doubled during 1988–1997, coupled with the decrease in pH values (Duan et al.
2011). The long-term increasing trend of SO 4
2À concentrations in the middle and
lower reaches of Yangtze River (Duan et al. 2011), as well as the forest streams in
Chongqing (Yu et al. 2017c) before 2007, coincided with the increase trend of SO 2
emission in that area. The acidification of surface waters may show distinct trends in
different regions. An analysis of 73 large water bodies across China showed
significantly decreasing trends in pH at 31 sites (mainly located in Haihe River,
Taihu Lake, and Yangtze River), significantly increasing trends at 22 sites (mainly
located in Songhua River and Pearl River), and no significant trend at the other
20 sites from 2004 to 2014 (Table 8.1; Qiao et al. 2016).
Although the decrease of surface water pH due to acid deposition had been
reported in some regions, most researches documented that surface waters in
China were generally insensitive to acid deposition (Duan et al. 2000), with high
pH and ANC (Yu et al. 2016b, 2017b; Qiao et al. 2016). Recently, the large water
bodies (e.g., rivers, lakes, and reservoirs) were neither acidic nor with significant
decline in pH under the elevated acid deposition, due to strong buffering capacity
(Duan et al. 2000, 2011; Hao et al. 2001; Ye et al. 2002). Monitoring results showed
that the pH of 73 large water bodies (including rivers, lakes, and reservoirs) ranged
from 6.5 to 9.0 in the period of 2004–2014 (Qiao et al. 2016; Fig. 8.5), which were
larger than the criteria for water acidification (pH <6.0; Henriksen et al. 1992). Longterm data of eutrophic Taihu Lake also showed high pH (7.2–8.5) and ANC
(1.1–2.2) during the period of 1950s–2010s (Yu et al. 2016b). Although small
forested streams are proved more sensitive to acid deposition than large water bodies
(Duan et al. 2000; Fig. 8.5), there were no significant decline in neither pH nor ANC
of streams (Duan et al. 2011; Yu et al. 2017b, c). According to the surveys of
255 headwater streams in the forest over China during 2010–2015, no regional
acidification of small forested streams occurred, because of the lower deposition in
the sensitive northeastern China, the strong buffering capacity coping with high
deposition in southwestern China, and both relatively low deposition and insensitivity to acidification in southeastern China (Yu et al. 2017b). Almost all the stream
waters had high pH values (>6.0, Fig. 8.5; Yu et al. 2017b). By contrast, a lot of
surface waters had lower pH values (<4.8) in the 1970s in North America and
Europe (Newell and Skjelkvale 1997). Even with the recovery for about two
decades, most of the waters still had the pH values less than 5.0 (Newell and
Skjelkvale 1997) and ANC values lower than 0.05 meq L
À1 . However, high pH
(>6.0) and ANC (>0.02 meq L
À1 ) occurred for almost all of the lakes, rivers, and
streams surveyed in China, implying that there is no widely acidification of surface
water both at present and in the future.
Due to the decline of SO 2 emissions, especially in eastern China, a subsequent
stop to the SO 4
2À concentration increase and pH decrease occurred (e.g., in the upper
reach of Yangtze River in the 2000s; Duan et al. 2011). However, the increase in
NO 3
À leaching would delay the recovery of surface water from acidification, which
was observed for the forested stream in Chongqing province after 2006, when the S
deposition began to decline (Yu et al. 2017c). An increasing contribution of leached
N to the surface water acidification (Davis 2014; Fang et al. 2011; Mnich and
8 Contribution of Atmospheric Reactive Nitrogen to Acid Deposition in China
163
2011). The long-term increasing trend of SO 4
2À concentrations in the middle and
lower reaches of Yangtze River (Duan et al. 2011), as well as the forest streams in
Chongqing (Yu et al. 2017c) before 2007, coincided with the increase trend of SO 2
emission in that area. The acidification of surface waters may show distinct trends in
different regions. An analysis of 73 large water bodies across China showed
significantly decreasing trends in pH at 31 sites (mainly located in Haihe River,
Taihu Lake, and Yangtze River), significantly increasing trends at 22 sites (mainly
located in Songhua River and Pearl River), and no significant trend at the other
20 sites from 2004 to 2014 (Table 8.1; Qiao et al. 2016).
Although the decrease of surface water pH due to acid deposition had been
reported in some regions, most researches documented that surface waters in
China were generally insensitive to acid deposition (Duan et al. 2000), with high
pH and ANC (Yu et al. 2016b, 2017b; Qiao et al. 2016). Recently, the large water
bodies (e.g., rivers, lakes, and reservoirs) were neither acidic nor with significant
decline in pH under the elevated acid deposition, due to strong buffering capacity
(Duan et al. 2000, 2011; Hao et al. 2001; Ye et al. 2002). Monitoring results showed
that the pH of 73 large water bodies (including rivers, lakes, and reservoirs) ranged
from 6.5 to 9.0 in the period of 2004–2014 (Qiao et al. 2016; Fig. 8.5), which were
larger than the criteria for water acidification (pH <6.0; Henriksen et al. 1992). Longterm data of eutrophic Taihu Lake also showed high pH (7.2–8.5) and ANC
(1.1–2.2) during the period of 1950s–2010s (Yu et al. 2016b). Although small
forested streams are proved more sensitive to acid deposition than large water bodies
(Duan et al. 2000; Fig. 8.5), there were no significant decline in neither pH nor ANC
of streams (Duan et al. 2011; Yu et al. 2017b, c). According to the surveys of
255 headwater streams in the forest over China during 2010–2015, no regional
acidification of small forested streams occurred, because of the lower deposition in
the sensitive northeastern China, the strong buffering capacity coping with high
deposition in southwestern China, and both relatively low deposition and insensitivity to acidification in southeastern China (Yu et al. 2017b). Almost all the stream
waters had high pH values (>6.0, Fig. 8.5; Yu et al. 2017b). By contrast, a lot of
surface waters had lower pH values (<4.8) in the 1970s in North America and
Europe (Newell and Skjelkvale 1997). Even with the recovery for about two
decades, most of the waters still had the pH values less than 5.0 (Newell and
Skjelkvale 1997) and ANC values lower than 0.05 meq L
À1 . However, high pH
(>6.0) and ANC (>0.02 meq L
À1 ) occurred for almost all of the lakes, rivers, and
streams surveyed in China, implying that there is no widely acidification of surface
water both at present and in the future.
Due to the decline of SO 2 emissions, especially in eastern China, a subsequent
stop to the SO 4
2À concentration increase and pH decrease occurred (e.g., in the upper
reach of Yangtze River in the 2000s; Duan et al. 2011). However, the increase in
NO 3
À leaching would delay the recovery of surface water from acidification, which
was observed for the forested stream in Chongqing province after 2006, when the S
deposition began to decline (Yu et al. 2017c). An increasing contribution of leached
N to the surface water acidification (Davis 2014; Fang et al. 2011; Mnich and
8 Contribution of Atmospheric Reactive Nitrogen to Acid Deposition in China
163