They measured gasoline car SOA formation at low temperatures (−7 °C), in addition to measurements at 22 °C. They found that low temperatures significantly increase vehicle total hydrocarbon (THC) emission, including SOA precursors, and favor condensation of gases to the aerosol phase. The team used these measurements to parameterize vehicular SOA formation, accounting for the full range of relevant ambient temperatures, background OA concentrations, and extent of atmospheric aging.

The parametrization method is new and does not require direct knowledge of precursor composition (which typically includes unknown and/or unquantifiable gases).

Their experiments were performed at the Vehicle Emissions Laboratory (VELA) of the European Commission Joint Research Centre. Time-resolved online measurements of particle composition were performed using a high-resolution time-of-flight aerosol mass spectrometer for organics, nitrate, and ammonium, and an aethalometer for equivalent black carbon (BC).

They found that emissions from new gasoline cars (both EU and US) produce up to 6.5 times more SOA than POA after 5–10 hours of atmospheric aging. New diesel cars produced no detectable SOA, contrasting with old diesels, for which SOA production is about equal to emitted POA.

Carbonaceous emissions/secondary organic aerosol formation from modern diesel and gasoline passenger cars. (a) Aerosol emission factors (g kg−1 fuel) measured in the study. Diesel vehicles did not produce measurable OA so gravimetric PM is given. For comparison the average and standard deviation of PM emission factors from non-DPF medium duty diesels is shown. (b) Average ratio of diesel/gasoline emission factors for OA, BC, PM, methane (CH4), total hydrocarbon (THC), aromatic hydrocarbons (Ar. HC), NOx and carbon monoxide (CO) at 22 °C and −7 °C. Euro 5 values are obtained using the NEDC, while US LEV2 and DPF-equipped vehicles use the US unified driving cycle, UC3. While a true ratio cannot be calculated for OC and BC, a maximum can be, based on detection limits, highlighted by the red asterisks (*). In contrast to CO and especially NOx, PM and hydrocarbon emissions from gasoline cars are higher than from diesel cars. (c)Averaged THC-normalized exhaust composition (temperature in parentheses) in the smog chamber with uncharacterized emissions in grey. The single largest fraction of gasoline THC consists of methyl benzenes present in the fuel, while the rest likely consists of surviving linear/branched, saturated/unsaturated hydrocarbons. Meanwhile, diesel emissions mainly comprise pyrolysis products, including small carbonyls (formaldehyde, acetaldehyde) and carboxylic acids (formic, acetic), which are not efficient SOA precursors. Pratt et al. (2017) Click to enlarge.

They explained the absence of observed SOA from the DPF-diesels by the chemical composition of the THC emissions; while emissions from non-DPF-diesels chemically resemble diesel vapors, DPF-equipped-diesel exhaust comprises a large fraction (>70%) of short-chain oxygenated compounds, mainly formaldehyde and acetaldehyde, which are much less efficient SOA precursors than the aromatic species found in gasoline emissions.

These results challenge the existing paradigm that diesel cars are associated, in general, with far higher PM emission rates, reflecting the effectiveness of recent, Euro 5–6, diesel after-treatments such as DPFs combined with diesel oxidation catalysts. Furthermore, while Gordon et al. report primary PM emissions (g kg⁻¹ fuel) during DPF regeneration (burn-off of accumulated PM) similar to those during regular driving of a non-DPF vehicle, DPF regeneration is activated infrequently (every few hundred km), lasts around one minute, and is more likely to occur at high speed (more likely outside densely populated areas). Thus, the net result of regeneration is unlikely to significantly diminish DPF efficacy.

… Our results suggest that as vehicle fleets modernize to include a higher fraction of DPF-diesels the relative contribution from gasoline passenger cars to carbonaceous aerosol will increase. Note that while we draw these conclusions from driving cycles designed to represent real-world driving and from ambient data reflecting an aggregate of all emissions, exceptions are possible. Critically, how much of the carbonaceous aerosol in any region is from one engine technology or the other will depend on the number of vehicles with each type of engine technology in operation and the age of the vehicles (i.e. the number of old, non-DPF vs modern, DPF equipped diesels), which may be region-dependent.

Specific cases where diesel passenger cars may emit more carbonaceous aerosol than gasoline may include very long journeys (relative importance of the cold start is lower), operating conditions not investigated here e.g. high speeds (>120 km h⁻¹⁾ or extremes of temperature, and outlier vehicles … Nevertheless, and since NO_x emissions are generally higher from diesel cars, there is a choice between new passenger cars that generally emit less PM and produce less SOA (diesel), or new passenger cars that emit less NO_x (gasoline).

—Platt et al.

Resources

• S. M. Platt, I. El Haddad, S. M. Pieber, A. A. Zardini, R. Suarez-Bertoa, M. Clairotte, K. R. Daellenbach, R.-J. Huang, J. G. Slowik, S. Hellebust, B. Temime-Roussel, N. Marchand, J. de Gouw, J. L. Jimenez, P. L. Hayes, A. L. Robinson, U. Baltensperger, C. Astorga & A. S. H. Prévôt (2017) “Gasoline cars produce more carbonaceous particulate matter than modern filter-equipped diesel cars” Scientific Reports 7, Article number: 4926 doi: 10.1038/s41598-017-03714-9