Seasonal variation of emission factors for ultrafine particles from on-highway vehicles.
- As urban populations increase, the need to estimate urban exposure to harmful aerosols increases. Simple Gaussian screening models can provide initial exposure estimates, but urban environments can create complex wind fields that exceed the capabilities of simple models. To fill this need, sophisticated models, such as the Quick Urban & Industrial Complex (QUIC) dispersion model, have been develop... read moreed to model pollutant flow through urban environments. One pollutant of recent concern is ultrafine particles (UFP), particles whose diameter is less than 100 nm, and vehicle emission factors (EF) have been developed for UFP, in units of # of UFP/vehicle-km. The validity of urban exposure estimates depends on the predictive capacity of the EF estimate, and it is unclear if an EF estimate generated from average annual data can be used to adequately predict UFP concentrations at all times during a year. The goal of this research was to examine if an EF estimate that allowed average annual monitored data, collected in Ten Hills, MA, to fall within a factor-of-2 of QUIC model predictions could be used to adequately predict UFP concentrations in the summer and winter months (June through August and December through February, respectively). Monitored concentrations along Temple Road, a road orthogonal to highway I-93 (the main source of UFP in Ten Hills), were compared to QUIC predicted UFP concentrations. For data collected from hours where the prevailing wind direction was from the southwest (i.e., orthogonal to I-93), an EF of 5*1014 #/veh-km captured 92% of the annual average data within a factor-of-2 estimate, 68% of the summer data, and 52% of the winter data. When the prevailing wind direction was from the northwest (parallel to I-93), an EF estimate of 5*1014 #/veh-km captured 91% of the annual average data within a factor-of-2 estimate, 50% of the summer data, and 60% of the winter data. The impact of varying the EF based on the season was also investigated. Measured concentrations of UFPs were higher in the winter than in the summer, and varying the UFP EF with season greatly improved the amount of data captured by the factor-of-2 envelope from model estimates. The results of this research highlight the modeling capacity of QUIC, and show that season-sensitive EF estimates could generate more accurate UFP exposure estimates.read less