20 times. For individual amines, more than 80% of them accumulated in the size
fraction of <1.5 μm. There are mainly two pathways for gaseous amines partitioned
into particles, including direct dissolution and neutralization reactions (Ge et al.
2011). The neutralization reactions mainly related to reactions of amines with HNO 3
and H 2 SO 4 or displacement of NH 4
+ in NH 4 NO 3 and (NH 4 ) 2 SO 4 to form aminium
nitrate and aminium sulfate (Qiu and Zhang 2013). The fine particles usually have
larger surface area, which is favorable of amines uptake and chemical reactions.
During haze episodes the size distribution peak of NO 3
À and NH 4
+ shifted from
fine mode to a larger particle size, where some organic compounds accumulated
(Sun et al. 2013; Tian et al. 2014). This shifting phenomenon was mainly caused by
heterogeneous reaction in droplet mode and hygroscopic growth of submicron
particles. Since haze pollution is always associated with high RH, the aqueous
phase on the particle surface is beneficial for the heterogeneous gas-liquid conversion of gaseous precursors to form SNA in droplet mode (Wang et al. 2012). As a
consequence, the content of SNA in 0.65–1.1 μm showed an increase in winter
compared to summer.
The size distributions of nitrogen compounds also greatly affect their contribution
to light extinction. The contributions of chemical species in PM 1 and PM 1–2.5 to the
extinction coefficient (b ext ) in Guangzhou in different seasons and pollution levels
were studied (Xia et al. 2017). On clean days, (NH 4 ) 2 SO 4 in PM 1 and PM 1–2.5
accounted for 19.8% and 3.0% of b ext in summer and 13.4% and 5.3% in winter. On
polluted days in summer, the contribution of (NH 4 ) 2 SO 4 in PM 1 to b ext increased to
27.9% but decreased to 1.6% in PM 1–2.5 . Clearly, (NH 4 ) 2 SO 4 plays a critical role in
light extinction during haze events.
6.4 Mitigation of Nr for PM 2.5 Pollution
6.4.1 Long-Term Trends of PM 2.5 , NO x , and NH 3 Over China
as Seen from Satellite and Surface Observations
Several studies have investigated the long-term trend of PM 2.5 over China. For
example, Han et al. (2016) estimated the PM 2.5 concentration in Beijing using
visibility data from 1973 to 2013. They found that under stable meteorological
conditions (wind speed is less than 4 m/s and stays stable), the PM 2.5 concentration
increased significantly in the 40 years from 1973 to 2013, indicating that meteorological conditions were not the main cause of the PM 2.5 increase over these years.
Seasonally, under stable meteorological conditions, the increase in the PM 2.5 concentration was largest in summer, whereas in winter, there was no significant trend
from 1973 to 2013. This result reveals that meteorological conditions may have large
impacts on PM 2.5 in the winter season, and in summer, PM 2.5 is affected mostly by
emissions of pollutants. They also found a high positive correlation between PM 2.5
and some human activity indicators such as urbanization, GDP (gross domestic
6 Contribution of Atmospheric Reactive Nitrogen to Haze Pollution in China
121
fraction of <1.5 μm. There are mainly two pathways for gaseous amines partitioned
into particles, including direct dissolution and neutralization reactions (Ge et al.
2011). The neutralization reactions mainly related to reactions of amines with HNO 3
and H 2 SO 4 or displacement of NH 4
+ in NH 4 NO 3 and (NH 4 ) 2 SO 4 to form aminium
nitrate and aminium sulfate (Qiu and Zhang 2013). The fine particles usually have
larger surface area, which is favorable of amines uptake and chemical reactions.
During haze episodes the size distribution peak of NO 3
À and NH 4
+ shifted from
fine mode to a larger particle size, where some organic compounds accumulated
(Sun et al. 2013; Tian et al. 2014). This shifting phenomenon was mainly caused by
heterogeneous reaction in droplet mode and hygroscopic growth of submicron
particles. Since haze pollution is always associated with high RH, the aqueous
phase on the particle surface is beneficial for the heterogeneous gas-liquid conversion of gaseous precursors to form SNA in droplet mode (Wang et al. 2012). As a
consequence, the content of SNA in 0.65–1.1 μm showed an increase in winter
compared to summer.
The size distributions of nitrogen compounds also greatly affect their contribution
to light extinction. The contributions of chemical species in PM 1 and PM 1–2.5 to the
extinction coefficient (b ext ) in Guangzhou in different seasons and pollution levels
were studied (Xia et al. 2017). On clean days, (NH 4 ) 2 SO 4 in PM 1 and PM 1–2.5
accounted for 19.8% and 3.0% of b ext in summer and 13.4% and 5.3% in winter. On
polluted days in summer, the contribution of (NH 4 ) 2 SO 4 in PM 1 to b ext increased to
27.9% but decreased to 1.6% in PM 1–2.5 . Clearly, (NH 4 ) 2 SO 4 plays a critical role in
light extinction during haze events.
6.4 Mitigation of Nr for PM 2.5 Pollution
6.4.1 Long-Term Trends of PM 2.5 , NO x , and NH 3 Over China
as Seen from Satellite and Surface Observations
Several studies have investigated the long-term trend of PM 2.5 over China. For
example, Han et al. (2016) estimated the PM 2.5 concentration in Beijing using
visibility data from 1973 to 2013. They found that under stable meteorological
conditions (wind speed is less than 4 m/s and stays stable), the PM 2.5 concentration
increased significantly in the 40 years from 1973 to 2013, indicating that meteorological conditions were not the main cause of the PM 2.5 increase over these years.
Seasonally, under stable meteorological conditions, the increase in the PM 2.5 concentration was largest in summer, whereas in winter, there was no significant trend
from 1973 to 2013. This result reveals that meteorological conditions may have large
impacts on PM 2.5 in the winter season, and in summer, PM 2.5 is affected mostly by
emissions of pollutants. They also found a high positive correlation between PM 2.5
and some human activity indicators such as urbanization, GDP (gross domestic
6 Contribution of Atmospheric Reactive Nitrogen to Haze Pollution in China
121