M ODELLING THE D ISPERSION OF N ANOPARTICLES IN S TREET C ANYONS P RASHANT K UMAR* & A LAN R OBINS HARMO 13 : P ARIS, F RANCE 1 – 4 J UNE 2010
O UTLINE B ACKGROUND M EASUREMENTS Application of a DMS500 for street canyon measurements M ODELLING Effect of wind speed and direction on the various size ranges of nanoparticles in street canyons (i.e. testing of inverse wind speed law; cut – off wind speed)? Role of particle dynamics in street scale modelling Formulation of a simple dispersion model (a modified Box model) Comparison of measured and modelled concentrations of nanoparticles using OSPM, CFD (Fluent) and the modified Box model Uncertainties in modelling due to particle number emission factors S UMMARY AND C ONCLUSIONS A CKNOWLEDGEMENTS HARMO 13, P ARIS, F RANCE 1 – 4 J UNE 2010 2
B ACKGROUND Stringent emissions: particle mass emissions (↓), number (↑) Ultrafine particles (< 100 nm); main component of ambient particles by number, produced mainly by vehicles, contribute most to PNC but little to PMC; these are more toxic than coarse particles per unit mass Current regulations address atmospheric particulate matter as PM 10 , PM 2.5 mass concentration; not particle number concentration (PNC) Recent inclusion of in vehicle emission standards Euro – 5 and Euro – 6 on a particle number basis – ambient air quality standards for nanoparticles also likely Progress hampered by lack of standard instruments for measurements, limited understanding of nanoparticles dispersion, and scientifically validated modelling tools HARMO 13, P ARIS, F RANCE 1 – 4 J UNE 2010 3
M EASUREMENTS 1 of 3 Measurement Campaigns: Street canyon (Pembroke Street, Cambridge) Instrument: Differential Mobility Spectrometer (DMS500) Response: 10 Hz, real time continuous Sampling flow rate: 8.0 lpm at 250 mb for 5-1000 nm 2.5 lpm at 160 mb for 5-2738 nm Movie : Diesel drive by (Courtesy: Cambustion Instruments) HARMO 13, P ARIS, F RANCE 1 – 4 J UNE 2010 4
M EASUREMENTS S AMPLING S ITE 2 of 3 NW 16.60 m Wind NE SW SE Chemical Engineering Department Winds from NW Site: Pembroke Street, Cambridge, UK 66 m H 11.60 m Measurement site 2.60 m Traffic flow (down-canyon) W = 11.75 m 1.60 m Kerb 2.50 m Leeward side Windward L 167 m side 3-cup vortex anemometer Pembroke College Building (Figures not to scale) HARMO13, P ARIS, F RANCE 1 – 4 J UNE 2010 5
M EASUREMENTS A PPLICATION OF DMS500 3 of 3 Check the sensitivity level of the instrument Identify the suitable operating conditions (mainly sampling frequency) of the instrument which maximised its utility 10 5 5 1.0 Smaller (1 Hz or lower) rather than 0.1 s Av Noise (10 Hz) 1 s Av Noise (1 Hz) maximal (10 Hz) sampling frequencies 10 s Av Noise (0.1 Hz) 0.8 4 0.1 s Av Roadside background (10 Hz) found appropriate, unless experiments 0.1 s Roadside (10 Hz) relied critically upon fast response d N /dlog D p (# cm –3 ) –3 data 4 0.6 0.4 4 Suggested sampling frequencies used in later experiments (Kumar et al., 0.2 4 2008a – d, 2009a-c): 0.0 0 measured PNDs well above 1 10 100 1000 D p (nm) instrument’s noise level Sensitivity of the DMS500. Both typical roadside and reduced size of data files to background PNDs were measured at the fastest (10 manageable proportions Hz) sampling frequency. HARMO13, P ARIS, F RANCE 1 – 4 J UNE 2010 6
E FECT OF W IND D IRECTIONS & S PEED ON N 10-30 and N 30-300 1 of 7 1 of 8 Measurements taken for 17 days continuously; sampling rate 1 Hz Range considered: N 10-30 (nucleation) and N 30-300 (accumulation) Measurements at 1.6 m with intention that effect of TPT’s can be observed Objective was to test inverse-wind speed law on N 10-30 and N 30-300 ; important information for nanoparticle dispersion models U r,crit is critical cut-off wind speed which divides zone of traffic-dependent and wind-dependent concentrations 1.2 m s -1 for gaseous pollutants (DePaul and U r,crit is generally considered as Sheih, 1986). What about U r,crit for N 10-30 , N 30-300 and overall N 10-300 during various wind directions and speed? HARMO13, P ARIS, F RANCE 1 – 4 J UNE 2010 7
E FECT OF W IND D IRECTIONS & S PEED ON N 10-30 and N 30-300 2 of 7 Two limiting cases for PNCs dilution N i-j = a T m U r -n + C b, i-j Traffic dependent PNCs case (during smaller U r ; n=0 & m=1 ) Wind dependent PNCs case (during larger U r ; n=1 & m=1 ) A model with two distinct regimes , reflecting the role of both TPT and WPT , was proposed and applied to the measured data: N N i j i j For U r << U r , crit ( n = 0; m = 1) m m T T crit Traffic dependent regime: Normalised PNCs are independent of U r up to U r,crit N N i j i j n U U For U r >> U r , crit ( n = 1; m = 1) r , crit r m m T T crit U r dependent regime: Norm PNCs are inversely dependent of U r after U r,crit HARMO13, P ARIS, F RANCE 1 – 4 J UNE 2010 8
E FECT OF W IND D IRECTIONS & S PEED ON N 10-30 and N 30-300 3 of 7 Wind directions during the measurements were: Cross Canyon (NW and SE) Along canyon (SW and NE) Period covered smaller and larger U r ’ s Direction of traffic Direction of traffic Direction of traffic Direction of traffic N (no winds) N (no winds) N (no winds) N (no winds) NE (3 %) NE (3 %) NE (3 %) NE (3 %) NW (16 %) NW (16 %) NW (16 %) NW (16 %) 40% 32% 24% 16% 8% E (no winds) E (no winds) E (no winds) E (no winds) W (16 %) W (16 %) W (16 %) W (16 %) WIND SPEED (m s -1) WIND SPEED (m s -1) WIND SPEED (m s -1) ≥ 6.5 ≥ 6.5 ≥ 6.5 5.5-6.5 5.5-6.5 5.5-6.5 4.5-5.5 4.5-5.5 4.5-5.5 3.5-4.5 3.5-4.5 3.5-4.5 SE (5 %) SE (5 %) SE (5 %) SE (5 %) 2.5-3.5 2.5-3.5 2.5-3.5 SW (38 %) SW (38 %) SW (38 %) SW (38 %) 1.2-2.5 1.2-2.5 1.2-2.5 ≤ 1.2 ≤ 1.2 ≤ 1.2 Street canyon orientation Street canyon orientation Street canyon orientation Street canyon orientation S (23 %) S (23 %) S (23 %) S (23 %) HARMO13, P ARIS, F RANCE 1 – 4 J UNE 2010 9
E FECT OF W IND D IRECTIONS & S PEED ON N 10-30 and N 30-300 4 of 7 1000 1000 1000 b c a -1 ) -3 )/(veh per h/2 -1 ) 100 100 100 -3 )/(veh h/2 10 10 10 NW Ni-j/ T (# cm N 10-300 / T (# cm 1 1 1 fit results (N 10-300 ) : fit results (N 10-30 ) : fit results (N 30-300 ) : n = 0 and 1 n = 0 and 1 n = 0 and 1 n = 0.64 n = 0.4 n = 0.98 0 0 0 0.1 1 10 0.1 1 10 0.1 1 10 U r (m s -1 ) 1000 1000 1000 c a b -1 ) -3 )/(veh h/2 -1 ) 100 100 100 -3 )/(veh h/2 SE N 10-300 / T (# cm 10 10 10 Ni-j/ T (# cm 1 1 1 fit results (N 10-300 ) : fit results (N 10-30 ) : fit results (N 30-300 ) : n = 0 and 1 n = 0 and 1 n = 0 and 1 n = 0.85 n = 1.04 n = 0.99 0 0 0 0.1 1 10 0.1 1 10 0.1 1 10 0 1 10 U r (m s -1 ) Norm PNCs against U r (logarithmic plots) for cross-canyon wind direction Different best-fit model tried; proposed model fitted data best which split data into wind-independent ( n =0) and wind-dependent ( n =1) regions. Minimising the diff. between model and experimental results yielded U r,crit HARMO13, P ARIS, F RANCE 1 – 4 J UNE 2010 10
E FECT OF W IND D IRECTIONS & S PEED ON N 10-30 and N 30-300 5 of 7 1000 1000 1000 a b c -1 ) -3 )/(veh h/2 -1 ) 100 100 100 -3 )/(veh h/2 N 10-300 / T (# cm 10 10 10 Ni-j/ T (# cm NE 1 1 1 fit results (N 10-300 ) : fit results (N 10-30 ) : fit results (N 30-300 ) : n = 0 and 1 n = 0 and 1 n = 0 and 1 0 0 0 0.1 1 10 0.1 1 10 0.1 1 10 U r (m s -1 ) 1000 1000 1000 b c a -1 ) N 10-300 / T (# cm -3 )/(veh per h/2 -1 ) 100 100 100 N i-j / T (# cm -3 )/(veh h/2 SE 10 10 10 1 1 1 fit results (N 10-300 ) : fit results (N 10-30 ) : fit results (N 30-300 ) : n = 0 and 1 n = 0 and 1 n = 0 and 1 n = 1.15 n = 1.35 n = 0.69 0 0 0 0 1 10 0.1 1 10 0.1 1 10 0.1 1 10 U r (m s -1 ) Along-Canyon winds Till U r,cri t - dilution independent of U r ; here TPT governs dilution After U r,crit - dilution independent of T ; here WPT governs dilution HARMO13, P ARIS, F RANCE 1 – 4 J UNE 2010 11
E FECT OF W IND D IRECTIONS & S PEED ON N 10-30 and N 30-300 6 of 7 1000 1000 1000 c b a -1 ) -1 ) N 10-300 / T (# cm -3 )/(veh h/2 100 100 100 Ni-j/ T (# cm -3 )/(veh h/2 S 10 10 10 1 1 1 fit results (N 10-30 ) : fit results (N 30-300 ) : fit results (N 10-300 ) : n = 0 and 1 n = 0 and 1 n = 0 and 1 n = 0.92 n = 1.19 n = 1.10 0 0 0 0 1 10 0.1 1 10 0.1 1 10 0.1 1 10 U r (m s -1 ) 1000 1000 1000 c b a -1 ) -1 ) N 10-300 / T (# cm -3 )/(veh h/2 100 100 100 Ni-j/ T (# cm -3 )/(veh h/2 W 10 10 10 1 1 1 fit results (N 10-300 ) : fit results (N 10-30 ) : fit results (N 30-300 ) : n = 0 and 1 n = 0 and 1 n = 0 and 1 n = 1.03 n = 1.27 n = 1.19 0 0 0 0.1 1 10 0.1 1 10 0.1 1 10 0 1 10 0 1 10 U r (m s -1 ) HARMO13, P ARIS, F RANCE 1 – 4 J UNE 2010 12
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