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

摘要

Natural gas usage in the traffic and energy production sectors is a growingtrend worldwide; thus, an assessment of its effects on air quality,human health and climate is required. Engine exhaust is a source ofprimary particulate emissions and secondary aerosol precursors, whichboth contribute to air quality and can cause adverse healtheffects. Technologies, such as cleaner engines or fuels, that produceless primary and secondary aerosols could potentially significantlydecrease atmospheric particle concentrations and their adverseeffects. In this study, we used a potential aerosol mass (PAM) chamberto investigate the secondary aerosol formation potential of naturalgas engine exhaust. The PAM chamber was used with a constant UV-lightvoltage, which resulted in relatively long equivalent atmospheric agesof 11 days at most. The studied retro-fitted natural gas engineexhaust was observed to form secondary aerosol. The mass of the totalaged particles, i.e., particle mass measured downstream of the PAMchamber, was 6–268 times as high as the mass of the emitted primaryexhaust particles. The secondary organic aerosol (SOA) formationpotential was measured to be9–20 mg kg. The total aged particlesmainly consisted of organic matter, nitrate, sulfate and ammonium,with the fractions depending on exhaust after-treatment and the engineparameters used. Also, the volatility, composition and concentration ofthe total aged particles were found to depend on the engine operatingmode, catalyst temperature and catalyst type. For example, a highcatalyst temperature promoted the formation of sulfate particles,whereas a low catalyst temperature promoted nitrateformation. However, in particular, the concentration of nitrate neededa long time to stabilize – more than half an hour – whichcomplicated the conclusions but also indicates the sensitivity ofnitrate measurements on experimental parameters such as emissionsource and system temperatures. Sulfate was measured to have thehighest evaporation temperature, and nitrate had the lowest. Theevaporation temperature of ammonium depended on the fractions ofnitrate and sulfate in the particles. The average volatility of thetotal aged particles was measured to be lower than that of primaryparticles, indicating better stability of the aged natural gasengine-emitted aerosol in the atmosphere. According to the results of thisstudy, the exhaust of a natural gas engine equipped with a catalystforms secondary aerosol when the atmospheric ages in a PAM chamber areseveral days long. The secondary aerosol matter has different physicalcharacteristics from those of primary particulate emissions.