U LTRAFINE PM IN NEAR GROUND LAYER OF URBAN ATMOSPHERE , P RAGUE 2002/2003

1. JAN HOVORKA, LUBOMÍR BETUŠ hovorka@cesnet.cz; lubomir.betus@messer.cz ; Institute for Environmental Studies, Faculty of Science, Charles University in Prague, Benátská 2, Prague 2, 128 01, Czech Republic ULTRAFINEPMINNEARGROUNDLAYEROF URBANATMOSPHERE, PRAGUE 2002/2003 • There is remarkable improvement of air quality of Prague city center within last decade which also includes PM10 mass concentrations. Contrary to PM10ultrafinePM concentrations either kept constant or increased probably because of the rise in transportation density • Ultrafine PM is of increasing concern nowadays due to its catalytic properties in smog build-up in urban atmosphere and strong effect on human health • We measured ultrafine PM in a breathing zone of urban atmosphere in a botanical garden, which serves as a “green refuge” area in the city centre • Quantitative and qualitative parameters of ultrafine PM, arerelated to simultaneously measured concentrations of gaseous atmospheric components, meteorology parameters, and transportation density sampling 2.5 mm cut-off inletat height 1.8 m, 21 days (10/11 working/weekend days) in 2002/2003, 7:00 – 18:00 size discrimination3 - 200 nm by diffusion battery DB (3040, TSI) coupled with ultrafine condensation particle counter UCPC (3025 A, TSI), 10s / 18s sampling / waiting time, scan time 298 s, average of 3 consecutive scans (15 mins) distributions recorded, 374 distributions evaluated AEROSOL sampling inlet at height 2.0 m mounted to the same pole as for aerosol sampling determinationNO, NO2, (APNA – 360), O3 (APOA – 360), CH4 and non-methane hydrocarbons NMHC (APHA – 360, all Horiba), 15 min averages GASES OTHER PARAMETERS wind speed/direction, sun radiation, temperature, humidity, traffic density at the nearest crossroads approx. 18000/24 h, mostly passenger cars, city center of Prague (1.2 mil population, 496 km2 urban territory) Number size distributions averaged over working days (yellow) and weekend days (green) • aerosol number size distributions were usually three modal • particles of the first mode (5 nm) in the range of 3-10 nm are referred as nanoparticles • particles of the second (20 nm) and third (80 nm) modes in the range of 10-200 nm as accumulation mode particles • there was statistically significant (a<0.05) more particles in working days than in weekends for the second mode only • nano and accumulation mode particles significantly differ in frequency distributions of number concentrations Frequency of nanoparticle number concentrations • high frequency of number concentration below 500 pp/cm-3 for the nanoparticles reflects their high reactivity in the atmosphere • fast plate-out on dense vegetation and coagulation are two main processes causing low nanoparticle concentrations • fast coagulation of nanoparticles leads to their residence times within the range of tens of seconds to couple of minutes • due to the fast coagulation and low wind speeds, sources of nanoparticles must have been close to the sampling site • possible nanoparticle sources are gas-to-particle conversion or emission of primarily nanoparticles from high temperature processes Frequency of accumulation mode particle number concentrations • accumulation mode particles form on average about 77% of total particle numbers • accumulation mode particles showed two modes (9000 pp/cm3) and (18000 pp/cm3) of frequency distribution of number concentrations which also reflects bimodality of number size distribution • contrary to nanoparticles, accumulation mode particles have much longer residence times in the atmosphere • sources of accumulation mode particles need not to be close to the sampling site • traffic should be considered as significant source of accumulation mode particles

2. Hourly averages of traffic density during working days (yellow) and weekends (green) • traffic density peaks at round 9 a.m. and 17 p.m. on working days; there is no such peak values during weekends in Prague • number concentrations of accumulation mode particles follow rush hours; nevertheless, high concentration values in weekend mornings point to other important sources of the particles than transportation • there were significantly more accumulation mode particles suspended in the air during working days than on weekends; this does not apply to nanoparticles • number concentrations of nanoparticles is significantly higher than daily averages just in late (5-6 p.m.) afternoons on working days Hourly averages of particle number concentrations during working days Hourly averages of particle number concentrations during weekends Tables of correlation coefficients between particle number concentrations and other measured variables during working days and weekends • red numbers denotes statistically significant (a<0.05) values of correlation coefficients • correlation coefficients between particle number and temperature, humidity, pressure were calculated regardless days of week CONCLUSIONS • ultrafine aerosol measurement in near-to-ground layer revealed low levels of nanoparticle number concentrations; this is result of nanoparticle fast plate-out on the dense vegetation and absence of strong sources emitting primary nanoparticles in the botanical garden • statistically significant correlation between nanoparticle number concentrations and temperature decrease indicate importance of gas-to-particle conversion as an important source of nanoparticles in near ground layer mainly on weekends; besides, statistically significant correlation between nanoparticle number concentrations and CH4, NO concentrations (within 0-3 mm m-3) suggests that natural gas burning boilers for heating are also an important nanoparticle sources • contrary to nanoparticles, number concentrations of accumulation mode particles did not differ significantly in botanical garden from the values measured on the roof of the nearby building, and are mainly associated with traffic density • the importance of traffic, as a source of accumulation mode particles, is confirmed by statistically significant correlation between the particle number concentrations and NOx concentrations both in low and peak values and by similar pattern of traffic density temporal variation during working days • similarly to nanoparticles, natural gas burning boilers for heating has to be considered as an important source of accumulation mode particles during weekend mornings, when the traffic density is low Acknowledgment The authors are grateful to professor R.F.Holub of Colorado School of Mines for lending us the DB-UCPC apparatus