F AST R ESPONSE M EASUREMENTS FOR THE D ISPERSION OF N ANOPARTICLES IN V EHICLE W AKE AND S TREET C ANYON 89 TH AMS M EETING, P HOENIX, J ANUARY 09 P RASHANT K UMAR A LAN R OBINS R EX B RITTER

P OINTS FOR D ISCUSSION  B ACKGROUND  M EASUREMENTS  Application of a DMS500 for ambient measurements  Street canyon measurements  Vehicle wake measurements  Hypothesis  S UMMARY AND C ONCLUSIONS  A CKNOWLEDGEMENTS P RASHANT K UMAR 89 TH AMS M EETING, P HOENIX, J ANUARY 09 2

B ACKGROUND  Stringent emissions: particle mass emissions (↓), number (↑)  Current regulations address atmospheric particulate matter as PM 10, PM 2.5 mass concentration; not number concentration (PNC)  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  Progress hampered by lack of proven methods and instrumentation to measure PNCs 1 of 1  This work addresses:  application of a fast response DMS500, its suitability and best operating conditions for the measurements of PNDs in operational (vehicle wake) and controlled (street canyons) conditions  to determine the time scale over which competing influences of transformation and dilution processes affect dispersion of nanoparticles  to discuss the importance of particle dynamics (hypothesis) during ambient measurements and modelling of nanoparticles P RASHANT K UMAR 89 TH AMS M EETING, P HOENIX, J ANUARY 09 3

M EASUREMENTS  Measurement Campaigns:  Street canyon (Pembroke Street)  Vehicle wake (Chemical Engineering Department)  Instrument: Differential Mobility Spectrometer (DMS500)  Response: 10 Hz, real time continuous  Sampling flow rate: 8.0 lpm at 250 mb for nm 2.5 lpm at 160 mb for nm  Movies : 2stroke-idle, diesel drive by2stroke-idlediesel drive by 1 of 10 P RASHANT K UMAR 89 TH AMS M EETING, P HOENIX, J ANUARY 09 4

A PPLICATION OF DMS500  Check the sensitivity level of the instrument  Identify the suitable operating conditions (mainly sampling frequency) of the instrument which maximised its utility 2 of 10 P RASHANT K UMAR 89 TH AMS M EETING, P HOENIX, J ANUARY 09 5 M EASUREMENTS  10 5 Sensitivity of the DMS500. Both typical roadside and background PNDs were measured at the fastest (10 Hz) sampling frequency.  Smaller (1 Hz or lower) rather than maximal (10 Hz) sampling frequencies found appropriate, unless experiments relied critically upon fast response data  Suggested sampling frequencies used in later experiments (Kumar et al., 2008a–e):  measured PNDs well above instrument’s noise level  reduced size of data files to manageable proportions

S TREET C ANYON 3 of 10 Site 1: Pembroke Street Kerb Winds from NW 1.60 m Traffic flow (down-canyon) W = m 66 m Chemical Engineering Department Measurement site H  m 2.60 m 2.50 m (Figures not to scale) 3-cup vortex anemometer Leeward side Windward side Pembroke College Building L  167 m NW NE SE SW Wind m P RASHANT K UMAR 89 TH AMS M EETING, P HOENIX, J ANUARY 09 6 M EASUREMENTS  Among several objectives (Kumar et al. 2008a-e, 2009), the goal of present work is to measure the lapse time (i.e. time between vehicular emissions and measurements at roadside).

S TREET C ANYON 4 of 10 P RASHANT K UMAR 89 TH AMS M EETING, P HOENIX, J ANUARY 09 7 Sample measurements showing lapse time at Pembroke Street. Winds were calm (  1.5 m s –1 ) during measurements.  Sample measurements: ~ 50 cars and vans and ~ 50 other vehicles (buses, trucks, LDVs). Average speed of vehicle  30±7 km h –1.  Average lapse time  45±6. M EASUREMENTS

V EHICLE W AKE  How does PNDs evolve in the vehicle wake?  Are the transformation processes important during street-scale measurements? 5 of 10 P RASHANT K UMAR 89 TH AMS M EETING, P HOENIX, J ANUARY 09 8 Pembroke College Chemical Engineering Road NMS Buildings Pembroke Street m 10 m 15 m 5 m Sampling point NW SE SWNE Vehicles (Figures not to scale) Site 2: Schematic diagram of sampling site showing sampling position. M EASUREMENTS

V EHICLE W AKE 6 of 10 dN/dlogD p (# cm -3 ) dM/dlogD p (μgm cm -3 ) Site background t=–0.7s t=–0.6s t=–0.5s t=–0.4s t=–0.3s t=– 0.2s t=–0.1s t=0.0s t=0.1s t=0.2s t=0.3s t=0.4s t=0.5s t=0.6s t=0.7s First evidence of exhaust emissionsClear bi-modal distribution Peak (number and mass) Evolution starts D p (nm) P RASHANT K UMAR 89 TH AMS M EETING, P HOENIX, J ANUARY 09 9 M EASUREMENTS

V ehicle W ake 7 of 10 dN/dlogD p (# cm -3 ) dM/dlogD p (μgm cm -3 ) D p (nm) Similar to site background Rapid Evolution t=0.8s t=0.9s t=1.0s t=1.1s t=1.2s t=1.3s t=1.4s t=1.5s t=1.6s t=1.7s Similar to site background  –0.7 s to 0.0 s (background PNC) – time to reach to DMS500  Evolution to reach to background takes  1.0 s P RASHANT K UMAR 89 TH AMS M EETING, P HOENIX, J ANUARY M EASUREMENTS

 10 – 5 Post–evolutionPre–evolution Evolution  10 5 Post–evolutionPre–evolution Evolution V EHICLE W AKE 8 of 10  Time to evolve PNDs in vehicle wake is far lesser (  1 s)  than time to measure PNDs at roadside (  45 ± 6 s)  Dilution is quick enough, and  effect of transformation processes is generally over by the time measurements made at road side. See Kumar et al. (2008d) for details P RASHANT K UMAR 89 TH AMS M EETING, P HOENIX, J ANUARY M EASUREMENTS

V EHICLE W AKE 9 of 10 P RASHANT K UMAR 89 TH AMS M EETING, P HOENIX, J ANUARY M EASUREMENTS Pre-evolutionEvolutionPost-evolution DistributionsIdenticalRapid changeIdentical Nucleation15.4 nm nm*15.4 nm Accumulation~87 nm † MassNegligible below 30 nm; small amount of mass between 30 and 300 nm Time~0.7 s~ 1.0 s~ 10 s (street ventilation) Temporal change in peak diameters of 0.1 s averaged PNDs for both nucleation and accumulation modes. *Shift in peak mode diameters shows influence of transformation processes since dilution should change the PNDs proportionally and should not change the peak mode diameters. † Nearly unchanged peak mode diameters in accumulation mode ( nm) suggested that dilution is the most dominant process to reduce the PNDs.

H YPOTHESIS 10 of 10 See Kumar et al. (2008b, c and d) for detailed testing of hypothesis P RASHANT K UMAR 89 TH AMS M EETING, P HOENIX, J ANUARY “The effect of transformation processes on the particles is nearly over by the time these particles are measured at roadside and total particle numbers are then assumed to be conserved”. M EASUREMENTS

S UMMARY AND C ONCLUSIONS  An advanced particle spectrometer was successfully applied to measure PNDs and PNCs in street canyons and in vehicle wake where fast response nature of an instrument is essential.  Vehicle wake measurements showed that the PNDs evolved rapidly in the wake of a moving diesel car and became similar to background PNDs within  1 s.  This evolution was significantly smaller than the typical time (  45  6 s) for traffic emissions to reach the roadside in a street canyon.  Comparison of these time scales suggested a hypothesis that the effect of transformation processes is generally complete by the time particles are measured at roadside and total particle numbers can then be assumed to be conserved.  The hypothesis allows to ignore the particle dynamics during street canyon measurements and found to be useful for the modelling of the dispersion of nanoparticles in street canyons (Kumar et al. 2008, 2009). 1 of 1 P RASHANT K UMAR 89 TH AMS M EETING, P HOENIX, J ANUARY 09 14

R ELATED A RTICLES FOR D ETIALED I NFORMATION 1 of 1 J OURNAL Kumar, P., Garmory, A., Ketzel, M., Berkowicz, R., Comparative study of measured and modelled number concentration of nanoparticles in an urban street canyon. Atmospheric Environment 43, Kumar, P., Fennell, P., Symonds, J., Britter, R., 2008e. Treatment for the losses of ultrafine aerosol particles in long sampling tubes during ambient measurements. Atmospheric Environment 42, Kumar, P., Fennell, P., Hayhurst, A., Britter, R., 2008d. Street versus rooftop level concentrations of fine particles in a Cambridge Street Canyon. Boundary–Layer Meteorology (in press, doi: /s ). Kumar, P., Fennell, P., Britter, R., 2008c. Effect of wind direction and speed of the dispersion of nucleation and accumulation mode particles in an urban street canyon. Science of the Total Environment 402, Kumar, P., Fennell, P., Britter, R., 2008b. Pseudo-simultaneous measurements for the vertical variation of coarse, fine and ultrafine particles in an urban street canyon. Atmospheric Environment 42, Kumar, P., Fennell, P., Britter, R., 2008a. Measurements of the Particles in the nm range close to the road level in an urban street canyon. Science of the Total Environment 390, C ONFERENCE Kumar, P., Ketzel, M., Robins, A., Britter, R., Street-scale modelling of nanoparticles using a simplified approach and an operational model. 7 th International Conference on Air Quality-Science and Application, Istanbul (Turkey), March Kumar, P., Fennell, P., Britter, R., 2008h. The influence of Ambient Meteorology on Nanoparticle Concentration in an Urban Setting. Cambridge Particle meeting, Cambridge (UK), 16 May Kumar, P., Britter, R., 2008g. Measurements and dispersion modelling on traffic-emitted particles in the urban environment. National Environment Research Institute (Denmark), 7 May Kumar, P., Fennell, P., Britter, R., 2007d. Measurement and dispersion behaviour of particles in various size (5 nm>Dp<1000 nm) ranges in a Cambridge Street Canyon. Proceedings of the 11 th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, Cambridge (UK), 2-5 July 2007, pp Kumar, P., Fennell, P., Britter, R., 2007c. The measurement of fine particles for the study of their dispersion and of street-scale air quality. UK Atmospheric Aerosol Network (UKAAN) Workshop, University of Reading, Berkshire (UK), 6-7 June 2007., University of Reading, Berkshire (UK), Kumar, P., Britter, R., 2007b. Particulate Matter: Importance, Regulations and Historical Perspective. ‘Nirmaan’, IIT Delhi Civil Engineering Society, Issue 2, May 2007 pp Nirmaan Kumar, P., Britter, R., Langley, D., 2007a. Street versus rooftop level concentrations of fine particles in a Cambridge Street Canyon. 6 th International Conference on Urban Air Quality Limassol (Cyprus), March 2007, pp on Urban Air Quality Limassol (Cyprus P RASHANT K UMAR 89 TH AMS M EETING, P HOENIX, J ANUARY 09 15

A CKNOWLEDGEMENTS  University of Surrey  Cambridge University Department of Engineering  Cambridge Philosophical Society  Cambridge Commonwealth Trust  Pembroke College, Cambridge  Dr. Paul Fennell (Imperial College, London) – help during experiments P RASHANT K UMAR 89 TH AMS M EETING, P HOENIX, J ANUARY T RAVEL G RANT 1 of 1

T HANK Y OU C ONTACT P RASHANT K UMAR Webpage:

V ehicle W ake EXTRA SLIDE P RASHANT K UMAR 89 TH AMS M EETING, P HOENIX, J ANUARY M EASUREMENTS Estimation of PMDs  For dM/dlogD p in  gm cm –3, M(Dp) is in Kg and D p is in nm,  (C’ = 6.95  10 –24 ) is a constant   p (= 1) is the assumed density of a particle in g cm –3  D f (= 3) is the fractal dimension for a spherical particle (Park et al. 2003).