|"Intra-urban spatial variability of surface ozone in Riverside, CA: viability and validation of low-cost sensors", Sadhighi et al.|
This manuscript has been improved dramatically. The decision to remove the CO2 analysis allows more focus and precision on the O3 testing, validation, and interpretation. I have very few mostly wording fixes/comments below.
P 3, L8: "these sensors" - is this referring to metal oxide sensors specifically, or any low-cost ozone sensor? Just repeat and make clear.
p3, L13: Seems odd to say "regional air quality body" without naming it. Is it the EPA region or the SCAQMD?
P3, L16: Also odd to mention funding source here - should this be better located in the acknowledgements at the end of the paper?
p3, L20, perhaps say "low-cost metal oxide ozone monitors"? Or low-cost ozone monitors at least, to differentiate from pm2.5 which is mentioned right before.
P6, prior to the results, I was expecting another short section on "deployment",explaining which sensors were deployed and where. Presumably the three sensors noted in the "validation" section were kept near reference monitors for "validation", but the rest were deployed throughout the region as in Fig.1. A few sentences could explain this and refer to the timeline in Fig.1 .
p8, L15, this is the first mention of Linear 4, 4T, etc. Perhaps what they are should mentioned here, or prior to Eq 1? (is Eq 1 Linear 4T?). [this was a comment in the first round, which the authors say they addressed, but not sufficiently].
p8, L18l should be Masson et al.
p9, L18 should have a comma after "contant pressure value"
p9, L16 - P10 L2. This seems odd and a bit arbitrary, to filter the UPod data just based on a maximum value seemed unrealistic. Do you have reason to think the sensors fail for some reason and read higher values than they should? Did this occur during the calibration phase, or for the validation data? Are they discontinuities from a time series (i.e. perhaps the ozone reading jumps suddenly)? At 170 are they correct?
I see the response to my original comments explain this a little more, but the amount of data points affected and final effect could be mentioned in the text as well for the other readers.
p10, lines 4-9 - this seems reasonable, so I wonder why the absolute magnitude was needed to be filtered as well. Did the high (> 171 ppb) values occur in the paired sensor in the same location as well? If not, then perhaps some agreement between pairs could be used as a data quality indicator?
P14 L5 & L9, should be "Gao et al." in both places.
P14 L9 should refer to Fig. 7
Here (Fig 7 and text) and elsewhere you should be clear with the word "concentration". I believe these are mole fractions or mixing ratios (not sure which). A concentration is a mass (or number of moles, if molar) per unit volume and would be in units of moles per liter or grams per liter. At the very least, they should define that they will be using the word concentration to mean moles of ozone per moles of dry air (or whatever is appropriate).
p15, L6-10: Although now I am clear that "collocated" = calibration period and spatially distributed = deployment period, this paragraph could be even clearer, for example:
"U-Pod ozone measurements are more correlated to each other during the calibration period (when they were collocated) than during deployment, when they were spatially distributed through the region. The R2 values between collocated pods are very high, with their medians varying from 0.92-0.99 ppbv. Conversely, during the deployment period, the spatially distributed pods were less correlated with each other, leading to R2 distribution medians between 0.52 and 0.86. "
P16 L23 should be "indicating"
Fig S1 typo on y-axis (magnitude)
In the text, often references refer to the authors and then include them again in the citation, that should be fixed. Example p7, line 10, "Barsan and Weimar (Barsan and Weimar, 2001)" should just read Barsan and Weimar (2001) ....
There seem to be some references in the final list that are not referred to in the paper; most seem to be left over from the original manuscript version which included CO2 (Kort, Hutyra, and others). They should be checked thoroughly and removed.