et al. 2010). The GEOS-Chem model simulation for 2008–2010 shows N deposition
rates of 0.8–20 kg Nha
À1 year
À1 over the northwestern Pacific (Zhao et al. 2015).
Simulated N deposition fluxes are on average 11.9 kg N ha
À1 year
À1 to the Yellow
Sea and 5.6 kg N ha
À1 year
À1 to the South China Sea. Source attribution analyses
show that N from fertilizer use, power plants and transportation contribute, respectively, 24%, 22% and 18% of the N deposition to the Yellow Sea (Zhao et al. 2015).
4.3.3 Dry vs. Wet Deposition Contributions
Chemical transport models provide valuable information to quantify N dry deposition fluxes as direct measurements are limited. The relative contributions of dry and
wet deposition vary in different models. A CMAQ model simulation for 2010
calculated that dry deposition accounted for 62% of total N deposition in China
(Zheng et al. 2014), while the GEOS-Chem simulations averaged over 2008–2012
estimated a smaller contribution from dry deposition of 39% (Zhao et al. 2017a). A
recent study using observations from a nationwide network reported a percentage
contribution of 45% via dry deposition (Xu et al. 2015). The different model
estimates of dry deposition contribution could be due to a number of factors
including model parameterization of wet and dry deposition, chemical mechanisms
and emissions.
Figure 4.2 shows simulated percentage contributions of wet deposition to total
deposition over China using the model results of Zhao et al. (2017a). The percentage
contributions of dry and wet deposition show considerable spatial variation. Over
most areas of China, except for some parts of the North China Plain and Western
China, wet deposition is higher than dry deposition. The higher contribution of wet
deposition over southeastern China is attributed to high precipitation (Zhao et al.
2017a). Regional model studies have also shown dry/wet deposition ratios of about
0.5 over the North China Plain (FRAME model results; Zhang et al. 2011) and over
the Yangtze River Basin (GEOS-Chem model results; Xu et al. 2015) and a higher
dry/wet deposition ratio of 2 over the Pearl River Delta (CMAQ model results;
Huang et al. 2015).
Both dry and wet deposition fluxes show distinct seasonality as driven by
emissions and meteorological conditions. During summer months, higher ammonia
emissions increase both wet and dry deposition (Zhang et al. 2012; Zheng et al.
2014), while higher precipitation can enhance wet deposition but suppress dry
deposition (Huang et al. 2015). Higher temperatures in summer also enhance
oxidization of NO x and decomposition of nitrate aerosol to HNO 3 that is more
efficiently removed via dry deposition (Zheng et al. 2014). On the national scale,
both dry and wet N deposition fluxes peak in July (Zheng et al. 2014), while regional
simulations focused on the Pearl River Delta show the highest dry deposition in
September and the highest wet deposition in August (Huang et al. 2015).
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L. Zhang et al.
rates of 0.8–20 kg Nha
À1 year
À1 over the northwestern Pacific (Zhao et al. 2015).
Simulated N deposition fluxes are on average 11.9 kg N ha
À1 year
À1 to the Yellow
Sea and 5.6 kg N ha
À1 year
À1 to the South China Sea. Source attribution analyses
show that N from fertilizer use, power plants and transportation contribute, respectively, 24%, 22% and 18% of the N deposition to the Yellow Sea (Zhao et al. 2015).
4.3.3 Dry vs. Wet Deposition Contributions
Chemical transport models provide valuable information to quantify N dry deposition fluxes as direct measurements are limited. The relative contributions of dry and
wet deposition vary in different models. A CMAQ model simulation for 2010
calculated that dry deposition accounted for 62% of total N deposition in China
(Zheng et al. 2014), while the GEOS-Chem simulations averaged over 2008–2012
estimated a smaller contribution from dry deposition of 39% (Zhao et al. 2017a). A
recent study using observations from a nationwide network reported a percentage
contribution of 45% via dry deposition (Xu et al. 2015). The different model
estimates of dry deposition contribution could be due to a number of factors
including model parameterization of wet and dry deposition, chemical mechanisms
and emissions.
Figure 4.2 shows simulated percentage contributions of wet deposition to total
deposition over China using the model results of Zhao et al. (2017a). The percentage
contributions of dry and wet deposition show considerable spatial variation. Over
most areas of China, except for some parts of the North China Plain and Western
China, wet deposition is higher than dry deposition. The higher contribution of wet
deposition over southeastern China is attributed to high precipitation (Zhao et al.
2017a). Regional model studies have also shown dry/wet deposition ratios of about
0.5 over the North China Plain (FRAME model results; Zhang et al. 2011) and over
the Yangtze River Basin (GEOS-Chem model results; Xu et al. 2015) and a higher
dry/wet deposition ratio of 2 over the Pearl River Delta (CMAQ model results;
Huang et al. 2015).
Both dry and wet deposition fluxes show distinct seasonality as driven by
emissions and meteorological conditions. During summer months, higher ammonia
emissions increase both wet and dry deposition (Zhang et al. 2012; Zheng et al.
2014), while higher precipitation can enhance wet deposition but suppress dry
deposition (Huang et al. 2015). Higher temperatures in summer also enhance
oxidization of NO x and decomposition of nitrate aerosol to HNO 3 that is more
efficiently removed via dry deposition (Zheng et al. 2014). On the national scale,
both dry and wet N deposition fluxes peak in July (Zheng et al. 2014), while regional
simulations focused on the Pearl River Delta show the highest dry deposition in
September and the highest wet deposition in August (Huang et al. 2015).
78
L. Zhang et al.