Diurnal and seasonal characteristics of particle volatility and chemical composition in the vicinity of a light-duty vehicle freeway

Abstract

During the winter of 2005, physicochemical properties of size-fractionated PM 10 particles were analyzed next to State Route 110 (Pasadena freeway in Los Angeles, CA, USA), a light-duty vehicle freeway, closed to heavy-duty traffic. We report volatility of ultrafine particles and chemical characteristics in the coarse, accumulation, and ultrafine modes very close to the freeway and at an urban background site. For measurement of mass and chemical composition, the study employed in each location a Micro-Orifice Uniform Deposit Impactor (MOUDI) and a modified high-volume sampler. Both instruments sampled with the same size cutpoints. A tandem differential mobility analyzer (TDMA) was used at the two sampling sites to analyze the semivolatile fraction of the aerosols. Size distributions of the ambient aerosol were measured using a DMA and total number concentration using a condensation particle counter (CPC). In this study, which was a continuation of a campaign during summer 2004, we compared the seasonal, i.e. winter versus summer, and diurnal, i.e. daytime versus evening, variation in PM characteristics in the vicinity of this freeway. Conditions in winter differed most from those in summer during the evening rush-hour traffic, with much lower temperatures in the winter campaign. Diurnal variations in particle volatility were observed, with higher content of volatile material in aerosols sampled in the evenings. Particle number concentrations were also much higher during the evening hours, increasing from 67,000 to 110,000 cm −3. The average total number concentration next to the freeway was 80,000 cm −3 during the sampling period, which was higher than in summer, when the average was 46,000 cm −3. External mixing was observed with increasing nonvolatile fractions for 40, 80, and 120 nm particles (39% of particles). In general the volatility increased in the evenings, while the nonvolatile fraction decreased.