The authors measured the emissions of air pollutants from fuel-operated auxiliary heaters (AHs) in passenger cars. They estimated the emission factors (EFs) both outdoors (−19 to −7 ℃) and in the laboratory (+25 ℃), and evaluated the temperature dependence of the EFs. They also estimated secondary emissions using an environmental chamber and an oxidation flow reactor. In general, this manuscript is well written, and provides useful information on the emission characteristics of AHs, which are not well understood to date. However, I have several concerns as listed below.

(1) The authors evaluated the EFs of secondary pollutants. As well known (e.g., Hallquist et al., 2009; doi:10.5194/acp-9-5155-2009), formation yields of secondary organic aerosol (SOA) are strongly affected by the air temperature, exposure to oxidants (e.g., OH exposure), NOx levels, concentrations of seed particles, and so on. In this manuscript, the EFs of secondary pollutants are summarized in Figure 9, but the experimental conditions are not properly reported. I recommend the authors to state the air temperature, OH exposure, and NOx levels (or NOx/HC ratio) of individual experiments for fair comparison.

For example, the emissions of fresh PM mass and OGCs from the renewable diesel case were greater than those from the regular diesel case, whereas the aged PM mass was greater for the renewable fuel (L584-585). Without the information on the experimental conditions, we cannot assess whether this difference is explained by the different experimental conditions or by the different precursor composition.

(2) Standard deviations (SDs) of EFs for both outdoor and laboratory measurement are shown in Tables 1 and 2, but the SDs of EFs for laboratory measurement are not shown in Figure 9. I recommend the authors to clearly state the numbers of laboratory measurements under individual conditions and the meaning of SDs in Tables 1 and 2.

(3) Information on aftertreatment devices on the AH (e.g., particulate filters, oxidation catalyst) is helpful to readers.

Specific comments:

L251-253: Let me confirm the meaning of the “preheating”. Is it the whole heating cycle of AH, including ignition and shutdown? I could not correctly understand the meaning of this word, and the clarification of this word is helpful.

L350: Is this temperature dependence consistent with that of gasoline and diesel vehicles?

L377: typo (“were had”)

L458-460: Do you mean enhanced new particle formation from extremely low volatile organic compounds (ELVOCs) or from VOC? A simple explanation with references is helpful to the readers. (If you mean ELVOCs, is the comparison between the HC emissions and new particle formation significant?)

L499-501: “9 and 1.1 mg kg-1”: Is this for fresh mass or for “fresh and aged mass”? I could not correctly understand the meaning of “the fresh and aged PM mass” for “fresh AH exhausts”.

L538: typo (“+ Impact of aging on AAEs?”)

The manuscript by Oikarinen et al. reports experimental data for emission of both gas and particulate matter species from fuel-operated auxiliary heaters. The experiments include operation of the heater under a winter-time condition, making the study to be relevant to understand the real environment. Overall, the experiments seem to be well-conducted, and the data are well organized. The dataset would be useful for understanding the urban environment. Though the manuscript is a good report of data, all the employed measurement methods are well-established ones (i.e., contribution to the advancement to atmospheric measurement technique is unclear). The reviewer is not sure if the manuscript fits well with the scope of the journal. The reviewer leaves this concern to the editor. Regarding the main content of the manuscript, I only have a few minor comments.

Line317

The authors report emission of HCl. Is Cl included in fuel, or other material of the heater?

Line336

Emission of SO2 is reported. Could the authors clarify the sulfur contents of the fuels that were employed for the experiment? It could influence the emission factor of SO₂.

Line380

It is interesting that usage of the renewable diesel influences the emission of OGCs. Could the authors add further descriptions about the cause?

Line 369

The reported particle number concentration is in the range of 10^7 # cm-3. Could the coincidence issue influence particle counting by the CPC for this high concentration range? If so, how much?

Line 430

Units are missing.

Figure 7

It would be interesting to show organic mass spectra, and discuss the data in detail.