Temporal variations in black carbon concentrations with different time scales in Helsinki during 1996–2005

Variations in black carbon (BC) concentrations over different timescales, including annual, weekly and diur- nal changes, were studied during ten years in Helsinki, Fin- land. Measurements were made in three campaigns between 1996 and 2005 at an urban area locating two kilometres of the centre of Helsinki. The first campaign took place from November 1996 to June 1997, the second from September 2000 to May 2001 and the third from March 2004 to Oc- tober 2005. A detailed comparison between the campaigns was only made for winter and spring months when data from all campaigns existed. The effect of traffic and meteorologi- cal variables on the measured BC concentrations was studied by means of a multiple regression analysis, where the me- teorological data was obtained from a meteorological pre- processing model (MPP-FMI). The BC concentrations showed annual pattern with max- ima in fall and late winter due to the weakened mixing and enhanced emissions. Between 1996 and 2005, the campaign median BC concentrations decreased slightly from 1.11 to 1.00µg m 3 . The lowest campaign median concentration (0.93µg m 3 ) was measured during the second campaign in 2000-2001, when also the lowest traffic rates were measured. The strongest decrease between Campaigns 1 and 3 was ob- served on weekday daytimes, when also the traffic rates are highest. The variables affecting the measured BC concentra- tions most were traffic, wind speed and mixing height. On weekdays, traffic had clearly the most important influence before the wind speed and on weekends the effect of wind speed diluted the effect of traffic. The affecting variables and their influence on the BC concentrations were similar in win- ter and spring. The separate examination of the three cam- paigns showed that the effect of traffic on the BC concentra- tions had decreased during the studied years. This reduction was caused by lower emitting vehicles, since between years 1996 and 2005 the traffic rates had increased.