|The authors have addressed most of my comments. However, there are two important points requiring author’s further consideration and follow up.|
First is on CIMS. More information on the sampling line needs to be provided. What is the length? How often was it cleaned? Although ClNO2 has small loss in the sampling line, N2O5 can be converted to ClNO2 (i.e., give positive bias) if the sample line is not washed frequently. Was this potential bias tested during the field study? If not, need to mention this interference to ClNO2. Also please provide information on CIMS’s sensitivity and detection limit or give a reference on them.
Another concern is on the attribution of second morning peak. I am still not convinced that it is caused by horizontal transport.
From the chemical point of view, the ClNO2 is formed after sunset, and such formation process continues throughout the night until sunrise. Starting from sunrise, the production of N2O5 and then ClNO2 should be negligible (due to the short lifetime of NO3 after sunrise), but the loss of ClNO2 from photolysis starts to govern the ClNO2 levels which results in decreasing ClNO2 in a certain bulk of air mass.
From the transport point of view, ClNO2 can be produced in the west coast of Korea during the night, and the westerly wind brings ClNO2 to the middle of Korea, and such transport should happen continuously. If the wind doesn’t change significantly during the second peak (as in the present study), the same bulk of air mass from the west coast should influence the observation site continuously. And according to the chemical budget of ClNO2 (in the previous point), the ClNO2 level in this bulk of air mass should decrease after the sunrise, and no second peak should be observed.
However, the second peaks were always observed between 7-8 a.m., not before nor after. Such phenomenon is most likely linked to the break-up of the nocturnal boundary layer and the entrainment process after the sunrise. In fact, there is evidence for this downward transport. In figure 6(b), for the second morning peak, ClNO2 increases concurrently with ozone and SO2, which can be interpreted as transport of ozone and SO2 (from elevated point source) from the nocturnal layer. The aircraft measurements at 8:00 did not record elevated ClNO2, maybe because the aircraft did not sample the ClNO2 rich airmass at right location/time.
Therefore, I advise the author to carefully re-consider the explanation of the second morning peak.
1. Response (1) and the corresponding changes in the manuscript, “during a severe have event” should be “during a severe haze event”.
2. Response (20) and the corresponding changes in manuscript, “ClONO2 and HOCl were set to 0.06”. Do you mean “the uptake coefficients of ClONO2 and HOCl were set to 0.06”?