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Variations in black carbon (BC) concentrations over different timescales,including annual, weekly and diurnal changes, were studied during ten yearsin Helsinki, Finland. Measurements were made in three campaigns between 1996and 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 secondfrom September 2000 to May 2001 and the third from March 2004 to October2005. A detailed comparison between the campaigns was only made for winterand spring months when data from all campaigns existed. The effect oftraffic and meteorological variables on the measured BC concentrations wasstudied by means of a multiple regression analysis, where the meteorologicaldata was obtained from a meteorological pre-processing model (MPP-FMI).  The BC concentrations showed annual pattern with maxima in fall and latewinter due to the weakened mixing and enhanced emissions. Between 1996 and2005, the campaign median BC concentrations decreased slightly from 1.11 to1.00 μg m<sup>−3</sup>. The lowest campaign median concentration (0.93 μg m<sup>−3</sup>)was measured during the second campaign in 2000–2001, whenalso the lowest traffic rates were measured. The strongest decrease betweenCampaigns 1 and 3 was observed on weekday daytimes, when also the trafficrates are highest. The variables affecting the measured BC concentrationsmost were traffic, wind speed and mixing height. On weekdays, traffic hadclearly the most important influence before the wind speed and on weekendsthe effect of wind speed diluted the effect of traffic. The affectingvariables and their influence on the BC concentrations were similar inwinter and spring. The separate examination of the three campaigns showedthat the effect of traffic on the BC concentrations had decreased during thestudied years. This reduction was caused by lower emitting vehicles, sincebetween years 1996 and 2005 the traffic rates had increased

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