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

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 kg −1 fuel . 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.

Comparative performance of a thermal denuder and a catalytic stripper in sampling laboratory and marine exhaust aerosols

“…Figure 4 shows the total number of particles and the number of neutral ones downstream of the TD, when this is challenged with 6.4 £ 10 5 particles/cm 3 of sulfuric acid with »30 nm mean particle size. The TD operated in a temperature ramp mode, i.e., it was first heated up at a high temperature and then it was allowed to gradually cool down, after heating had been switched off (Alanen et al 2017).…”

Formation and evolution of secondary organic aerosols derived from urban-lifestyle sources: vehicle exhaust and cooking emissions

“…Vehicle and cooking emissions are important sources of OAs in urban areas (Rogge et al, 1991(Rogge et al, , 1993Hu et al, 2015;Hallquist et al, 2016;Crippa et al, 2013;Mohr et al, 2012;Guo et al, 2013Guo et al, , 2012. Take the megacity (defined as a total metro area population of more than 3 million) for example, in London, where these two lifestyle sources contribute 50 % of OAs on average (Allan et al, 2010). In addition, vehicles themselves could even contribute 62 % of OA mass in the rush hour of New York City (Sun et al, 2012).…”

“…According to seasonal observations in Beijing, at least 30 % of OAs there come from vehicle and cooking emissions (Hu et al, 2017). Briefly, these two urban-lifestyle sources are closely related to the daily life of city residents and could account for 20 %-60 % of ambient OA mass in urban areas when only considering their contributions to POAs (Allan et al, 2010;Ge et al, 2012;Sun et al, 2012;Lee et al, 2015;Hu et al, 2017). Furthermore, it has been speculated that vehicle and cooking emissions might even contribute over 90 % of SOAs in downtown Los Angeles by applying hypothetical model parameters with a certain degree of uncertainty (Hayes et al, 2015).…”

“…Several smog chamber studies have found that the mass loading of SOAs exceeds that of POAs when the equivalent photochemical age is more than 1 d (Gordon et al, 2013;Chirico et al, 2010;Nordin et al, 2013). Besides, an OFR could simulate a higher OH exposure, and the peak SOA production occurs after 2-3 d of equivalent atmospheric oxidation (Tkacik et al, 2014;Zhao et al, 2018;Timonen et al, 2017;Watne et al, 2018;Alanen et al, 2017). The mass spectra of vehicle SOAs has shown both semi-volatile and low-volatility oxygenated organic aerosol (SV-OOA and LV-OOA) features along with the growth of oxidation degree (Tkacik et al, 2014).…”

Aerosol gas exchange system (AGES) for nanoparticle sampling at elevated temperatures: Modeling and experimental characterization

“…residence time and temperature in the sampling system. These extremely challenging aerosol measurement fields include for example engine emission studies 1–4 and atmospheric studies 5–7 . The sampling phase must (1) reduce particle concentrations to levels suitable for the measurement instruments, and (2) prevent condensation of gases on the surfaces of the sampling system itself or inside the measurement instruments.…”