Vertical gradients of benzene concentration in a deep street canyon in the urban area of Naples

The results of an about 2-week monitoring campaign of benzene concentration in a deep street canyon in the city of Naples are reported. Passive samplers were located at different heights on the road level (3–14–27 m). The geometry of the canyon is: width W=5.8 m and height H=33 m (Aspect Ratio, AR=H/W=5.7). Monitoring was carried out in two 6-day campaigns: 10–16 December 2002 and 14–20 January 2003. Traffic levels in the same street were measured to obtain typical daily curves of average vehicle flow vs. time. Sampling time was fixed in 2 days and 6 days. Therefore, 2-day and 6-day average benzene concentrations were measured. The results show that benzene is present at high concentration level in the street canyon. Two-day average concentration up to 23.8 μg m−3 was detected. In all the samples the benzene concentration at 3 m on the road level was higher than the threshold limit value of 10 μg m−3. The vertical concentration profiles of benzene were fitted assuming a first-order exponential decay. Values of the decay coefficient vary in the range 8×10−2–1.6×10−1 m−1.