CO Editor

Suggestions for revision or reasons for rejection

The current version of the manuscript is lack of solid evidences and full of speculation and arguments. What the authors selected the statistical analysis tool appears to be quietly at will, at least to this reviewer. The authors failed to demonstrate why the selected statistical analysis tool can allow addressing the scientific questions associated with their specific data sets. The modeling results were not fully used while the authors presented a bunch of non-evidence supported arguments. The analysis of formation of particulate NH4+ is also not convincing. This reviewer cannot recommend the current version for publishing in ACP, a high quality Journal. A few comments are listed, but not just limited these.

Page 1, Line 37, “associated with a slower formation of particulate NH4+ in the atmosphere from gas-phase NH3.” Formation rate of particulate NH4+ in the atmosphere highly varies, depending on formation pathways. Gas-particle condensation is only one, but not necessary to be the major one. Did the authors verify the importance of formation of particulate NH4+ via the route?

Page 2, lines 32-34, “While it is clear that reductions in NH3 emissions will lead to reductions in overall NH4+ concentrations, the relative changes in gaseous NH3 and NH4+ particles remains poorly quantified” It is only true for the NH3-limited formation of ammonium salts. The reference is thereby needed to demonstrate when and where.

Page 8, lines 31-32 “The observed variability is consistent with the large regional variability in NH3 emissions and sources (Figure 4c & d).” At least to this reviewer, it is hard to find this.

Page 9, lines 7-9 “The limited variation across the UK for the annual average NH4+ concentrations can be attributed to the atmospheric formation process (providing a diffuse source) and its longer atmospheric lifetime” This is very speculative, although the argument is one of potential causes.

Page 10, line 10, why use “for an example year of 2012 s” but not 2005 as used early?

Page 10, lines 13-15 “The scatter may be explained by the large local spatial variability of NH3, related primarily to rapid decreases of NH3 concentrations with distance from a source (see e.g. Pitcairn et al., 1998; Dragosits et al., 2002),” How come the references in ten year ago can support the interpretation of the difference in 2012? Is it still true and any evidence to say this?

Page 10, lines 10-29, the agreement is of course more important than the correlation and should be discussed.

Page 10, lines 39-41 “This suggests either too low a formation rate for NH4+ in the model at cleaner sites, or too high a removal rate for NH4+, or a combination of both. The presence of higher measured NH4+ concentrations in remote areas than shown by the model may also indicate that NH4+ has a longer residence time than treated in the model.” This is very speculative. Under prediction of particulate NH4+ by air quality models is very common in the remote clean atmosphere due to those simple assumptions.

Page 11, lines 19-24, all arguments are absolutely correct, but then what? In other words, how these directly explain the seasonal variation?

Page 11, lines 28-29 “A smaller peak in NH3 can also be seen annually in April, which indicates potential longer range influences of manure spreading in spring, even at this remote location (Figure 8b).” The argument is also very speculative. Solid evidence is needed.

Page 12, lines 5-6, “Interestingly, the dip in concentrations in June matches a period when crops will be actively growing with possible uptake and removal of NH3 from the atmosphere.” Evidence but not just arguments!

Page 12, lines 10-13 “Although the formation of particulate NH4+ primarily depends on the occurrence of NH3 in the atmosphere, synoptic meteorology and long range transboundary transport from continental Europe are important drivers influencing the seasonal variations of NH4+ across the UK, due to its’ longer lifetime.” The part is totally confused and which studies support these?

Page 12, lines 10-35, it is odd. Why not use modeling results,.e.g., processing analysis, to interpret? It is very straight forward by comparing with current arguments.

The analysis in Section 3.5.1 does not sound scientific when analytic errors were considered. The same concern is applicable for Section 3.5.4.

Section 3.5.6, a bunch of arguments are redundant.

Interactive discussion

Peer-review completion