Review of “Locating and quantifying CH4 sources within a wastewater treatment plant based on mobile measurements” by Yang et al. 

General summary

This study conducted CH4 mobile measurements in a wastewater treatment plant in summer and winter of 2023 and utilized a multi-source Gaussian plume model along with a genetic algorithm inversion framework to locate major sources within the plant and quantify the corresponding CH4 emission fluxes. Similar to previous studies, they found that emission estimates based on their inversion framework were higher than those estimated using traditional IPCC methods. They also found that the emissions are higher during summer than winter. Given the important role of CH4 in climate change, these types of studies are essential. So, the paper is within the scope of ACP. However, the paper lacks some key details which makes it hard to understand the experimental design and the inversion framework. 

Major comments

Figure 1: Can you please explain how the numbers for different components are defined in Figure 1? I see secondary clarifiers 1, 2, and 5 but not 3 and 4. It would also be helpful to show the roads on which you drove the mobile van. 

Lines 147-149: Why were two days of data left out? How were the monitoring days determined? What were the meteorological conditions during the measurement days and were those conditions representative of typical summer and winter conditions?

Lines 210-211: Can you please provide more details about how the initial emission estimates are derived? Are they derived in Section 2.3? Since the sources are so close to each other, there is a high possibility of plumes overlapping with each other and the observed concentrations being affected by multiple sources. Can you please explain how this overlapping issue was addressed for the 12 sources considered in the multi-source Gaussian plume? 

Line 223: What about the emissions upwind (e.g., secondary clarifier 2, power sanitation 1 etc.) of primary qualifier 1? How are those removed from this line source? 

Equation (2) and lines 240-253: How are the values of different parameters determined using the observations? 

Minor comments

Line 26: What do you mean by “emission data” here? Activity data, emissions factors? 

Line 28: Suggest replacing “ in combination with” by “using” 

Line 35: Since measurements are done only during 10 days, I recommend reporting the emissions in tons/day rather than tons/annum. 

Line 63: Suggest adding “,” after “ emission factors” because emission factors and activity data are different parameters. 

Line 195-196: Please mention the TOW value deduced from the workbook. 

Lines 220-221: Screen 1 and primary qualifier 1 are located diagonally from each other. Can you mark this road in Figure 1? 

Lines 316-317: Were higher background concentrations in December due to shallower boundary layer? 

Lines 361-362 and Figures 3-4: How are the emission source locations determined? Are they known a priori?