On the validity of the incremental approach to calculate the impact - - PowerPoint PPT Presentation

On the validity of the incremental approach to calculate the impact of cities on air quality Philippe Thunis Athens June 2017

• Determine at which level/scale air quality measures should be taken

to abate air pollution in the most efficient manner.

Motivations

• How can we quantify the

contribution of city emissions on its own air pollution?

• Two main approaches:

 Incremental  CTM scenarios

Urban impact &urban increment

Urban impact &urban increment

5

?

• 1. CTM-scenario
• 2. Lenschow

) CTM (

city cf city cf

B B 

 

d C B

rur city cf



Urban impact &urban increment

Urban imoact Urban increment Lenschow City spread Background deviation

     

d I d B d C I B C

rur cf rur cf rur city cf city cf city

   

 

 

d C C

rur city 



 

 

city cf rur cf

B d B  

 

d I rur

cf



city cf

I

Urban impact &urban increment

7



 

 

   

 

                

. . Impact Urb. dev Background city cf rur cf spread city rur cf Increment Urb rur city city cf

B d B d I d C C I     

Not measurable Not measurable Measurable

Assumption I: the city spread is negligible

 

 d I rur

cf

The rural background location is far enough from the city not to feel its influence

Assumption II: the background is homogeneous

The city and rural background locations should not be too far from each other

 

city cf rur cf

B d B 

Urban impact &urban increment

How do these components vary

• with distance (d)
• With city fraction (cf)
• With city: Berlin, Paris, London,

Bruxelles

• With pollutant: PM2.5 and NO2

SHERPA



 

 

   

 

                

. . Impact Urb. dev Background city cf rur cf spread city rur cf Increment Urb rur city city cf

B d B d I d C C I     

SHERPA assessment in 4 cities

12 9 2 2.4 1.1

9

12 8 3 5 3.5

City fractions FUA Urban core Inner city

SHERPA assessment in 4 cities

10

PM2.5 for cf = FUA

Background deviation Lenschow increment City spread

11

PM2.5 for cf = urban core

Background deviation Lenschow increment City spread

) (d B B

rur cf city cf



PM2.5 for cf = inner city

PM2.5 for cf = inner city

Background deviation Lenschow increment City spread

14

Summary overviews

15

Comparison of obs. and mod. Increments (PM2.5)

Conclusions

 The urban increment (LUI) is an appropriate estimate of the urban impact (I) only when two assumptions are fulfilled:

 The city spread is negligible  The background deviation is negligible

 For PM2.5, these two assumptions are never fulfilled for large or medium cities and the LUI underestimates the urban impact by 30 to 50%. Although it works better for NO2 some underestimation is also found for this pollutant.

16

Conclusions (cont.)

 Given that:

• The urban impact is very sensitive to the size of the city fraction
• The urban increment is very sensitive to distance (d) and location

the urban increment seems to be a poor proxy for estimating the urban impact.  Studies based on the incremental approach are very likely to underestimate (heavily for PM2.5) the impact of cities to their air pollution

17