3.2 Measurement Methods of N Deposition
3.2.1 Measurement of Dry Deposition
Nitrogen dry deposition is the transfer to the landscape of gaseous and particulate N r
species through a number of atmospheric processes in the absence of precipitation.
Dry deposition contributes a large part to the total N deposition (Flechard et al. 2011;
Pan et al. 2012; Shen et al. 2013; Xu et al. 2015; Kuang et al. 2016) and should not
be neglected in N deposition monitoring. Due to their relative high concentrations in
the atmosphere and high chemical and biological activity, NH 3 , NO 2 , HNO 3 and
particulate NH 4
+ and NO 3
À are the main N r species studied in monitoring the dry
deposition of N. It can be measured directly using micrometeorological methods and
also using the inferential method. For micrometeorological methods, a uniform and
even underlying surface with enough area is needed to form a constant flux layer
(Fowler et al. 2001). The height of the constant flux layer and fetch (the horizontal
distance of the even surface in the upwind direction to the measuring tower) usually
has a ratio of 1:100.
3.2.1.1 Monitoring Dry Deposition Using Micrometeorological Methods
Micrometeorological methods for dry deposition measurement include gradient,
eddy covariance, relaxed eddy accumulation and time-averaged gradient. For the
gradient method, the dry deposition flux (F) is calculated as the production of eddy
diffusivity (K ) and the vertical concentration gradient (∂c/∂z) using the following
equation:
F ¼ K ∙
∂c
∂z
ð3:1Þ
where c is the atmospheric concentration of reactive N species and z is the measuring
height. Eddy diffusivity can be measured using the aerodynamic method (Fowler
et al. 2001) and the Bowen-ratio method (Monteith and Unsworth 1990). For the
eddy covariance method, the dry deposition flux is calculated by measuring the
vertical wind velocity fluctuation and concentration fluctuation of reactive N species.
The flux equation is
F ¼ w 0 ∙ c 0
ð3:2Þ
where w
0 is the instantaneous variation of vertical wind velocity and c’ is the
instantaneous concentration variation of N r species. Eddy covariance needs fast
response instruments to measure concentration and wind velocity at a time scale of
less than 0.1 s (Fowler et al. 2001). For this, the wind velocity is measured using
ultrasonic anemometers, and the concentrations of reactive N species using tunable
3 Monitoring Atmospheric Nitrogen Deposition in China
43
3.2.1 Measurement of Dry Deposition
Nitrogen dry deposition is the transfer to the landscape of gaseous and particulate N r
species through a number of atmospheric processes in the absence of precipitation.
Dry deposition contributes a large part to the total N deposition (Flechard et al. 2011;
Pan et al. 2012; Shen et al. 2013; Xu et al. 2015; Kuang et al. 2016) and should not
be neglected in N deposition monitoring. Due to their relative high concentrations in
the atmosphere and high chemical and biological activity, NH 3 , NO 2 , HNO 3 and
particulate NH 4
+ and NO 3
À are the main N r species studied in monitoring the dry
deposition of N. It can be measured directly using micrometeorological methods and
also using the inferential method. For micrometeorological methods, a uniform and
even underlying surface with enough area is needed to form a constant flux layer
(Fowler et al. 2001). The height of the constant flux layer and fetch (the horizontal
distance of the even surface in the upwind direction to the measuring tower) usually
has a ratio of 1:100.
3.2.1.1 Monitoring Dry Deposition Using Micrometeorological Methods
Micrometeorological methods for dry deposition measurement include gradient,
eddy covariance, relaxed eddy accumulation and time-averaged gradient. For the
gradient method, the dry deposition flux (F) is calculated as the production of eddy
diffusivity (K ) and the vertical concentration gradient (∂c/∂z) using the following
equation:
F ¼ K ∙
∂c
∂z
ð3:1Þ
where c is the atmospheric concentration of reactive N species and z is the measuring
height. Eddy diffusivity can be measured using the aerodynamic method (Fowler
et al. 2001) and the Bowen-ratio method (Monteith and Unsworth 1990). For the
eddy covariance method, the dry deposition flux is calculated by measuring the
vertical wind velocity fluctuation and concentration fluctuation of reactive N species.
The flux equation is
F ¼ w 0 ∙ c 0
ð3:2Þ
where w
0 is the instantaneous variation of vertical wind velocity and c’ is the
instantaneous concentration variation of N r species. Eddy covariance needs fast
response instruments to measure concentration and wind velocity at a time scale of
less than 0.1 s (Fowler et al. 2001). For this, the wind velocity is measured using
ultrasonic anemometers, and the concentrations of reactive N species using tunable
3 Monitoring Atmospheric Nitrogen Deposition in China
43