The paper deals with a specific issue of emission factor determination using plume chasing. Typically, the emission factors of the pollutants are determined by assuming dilution factors based on CO2 measurements. In special cases (e.g. downhill driving, engine motoring, hybrid operation) or for non-exhaust emissions, where CO2 emissions are low or non-existent, this approach fails. To overcome this problem, the authors present two methodologies, one based on a multivariate regression algorithm and a second model-based approach that takes into account speed, exhaust mass flow, vehicle shape, and fuel used. The methods have been tested against conventional methods using experimental data from a designated measurement campaign.

The methods developed in this work are important for remote emission sensing and address relevant scientific issues within the AMT, namely the determination of exhaust plume dilution factors also in the absence of CO2 emissions for individual vehicle emission factor measurements.

However, the paper only deals with particulate emissions, which is not mentioned in the title or abstract.

It would be very interesting for the reader to know if the methods are also applicable to gaseous emissions and, if so, what the limitations are.

It is mentioned, but not described, how the methods could be applied to non-exhaust emissions.

In particular, it would be interesting to know whether the dilution of exhaust and non-exhaust emissions is comparable.

It would also be interesting to know how the methodologies deal with hybrid operation of vehicles.

Please revise accordingly.

Minor:

Line 172: particle concentration: abbreviation N is missing

Line 204: typo: without no 

The paper presents an interesting study where several methods for calculating the dilution ratio and the emission factor from a chased vehicle are tested and compared. It is an important contribution to the remote emission sensing research field, namely to the emission factor determination using the plume chasing technique.

134 Could you estimate how good the assumption that the vehicle distance was constant between the two vehicles is since you had GPS on both the vehicle and the measurement station?

In line 220 you state that Nraw uses CO₂ calculated from OBD data while in Table 2 it is in a row stating it is not using OBD data. Can you make this consistent?

Is Audi type-approved as Euro 5? Euro 5 type approval started in September 2009, while Audi's registration year was 2008. Where is the information on the studied vehicles from?

186 – 193 In the N/CO2 Traficom description you say you used the Traficom data, can you also state how the values you got there compared to what you measured and what the producer of the vehicle states the values are?

195-213 One-minute constant EF seems long. Looking at Figure 1, could you use a shorter time maybe 30s? How long did it take you to make a single round of 13.8 km? Was there much variability between runs?  Would this affect the result? How are the zero values included in the final EF distributions since these are on log plots?

What values do whiskers on box plots represent (Figures 5 and 6)?

Figure 4 is not clear – which splines are overlapping? Are all vehicles on all plots? It is difficult to see which vehicle is which with this color code.

Maybe add a time resolution column to Table 3. Would different time resolutions influence your results?

Minor issues

Use subscripts consistently throughout the text for “2” in CO₂ (there are two cases in Table 3 and in l. 175). Also degrees for degrees C in the caption of Figure 5.

204 “without no need to” do you mean double negative or “with no need to” or “without a need to”

The paper presented by Leinonen et al. summarizes different methods to determine dilution ratios and emission factors from vehicle chase studies. The study location was a well-suited test road for wintertime investigation. Mostly, no other vehicle intervened the setup.  The study appears to be scientifically sound and adds valuable data to the literature of exhaust chasing. It should be accepted for publication after some revision.

The study would have been much more useful and credible if the test vehicle would have been equipped with a  PN-PEMS. This would have proven if the order of magnitude for PN of the Diesel vehicle (assumed with DPF) is correct. An average of 5x10E12 #/km at -11C to -23C could have easily been confirmed, or disprovem.  The same question arises for the gasoline vehicle: 4x10E11 – 1x10E12 #/km (Skoda-24C to -26C ). How does this compare to PN PEMS during the same drive?

The results for the Skoda-2 -24C/-26C selecting downhill driving are puzzling: Why is the EF not substantially lower (downhill Figure 7) if compared to the overall trip?  Rather the opposite occurs comparing Skoda -24C Figure 5 (about 4x10E11 #/km) with downhill Figure 7 (about 6x10E11 #/km)?



What were the chasing distances and can could the NWD, or MARS be applied in open traffic, where this short distance would be potentially unsafe?

The authors are measuring total PN, and no size distributions. Therefore, it is left entirely open if nucleation particles would have occurred at the low temperatures. If nucleation would have occurred, if would question the method of NWD, particle formation could not have been completed in the exhaust plume.

Maybe, for a next study a setup w and w/o thermal treatment of the PN will be included?



Other editorial:

Line 23: please clarify what is meant with “but the regulation even for new vehicles is still under development and the new regulations do not completely cover the existing fleet”? If a future regulation is meant, it is normal. It has to be developed and can only apply to vehicles coming into production at that date.

Line 28-30: “The limits for PN only consider nonvolatile particles, and the particle mass (PM) formed from the precursor gases via nucleation and condensation as the exhaust gas dilutes and cools upon exiting the tailpipe is mostly neglected. The amount of particle matter (both in terms of PN and PM) formed this way can be considerable.”

These sentences are misleading:  With the elimination of fuel sulfur the occurrence of volatile PN formed upon cooling and nucleation has been decreased. The hydrocarbons potentially nucleating are measured and regulated via gaseous HC requirements. Also, potential health effects are more likely to be related to solid PN (i.e. more long-lived, and not dissolved and diluted in the alveoli).

Therefore, until data is available, or a strong reference is added, the sense of the statement “neglected” should be revised.

Tab 1: Add if particle filter was installed. Euro-5 should have DPF, for gasoline vehicles not clear.

Line 377: “NWD and MARS Method would be extendable to non-exhaust emissions”

This statement should be further explained. How to differentiate from Exhaust PM? How to differentiate tire wear, and brake wear? Applicable to electric vehicles only?