Most of the air quality standards available today are mass based and confined to PM2.5 and PM10 fractions. Size of most particles released from combustion sources is of submicron range, which has minor contribution to mass concentration. Therefore it is essential to obtain inventories for particulate number concentrations in this range.
The study was mainly focused on in-vehicle particulate number concentrations in public transport buses running on alternative fuels in the city of Toledo. The in-vehicle particulate number concentrations were collected over a period of one year from July 2008 to June 2009, in Biodiesel and Ultra low sulfur diesel fueled buses. The size of particulates found was in the range of 0.3µm and 20µm. Using the above measured particulate concentration data, the diurnal, monthly, and seasonal variations were studied. Various factors effecting in-vehicle particulate concentrations like number of passengers in the bus, vehicles moving near the bus, ambient temperature, relative humidity, wind speed, wind direction, precipitation were also analyzed using regression tree analysis.
It was found that 65-70 % of particulates observed were in the size range of 0.3-0.4 µm. From this we were able to conclude that particulates emitted from diesel vehicles mostly consisted of fine particles. It was observed that particulate concentrations in biodiesel bus were slightly more when compared to ultra low sulfur diesel bus concentrations. From diurnal graphs, it was found that maximum particulate concentrations were obtained during the early mornings, when bus starts its run. From monthly and seasonal trends, it was obtained that maximum concentrations were found during the winter season, because of limited air exchange rate within the bus compartment. From the above trends it was clearly understood that in-vehicle particulate number concentrations were mainly influenced by peak hours, vehicular traffic, positioning of doors and windows, and passengers travelling. Regression analysis showed that in-vehicle particulate concentrations were influenced by meteorology. Wind speed and wind direction were found to have a significant impact on particulate concentrations. Various combinations of variables explained the pattern of monitored concentrations.
The measured in-vehicle particulate number concentrations in B20 and ULSD buses were converted in to mass concentrations of PM1.0, PM2.5 and PM10. These PM mass concentrations were compared with previously measured two years PM concentrations in the same buses. Using all the above data annual and seasonal PM trends were studied. It was observed that PM mass concentrations increased in the year 2009 compared to 2008 concentration levels. In all the three years, particulate matter concentrations were found to be more in winter season when compared to other seasons in both BD and ULSD buses.
A screening mass balance model was developed for modeling of in-vehicle PM2.5 concentrations for buses. The model was tested over four different seasons during a one year period. The air exchange rate and, deposition loss rate were estimated from literature review and from the analysis of monitored concentrations when developing the mass balance model. The developed model predicts the in-vehicle PM2.5 levels inside buses for four seasons performed well up to 1:00 PM. It is suggested that a forecasting model should be used for ambient concentrations to improve the accuracy during afternoon hours.