The manuscript reports a comprehensive study of urban air quality in Helsinki, Finland. The work was carried out at two supersites using a bunch of instrumentation: traffic pollutant dominated measurement site and urban background station less affected by traffic. Interestingly, the urban station is less that one hundred meters further from the major traffic roads than the traffic station, but the observations were quite different. I have a few minor comments before its final publication.

Throughout the manuscript, measurement uncertainties should be added.

Line24-27, long sentence, please reword it.

L94-95, Please add references for the mentioned figures.

Section 2.2.1 Please make it clear if the intracavity Nd-YAG laser was used in the AMS. If laser was used, the RIE for BC should be mentioned too. How well does AMS-derived BC compare to other BC results?

L141, are you saying the transmission efficiency for the particles in the size range of 76-650 nm is 50%? This is not true, as the transmission efficiency is nearly 100% in the size range of 60 – 400nm.

Line 163, consist->consists

Line195-204, are the data used anywhere in the manuscript?

Section 2.2.1. The section does a good job describing the instrumentations. For a quick grab of the idea of this section, it could also be nice to tabulate all the instruments used at the traffic superstation. You can also add additional instruments not listed in Table S2 at UB station.

L376, Add a period at the end of bracket.

L385, it is unclear how the traffic frequencies were counted.

L410, which instrument was PN measurement from?

L422 Is SP-AMS derived BC concentration comparable to MAAP results?

L459, Please add reference(s) for your argument.

L472, can you guess the sources of a-pinene? In addition to the background concentrations, is it from regional or long-range transport?

L531, …are due to..

Section 3.1.6 PMF did a nice job in distinguishing six organic factors. A few more words describing the Tr-LVOOA, LVOOA and LVOOA-BB would also help to understand these three factors and their sources.

 General Comments

The study's goal of examining how cold conditions affect pollutants in Helsinki, Finland, using a variety of monitoring techniques, was evidently investigated. A comprehensive and well-rounded research plan is demonstrated by the use of two measurement sites and the integration of diverse methodologies such as source apportionment and particular analysis algorithms.  Even though it describes the pollution events that were observed and their possible causes, it could do a better job of highlighting the unique viewpoints and contributions that this specific study adds to the body of knowledge already available on urban air pollution in the winter.

Specific Comments

1. It would be beneficial to mention if there was any specific reason for choosing that particular "five-weeks" in January–February 2022 or if it was a random selection. Also, details about how representative that period is for wintertime conditions in Helsinki could strengthen the study's context.
2. The authors need to clearly explain the reason for restricting the mobile measurements (ATMo - Lab) to the daytime between 6:30 and 19:30. Since air pollution patterns can vary significantly between daytime and nighttime, and different sources might have different activity levels during these periods, it is essential to know how the exclusion of nighttime measurements might affect the source apportionment and understanding of overall pollution dynamics.
3. The authors must clearly describe the specific calculation method employed for determining the mixing height. Whether it's based on a particular theoretical model (e.g., a thermodynamic model, a boundary layer parameterization method, etc.), an empirical formula, or a combination of both, full relevant data and details should be provided.
4. The suggestion to incorporate a wind rose diagram at Fig. 2 has the potential to enhance the comprehensiveness and interpretability of the presented data.
5. The omission of standard values for the pollutant concentrations presented in lines 400 - 407. Without these reference values, it becomes extremely difficult for readers to assess the severity and significance of the measured pollutant levels. In addition to local standards, providing world or international standards (such as those set by organizations like the World Health Organization for certain key pollutants) would offer a broader perspective.
6. Apart from summarizing the primary elements derived from the source analysis, it is advised to conduct additional analysis on the variations in the contributions of various sources in various pollution incidents and time periods (e.g., weekdays versus weekends, and daytime versus nighttime), as well as to thoroughly investigate the dynamics of the sources and their connections with weather and traffic patterns.
7. Validate and supplement the source analysis results with other research methods (e.g., emission inventory data, PCA, etc.) to enhance the credibility and persuasiveness of the results.
8. The article has relatively little coverage on the chemical transformation mechanisms of pollutants during long-range transport. Research in this aspect should be strengthened. For example, by measuring the concentration changes of relevant precursors and products during the transport process, combined with the simulation of atmospheric chemistry models, analyze the generation processes and rates of secondary pollutants (such as sulfates, nitrates, secondary organic aerosols, etc.), and clarify the main chemical transformation reactions at different transport stages, so as to evaluate the impact of transport on the chemical composition and properties of pollutants more comprehensively.
9. For regional transport, insufficient attention has been paid to the impact exerted by the terrain and the characteristics of the underlying surface in the vicinity of the measurement sites on the transport of pollutants.

Technical Corrections

1. The lack of uniformity in the font used for pictures throughout the paper detracts from the overall professionalism and polish of the work.
2. Some of the sentences are longer and more complex in structure, which may cause some difficulty for readers to understand. For example: line 133–135.