Comparison of primary and secondary particle formation from natural gas engine exhaust and of their volatility characteristics

Natural gas usage in traffic and energy production sector is a growing trend worldwide, thus an assessment of its effects on air quality, human health and climate is required. Engine exhaust is a source of primary particulate emissions and secondary aerosol precursors that both contribute to air quality and can cause adverse health effects. Technologies, such as cleaner engines or fuels, that produce less primary and secondary aerosol could potentially significantly decrease the atmospheric particle concentrations and their adverse effects. In this study, we used a potential aerosol mass (PAM) chamber to investigate the secondary aerosol formation potential of natural gas engine exhaust. The PAM chamber was used with a constant UV-light voltage that resulted in an equivalent atmospheric age of 11 days at a maximum. The studied passenger car engine, retrofitted to run with natural gas, was observed to have a low or moderate secondary particle formation potential, although the simulated atmospheric ages were relatively long. The secondary organic aerosol (SOA) formation potential was measured to be 8–18 mg kgfuel−1. However, the mass of total aged particles, i.e. particle mass measured downstream the PAM chamber, was 6–184 times as high as the mass of the emitted primary exhaust particles. The total aged particles consisted mainly of nitrate, organic matter, sulfate and ammonium, the fractions depending on exhaust after-treatment and used engine parameters. Also the volatility, composition and concentration of the total aged particles were found to depend on the engine operating mode, catalyst temperature and catalyst type. For example, a high catalyst temperature promoted the formation of sulfate particles, whereas a low catalyst temperature promoted nitrate formation. However, especially the concentration of nitrate needed a long time, more than half an hour, to stabilize, which complicated the conclusions but also indicates the sensitivity of nitrate measurements on experimental parameters such as emission source and system temperatures. Sulfate was measured to have the highest evaporation temperature and nitrate the lowest. The evaporation temperature of ammonium depended on the fractions of nitrate and sulfate in the particles. The average volatility of the total aged particles was measured to be lower than that of primary particles, indicating better stability of the aged natural gas engine emitted aerosol in the atmosphere. According to the results of this study, the shift from traditional liquid fuels to natural gas can have a decreasing effect on total particle pollution in the atmosphere; in addition to the very low primary particle emissions, also the secondary organic aerosol formation potential of natural gas exhaust is lower or on the same level as the SOA formation potential measured on liquid fuels in previous studies.