Abstract
Abstract. Natural gas usage in the traffic and energy production sectors 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, which both contribute to air quality and can cause adverse health effects. Technologies, such as cleaner engines or fuels, that produce less primary and secondary aerosols could potentially significantly decrease 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, which resulted in relatively long equivalent atmospheric ages of 11 days at most. The studied retro-fitted natural gas engine exhaust was observed to form secondary aerosol. The mass of the total aged particles, i.e., particle mass measured downstream of the PAM chamber, was 6–268 times as high as the mass of the emitted primary exhaust particles. The secondary organic aerosol (SOA) formation potential was measured to be 9–20 mg kgfuel−1. The total aged particles mainly consisted of organic matter, nitrate, sulfate and ammonium, with the fractions depending on exhaust after-treatment and the engine parameters used. 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, in particular, the concentration of nitrate needed a long time to stabilize – more than half an hour – 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 had 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 exhaust of a natural gas engine equipped with a catalyst forms secondary aerosol when the atmospheric ages in a PAM chamber are several days long. The secondary aerosol matter has different physical characteristics from those of primary particulate emissions.
Highlights
Primary aerosol particles are directly emitted into the atmosphere by various anthropogenic sources, such as vehicles, engines and power plants, and biogenic sources
Primary particle mass has not been subtracted from the mass measured downstream of the potential aerosol mass (PAM) to calculate the secondary particle mass separately because doing so would have created inconsistency in representation of the results, since, for example, particle size distributions or volatility behavior cannot be presented in this way
Exceptions were made by the high temperature catalyst cases (M2, C2, 500 ◦C and mode 2 (M2), C1, 450 ◦C) during which more primary particle mass was formed, especially on the size ranges of the HRLPI and EEPS: a high catalyst temperature favors the conversion of SO2 into SO3 and further into sulfuric acid, which can nucleate and condense on existing particles in the sampling process or when released into the atmosphere
Summary
Primary aerosol particles are directly emitted into the atmosphere by various anthropogenic sources, such as vehicles, engines and power plants, and biogenic sources. Secondary aerosol particle mass forms as a consequence of the atmospheric oxidation of emitted precursor gases. In this process, the saturation vapor pressure of the organic and inorganic gases becomes lower, allowing them to transfer into particle phase through condensation and nucleation (Hallquist et al, 2009; Murphy et al, 2014). In addition to biogenic sources, traffic and other anthropogenic sources con-. J. Alanen et al.: Comparison of primary and secondary particle formation tribute to secondary aerosol formation (Kanakidou et al, 2005)
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