Time variations in traffic and traffic emissions

Hourly traffic counts have been used to calculate traffic emissions at Strømsveien, a part of the E6-road in Oslo, Norway. The road is a major radial road to Oslo with a workday traffic of about 58 000 vehicles/day and a weekend traffic of 33 000 vehicles/day. The traffic pattern is typical for many main roads in Oslo, with a morning peak in the direction of the center, and an afternoon peak for traffic leaving the center. The heavy traffic is almost bell-shaped, and varies between 5 and 25% of the total traffic. From the traffic data the emissions of CO, total NOx and NO2 were calculated, using emission routines from the NILU model RoadAir. The CO-emissions show a pattern similar to the traffic variation, but with a higher afternoon peak from the center than towards the center, mainly due to higher emissions uphill. The total NOx-emissions are almost constant in the period from 08:00–17:00 h for the traffic from the center, due to a combination of emissions from light and heavy vehicles. In the direction of the center, the emissions are only 20% of the NOx-emissions from the center, due to low emissions downhill. The NO2-emissions show another pattern, with almost the same emissions in each direction, due to a high NO2-proportion (20%) downhill. For hourly model calculations of air pollution the results of this study show that the emission calculations should be based upon counts or simulations of both hourly traffic composition and driving conditions. It may lead to erroneous results if the dispersion calculations are based upon emissions from annual average daily traffic, scaled according to the traffic intensity variations. Local emissions are sensitive to daily variations of driving speed, road gradient and to the traffic composition, in particular considering light and heavy vehicles.