Traffic NO2 formula (Traffic NO2 Overlay)

The calculations follow the standard rekenmethode 1 (SRM 1).

$Cjm[NO_{2}]=Ca,jm[NO_{2}]+Cb,jm[NO_{2}]$

where:

$Cjm[NO_{2}]$ = annual average concentration [μg/m3],

$Ca,jm[NO_{2}]$ = annual average background-concentration (default = 0 [μg/m3], can be adjusted via the overlay attributes),

$Cb,jm[NO_{2}]$ = annual average traffic contribution to NO2 concentration [μg/m3].

Contribution NO2

To compute the annual average traffic contribution to NO2, $Cb,jm[NO_{2}]$ [μg/m3], the following formula from SRM1 is applied:

$Cb,jm[NO_{2}]=fNO_{2}\cdot Cb,jm[NO_{x}]+{\frac {B\cdot Ca,jm[O_{3}]\cdot Cb,jm[NO_{x}]\cdot (1-fNO_{2})}{Cb,jm[NO_{x}]\cdot (1-Fno2)+K}}$

where:

$Cb,jm[NO_{x}]$ = annual average traffic contribution to NOx concentration [μg/m3],

$Cb,jm[O_{3}]$ = annual average concentration ozon [μg/m3] (default = 42),

$fNO_{2}$ = weighted fraction direct emitted NO2 [-],

$B$ = parameter (default 0.6),

$K$ = parameter for conversion NO to NO2 (default = 100 μg/m3).

Contribution NOX

To compute the annual average traffic contribution to NOX, Cb,jm[NOX] [μg/m3], the following formula from SRM1 is applied:

$Cb,jm[NO_{x}]=Fk\cdot ENO_{x}\cdot \theta \cdot Fb\cdot Fmeteo$

where:

$Fk$ = calibration factor [-] (default = 0.62]

$ENO_{x}$ = emission number NOx [μg/m/s]

$\theta$ = dilution factor [-]

$Fb$ = tree factor (default = 1)

$Fmeteo$ = 5/windspeed. Default value for windspeed is 5 m/s

The computation of the dilution factor, $\theta$, is split into two situations.

For surrounding cells within a distance of 30m:

$\theta =a\cdot S^{2}+b\cdot S+c$

And for cells within a distance of 30-60m (for road-type 1 and 4 only):

$\theta =\alpha \cdot S^{p}$

where:

$S$ = the calculation distance

$a$ = configurable using model attribute: ROAD1_30M_A

$b$ = configurable using model attribute: ROAD1_30M_B

$c$ = configurable using model attribute: ROAD1_30M_C

$\alpha$ = configurable using model attribute: ROAD1_60M_ALPHA

$p$ = configurable using model attribute: ROAD1_60M_POWER

The weighted fraction of direct emitted NO2 [-] is computed as:

$fNO_{2}={\frac {ENO_{2}}{ENO_{x}}}$

where:

$ENO_{2}$ = emission number NOs [μg/m/s]

Tree Factor

The tree factor is calculated as followed:

$Fb=1+{\frac {t}{2\cdot A_{t}}}$

$A_{t}=\pi \cdot {\frac {d^{2}}{A_{c}}}$

where:

$Fb$ = Tree factor, value between 1 and 1.5,

$t$ = Amount of tree cells within the search distance,

$A_{t}$ = Search area measured in amount of cells,

$d$ = Search distance of trees, currently set on 30 meters,

$A_{c}$ = Grid cell area in square meters.

Emission numbers

The emission numbers (E) for both NOx and NO2 are computed as:

$E=N\cdot [(1-FS)\cdot ((1-(fm+fz+fb))\cdot El+fm\cdot Em+fz\cdot Ez+fb\cdot Eb)+FS\cdot ((1-(fm+fz+fb))\cdot El,d+Fm\cdot Em,d+fz\cdot Ez,d+fb*Eb,d)]\cdot {\frac {1000}{(24\cdot 3600)}}$

where:

• N = traffic intensity [units per day]
• fm = fraction vans [-]
• fz = fraction trucks [-]
• fb = fraction buses [-]
• El = emission value cars [g/km]
• Em = emission value vans [g/km]
• Ez = emission value trucks [g/km]
• Eb = emission value buses [g/km]
• FS = fraction congested traffic [-]
• E*,d = emission factor per class (cars, vans, trucks or buses) if congested

Notes

• If the traffic amount is defined as a single value, that value is used for all defined hours. However, if the amount if an array of values, each of the 24 indexes of that array are assumed to correspond to the hour denoted by those indexes, and as such that specific value from the array is used.
• The SRM1 calculation method is only applicable for urban environments.

See also

• Traffic NO2 Overlay

References