.7 μg N m
À3 ) annual NO 3
À concentrations were close, while the averages of
satellite-derived (2.53 μg N m
À3 ) and directly measured (2.62 μg N m
À3 ) ground
NO 3
À concentration at 43 sites in NNDMN were almost the same.
3.4 Nitrogen Deposition in China
3.4.1 Dry Deposition
Dry deposition constitutes about two-thirds of overall N deposition (Flechard et al.
2011; Pan et al. 2012; Shen et al. 2013). The five main inorganic N r species, viz.
NH 3 , NO 2 , HNO 3 , NH 4
+ and NO 3
À , constitute a majority of overall dry N r mass, and
gaseous species are the principal contributors to overall dry-deposited N. NH 3 is
generally the predominant N r species in dry deposition, although its quantification is
subject to uncertainties owing to its bidirectional exchange between atmosphere and
biosphere (Pleim et al. 2013). In addition, HNO 3 may also contribute substantially to
dry deposition even in areas with comparatively lower concentration (Flechard et al.
2011) owing to greater reactivity and deposition rate.
A 5-year (2010–2014) study from NNDMN revealed that dry N deposition at
43 monitoring sites across China varied from 1.1 to 52.2 kg N ha
À1 year
À1 (Xu et al.
2015). Modelling showed that eastern China experiences the maximum global levels
of dry N deposition (Vet et al. 2014). Other modelling studies focused on the
simulation of dry N depositions at a regional and/or national level in China (Zhao
et al. 2017; Han et al. 2017; Xu et al. 2018a, b). Zhao et al. (2017) utilized the GEOSChem model to simulate N deposition to China averaged over 2008–2012, showing
that dry deposition (6.4 kg N ha
À1 year
À1 ) includes 35% of reduced (NH x ), 45% of
oxidized (NO y ) and 39% of total inorganic N depositions to China. Using the
RAMS-CMAQ system, Han et al. (2017) determined that total dry N deposition
over China was approximately 9.3 kg N ha
À1 year
À1 between 2010 and 2014,
including 4.3 kg N ha
À1 year
À1 as NO y and 4.4 kg N ha
À1 year
À1 as NH 3 . In
addition to ground observation and modelling methods, satellite monitoring captures
spatial heterogeneities and provides constant chronological coverage of gaseous N r
species, which is also an effective technique for determining dry deposition of N r
species (Jia et al. 2016; Liu et al. 2017d). Jia et al. (2016) found that dry deposition
was only 7.5 kg N ha
À1 year
À1 from 2005 to 2014, rising over that period, based on a
combination of NO 2 satellite measurements and empirical models. Although they
underestimated dry N deposition, Jia et al.’s (2016) approach proved the possibility
of estimating dry deposition using satellite measurements and empirical models
jointly.
So far, few monitoring and modelling studies have investigated the spatiotemporal pattern of dry deposition at a national level (Han et al. 2017). According
to Xu et al. (2015), the spatial variability of dry deposition followed the order
northern > southern > northeastern > northwestern China > Tibetan Plateau
52
X. Liu et al.
À3 ) and measured
(0.2–7.7 μg N m
À3 ) annual NO 3
À concentrations were close, while the averages of
satellite-derived (2.53 μg N m
À3 ) and directly measured (2.62 μg N m
À3 ) ground
NO 3
À concentration at 43 sites in NNDMN were almost the same.
3.4 Nitrogen Deposition in China
3.4.1 Dry Deposition
Dry deposition constitutes about two-thirds of overall N deposition (Flechard et al.
2011; Pan et al. 2012; Shen et al. 2013). The five main inorganic N r species, viz.
NH 3 , NO 2 , HNO 3 , NH 4
+ and NO 3
À , constitute a majority of overall dry N r mass, and
gaseous species are the principal contributors to overall dry-deposited N. NH 3 is
generally the predominant N r species in dry deposition, although its quantification is
subject to uncertainties owing to its bidirectional exchange between atmosphere and
biosphere (Pleim et al. 2013). In addition, HNO 3 may also contribute substantially to
dry deposition even in areas with comparatively lower concentration (Flechard et al.
2011) owing to greater reactivity and deposition rate.
A 5-year (2010–2014) study from NNDMN revealed that dry N deposition at
43 monitoring sites across China varied from 1.1 to 52.2 kg N ha
À1 year
À1 (Xu et al.
2015). Modelling showed that eastern China experiences the maximum global levels
of dry N deposition (Vet et al. 2014). Other modelling studies focused on the
simulation of dry N depositions at a regional and/or national level in China (Zhao
et al. 2017; Han et al. 2017; Xu et al. 2018a, b). Zhao et al. (2017) utilized the GEOSChem model to simulate N deposition to China averaged over 2008–2012, showing
that dry deposition (6.4 kg N ha
À1 year
À1 ) includes 35% of reduced (NH x ), 45% of
oxidized (NO y ) and 39% of total inorganic N depositions to China. Using the
RAMS-CMAQ system, Han et al. (2017) determined that total dry N deposition
over China was approximately 9.3 kg N ha
À1 year
À1 between 2010 and 2014,
including 4.3 kg N ha
À1 year
À1 as NO y and 4.4 kg N ha
À1 year
À1 as NH 3 . In
addition to ground observation and modelling methods, satellite monitoring captures
spatial heterogeneities and provides constant chronological coverage of gaseous N r
species, which is also an effective technique for determining dry deposition of N r
species (Jia et al. 2016; Liu et al. 2017d). Jia et al. (2016) found that dry deposition
was only 7.5 kg N ha
À1 year
À1 from 2005 to 2014, rising over that period, based on a
combination of NO 2 satellite measurements and empirical models. Although they
underestimated dry N deposition, Jia et al.’s (2016) approach proved the possibility
of estimating dry deposition using satellite measurements and empirical models
jointly.
So far, few monitoring and modelling studies have investigated the spatiotemporal pattern of dry deposition at a national level (Han et al. 2017). According
to Xu et al. (2015), the spatial variability of dry deposition followed the order
northern > southern > northeastern > northwestern China > Tibetan Plateau
52
X. Liu et al.