Urban aerosol number size distributions

Abstract

Abstract. Aerosol number size distributions have been measured since 5 May 1997 in Helsinki, Finland. The presented aerosol data represents size distributions within the particle diameter size range 8-400 nm during the period from May 1997 to March 2003. The daily, monthly and annual patterns of the aerosol particle number concentrations were investigated. The temporal variation of the particle number concentration showed close correlations with traffic activities. The highest total number concentrations were observed during workdays; especially on Fridays, and the lowest concentrations occurred during weekends; especially Sundays. Seasonally, the highest total number concentrations were observed during winter and spring and lower concentrations were observed during June and July. More than 80% of the number size distributions had three modes: nucleation mode (D p <30 nm), Aitken mode (20-100 nm) and accumulation mode (D p >90 nm). Less than 20% of the number size distributions had either two modes or consisted of more than three modes. Two different measurement sites were used; in the first (Siltavuori, 5.5.1997(Siltavuori, 5.5. -5.3.2001, the arithmetic means of the particle number concentrations were 7000 cm −3 , 6500 cm −3 , and 1000 cm −3 respectively for nucleation, Aitken, and accumulation modes. In the second site (Kumpula, 6.3.2001(Kumpula, 6.3. -28.2.2003) they were 5500 cm −3 , 4000 cm −3 , and 1000 cm −3 . The total number concentration in nucleation and Aitken modes were usually significantly higher during workdays than during weekends. The temporal variations in the accumulation mode were less pronounced. The lower concentrations at Kumpula were mainly due to building construction and also the slight overall decreasing trend during these years. During the site changing a period of simultaneous measurements over two weeks were performed showing nice correlation at both sites.

Winter and summer time size distributions and densities of traffic-related aerosol particles at a busy highway in Helsinki

“…The average nighttime background concentrations, calculated using short-time measurements, were ∼6000 #/cm 3 and 10 000 #/cm 3 for summer and winter, respectively. Hussein et al (2004) reported rather similar nighttime background concentration values of 8000 #/cm 3 and 10 000 #/cm 3 during the summer and winter seasons, respectively. The average daytime background concentrations are shown by the dashed lines in Fig.…”

“…2a. Hussein et al (2004) reported maximum morning rush hour concentrations of approximately 25 000 #/cm 3 and 17 500 #/cm 3 , during the summer and winter seasons, correspondingly. As expected, the roadside concentrations during the morning rush hours at Itäväylä were much higher, averaging approximately 100 000 #/cm 3 in the winter and 50 000 #/cm 3 in the summer.…”

“…In this case, the density value cannot be found at all or its value can be unreasonable. Hussein et al (2004) found mostly 3 modes in Helsinki: average geometric mean diameter (GMD) of nucleation mode was at ∼10-15 nm, Aitken mode at 40-50 nm and accumulation mode at ∼150 nm. Their measurement sites were at Kumpula and Siltavuori, representing background sites rather than roadside sites.…”

“…The roadside concentrations followed the same temporal pattern as the traffic rate. Hussein et al (2004) give the same hours for the minimum (03:00) and maximum (07:00) concentrations measured at the urban background stations located at Kumpula and Siltavuori in Helsinki. The minimum concentration values at the roadside were of the same order as the urban background values.…”

“…This has been shown earlier in many studies mostly based on measurements of the particle mass concentration. Recently, also the number concentration of fine particles measured at an urban background has been related to the traffic rate (Van Dingenen et al, 2004;Hussein et al, 2004). The size and concentrations of traffic related particles at roadsides have been widely studied during recent years.…”