twelfth “Five-Year Plan” on NO x emission controls. The proportion of NO x emission from industry was 83% on average during 1980–2015. Soil and biomass
burning emissions account for 9% and 8% of national surface NO x emissions,
respectively. Many top-down approaches such as studies of the seasonality of NO x
emissions (Gu et al. 2014b) and satellite inversions (Mijling and van der A 2012)
were used for improving estimations of national NO x in China. Generally, results
show that our bottom-up inventories of NO x emission derived from CHANS could
explain over 75% of satellite measurements on tropospheric columns of NO 2 .
N 2 O emission in China showed a steadily rising trend since 1980 (0.6 Tg N
year
À1 ) and the emission more than doubled by 2015 (1.5 Tg N year
À1
). The
elevated emission can be attributed largely ($90%) to the increases in Nr flux
from agriculture, forest and grassland, and surface water, and the rest mainly results
from fossil fuel combustion ($6%), wastewater treatment (3%), and industrial
processes ($1%) (Fig. 5.4c). The recent increases of N deposition and eutrophication also promoted the N 2 O emission from forest and grassland and surface water
(Tian et al. 2011; Wang et al. 2006).
5.4.2 Spatial Variations
Figure 5.5 displays the provincial breakdown of the N fluxes in 2015, revealing
considerable spatial heterogeneity across China. Generally, N inputs were larger in
Southeast China and decreased sharply towards the northwest. It is obvious that
several hotspots of N inputs, notably the North China Plain, the Middle and Lower
Yangtze River Plain, Sichuan Basin, and the southeast coastal area, are also the N
surplus hotspots. Most of these hotspots have higher per capita GDP and urbanization level, which are usually accompanied by higher synthetic fertilizer use and
higher atmospheric N deposition.
The NH 3 emission in China shared the similar spatial patterns with N input, NH 3
emission density in hotspots like the North China Plain has reached over 50 kg N
ha
À1 year
À1 in 2010s (Fig. 5.5c), while the most intense hotspot occurred in Henan
(~89 kg N ha
À1 year
À1 ) where it played a role in the enhancement of airborne PM 2.5
during haze episodes (Gu et al. 2014a; Ye et al. 2011). NO and N 2 O emissions also
have the similar emission patterns, but their emissions were less than 10% of those of
NH 3 . Generally, the spatial variability of the NH 3 emission densities agrees well
with the IASI-NH 3 VCDs distribution; however, relatively high NH 3 VCDs were
detected in Xinjiang by satellite IASI instrument, which is not captured by our
emission map. These hotspots of NH 3 emission mainly related to agriculture in
Xinjiang, which only account for a small proportion of the land area in Xinjiang, and
majority of the remaining land uses are classified as deserts or mountains. There is
little emission of SO 2 and NO x from industrial sources in Xinjiang compared to that
in Eastern China (Liu et al. 2017). Therefore, a reduced conversion rate to aerosol is
expected. This, and the dry climate, is probably responsible for a longer lifetime of
NH 3 in the atmosphere in Xinjiang compared to other regions in China. This will
5 Reactive Nitrogen Budgets in China
99
burning emissions account for 9% and 8% of national surface NO x emissions,
respectively. Many top-down approaches such as studies of the seasonality of NO x
emissions (Gu et al. 2014b) and satellite inversions (Mijling and van der A 2012)
were used for improving estimations of national NO x in China. Generally, results
show that our bottom-up inventories of NO x emission derived from CHANS could
explain over 75% of satellite measurements on tropospheric columns of NO 2 .
N 2 O emission in China showed a steadily rising trend since 1980 (0.6 Tg N
year
À1 ) and the emission more than doubled by 2015 (1.5 Tg N year
À1
). The
elevated emission can be attributed largely ($90%) to the increases in Nr flux
from agriculture, forest and grassland, and surface water, and the rest mainly results
from fossil fuel combustion ($6%), wastewater treatment (3%), and industrial
processes ($1%) (Fig. 5.4c). The recent increases of N deposition and eutrophication also promoted the N 2 O emission from forest and grassland and surface water
(Tian et al. 2011; Wang et al. 2006).
5.4.2 Spatial Variations
Figure 5.5 displays the provincial breakdown of the N fluxes in 2015, revealing
considerable spatial heterogeneity across China. Generally, N inputs were larger in
Southeast China and decreased sharply towards the northwest. It is obvious that
several hotspots of N inputs, notably the North China Plain, the Middle and Lower
Yangtze River Plain, Sichuan Basin, and the southeast coastal area, are also the N
surplus hotspots. Most of these hotspots have higher per capita GDP and urbanization level, which are usually accompanied by higher synthetic fertilizer use and
higher atmospheric N deposition.
The NH 3 emission in China shared the similar spatial patterns with N input, NH 3
emission density in hotspots like the North China Plain has reached over 50 kg N
ha
À1 year
À1 in 2010s (Fig. 5.5c), while the most intense hotspot occurred in Henan
(~89 kg N ha
À1 year
À1 ) where it played a role in the enhancement of airborne PM 2.5
during haze episodes (Gu et al. 2014a; Ye et al. 2011). NO and N 2 O emissions also
have the similar emission patterns, but their emissions were less than 10% of those of
NH 3 . Generally, the spatial variability of the NH 3 emission densities agrees well
with the IASI-NH 3 VCDs distribution; however, relatively high NH 3 VCDs were
detected in Xinjiang by satellite IASI instrument, which is not captured by our
emission map. These hotspots of NH 3 emission mainly related to agriculture in
Xinjiang, which only account for a small proportion of the land area in Xinjiang, and
majority of the remaining land uses are classified as deserts or mountains. There is
little emission of SO 2 and NO x from industrial sources in Xinjiang compared to that
in Eastern China (Liu et al. 2017). Therefore, a reduced conversion rate to aerosol is
expected. This, and the dry climate, is probably responsible for a longer lifetime of
NH 3 in the atmosphere in Xinjiang compared to other regions in China. This will
5 Reactive Nitrogen Budgets in China
99