Primary Gas- and Particle-Phase Emissions and Secondary Organic Aerosol Production from Gasoline and Diesel Off-Road Engines Timothy D. Gordon, †,‡,○ Daniel S. Tkacik, † Albert A. Presto, † Mang Zhang, § Shantanu H. Jathar, †,‡ Ngoc T. Nguyen, † John Massetti, § Tin Truong, § Pablo Cicero-Fernandez, § Christine Maddox, ∥ Paul Rieger, ∥ Sulekha Chattopadhyay, ⊥ Hector Maldonado, # M. Matti Maricq, ▽ and Allen L. Robinson* ,† † Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States ‡ Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States § Mobile Source Operations, California Air Resources Board, El Monte, California 91731, United States ∥ Monitoring and Laboratory, California Air Resources Board, El Monte, California 91731, United States ⊥ Planning and Technical Support, California Air Resources Board, El Monte, California 91731, United States # Research Division, California Air Resources Board, Sacramento, California 95814, United States ▽ Research and Advanced Engineering, Ford Motor Company, Dearborn, Michigan 48120, United States * S Supporting Information ABSTRACT: Dilution and smog chamber experiments were performed to characterize the primary emissions and secondary organic aerosol (SOA) formation from gasoline and diesel small off-road engines (SOREs). These engines are high emitters of primary gas- and particle-phase pollutants relative to their fuel consumption. Two- and 4-stroke gasoline SOREs emit much more (up to 3 orders of magnitude more) nonmethane organic gases (NMOGs), primary PM and organic carbon than newer on-road gasoline vehicles (per kg of fuel burned). The primary emissions from a diesel transportation refrigeration unit were similar to those of older, uncontrolled diesel engines used in on-road vehicles (e.g., premodel year 2007 heavy-duty diesel trucks). Two-strokes emitted the largest fractional (and absolute) amount of SOA precursors compared to diesel and 4-stroke gasoline SOREs; however, 35−80% of the NMOG emissions from the engines could not be speciated using traditional gas chromatography or high- performance liquid chromatography. After 3 h of photo-oxidation in a smog chamber, dilute emissions from both 2- and 4-stroke gasoline SOREs produced large amounts of semivolatile SOA. The effective SOA yield (defined as the ratio of SOA mass to estimated mass of reacted precursors) was 2−4% for 2- and 4-stroke SOREs, which is comparable to yields from dilute exhaust from older passenger cars and unburned gasoline. This suggests that much of the SOA production was due to unburned fuel and/or lubrication oil. The total PM contribution of different mobile source categories to the ambient PM burden was calculated by combining primary emission, SOA production and fuel consumption data. Relative to their fuel consumption, SOREs are disproportionately high total PM sources; however, the vastly greater fuel consumption of on-road vehicles renders them (on-road vehicles) the dominant mobile source of ambient PM in the Los Angeles area. ■ INTRODUCTION After decades of regulatory focus on reducing on-road vehicle emissions, gasoline and diesel off-road engines are becoming increasingly important sources of air pollutants. Although responsible for only 2% of gasoline consumption, small (<19 kW) off-road engines (SOREs) used in lawn and garden applications (e.g., lawnmowers, leaf blowers, trimmers) emitted 8 million tons of carbon monoxide (CO), nitrogen oxides (NO x ), hydrocarbons (HC), and fine particulate matter (PM 2.5 ) in 2007, accounting for 13% of the total U.S. mobile source emissions (excluding commercial air, rail, and marine vessels). 1−3 While the primary pollutant emissions from off-road engines are known to be high, their contribution to secondary particulate matter (PM) is poorly understood. Secondary PM forms when gas- and/or aqueous-phase oxidation of gas-phase precursors creates low volatility products. When the precursor gases are organic, the secondary PM is referred to as secondary organic aerosol (SOA). Numerous studies have shown that SOA is the largest component of the ambient organic aerosol budget, even in urban areas with substantial primary emissions. 4−6 Recent experiments report significant SOA formation from dilute exhaust from small diesel 7,8 and gasoline Received: August 9, 2013 Revised: November 21, 2013 Accepted: November 21, 2013 Published: November 21, 2013 Article pubs.acs.org/est © 2013 American Chemical Society 14137 dx.doi.org/10.1021/es403556e | Environ. Sci. Technol. 2013, 47, 14137−14146