Manuscript is very well-written, the study is a good exercise in source apportionment, that applies new concepts (land-water meteorology, stagnation events, and NH3 impacts to PM composition) to improve the hypothesized results. Only minor revisions are needed. 

FIgure 1 should include the NADP site ID for Beltsville site. The NH4 and NO3 data mentioned if taken from the CASTNET site is not regulatory (line 279). Only O3 is regulatory for that network. CASTNET should identifed as the data source.

Figure 7 mentions only dairy in PA and poultry in Maryland. Dairy is not large in Maryland, but there are some farms, since you are identifying moderate sources in downtown that are amplified due to the stagnation/low WS boundary layer events, could some of the MD dairy also be along the trajectory path? 

Where did the CO data come from that was used to eliminate the possible traffic source? 

NH3 needs a subscript in line 187 & 266. 

This paper describes measurements of ammonia and inorganic particulate matter constituents during the 2018 OWLETS-2 campaign. Overall, it is a clear, well-written paper that provides insight into the sources of ammonia, and impacts on particle composition in a coastal, peri-urban environment. I recommend that it be accepted for publication after the authors clarify a few points, listed below.

The use of HYSPLIT back trajectories to interpret the data and identify the likely source of the elevated ammonia is valuable, though as the authors note, it can be challenging in near-shore environments. I have two questions related to the trajectory analysis:

The authors indicate that the 3 km resolution HRRR data were used for trajectories (except when they weren’t). Then the analysis in Figure 4 shows trajectories from the lower resolution NAM dataset. Why was the NAM data used instead of the HRRR data in some cases? It seems like it would generally be less reliable.

On lines 111-115, the authors note that they compared the trajectory analysis to the local observations, but they do not indicate what, if anything, was done in the analysis if the observations and modelled trajectory disagreed.

A major factor that is emphasized in the analysis is that the gas phase ammonia concentrations at the Hart Miller Island (HMI) site are much higher than the three closest AMoN sites. Figure 2, and the corresponding text shows summertime (June, July, August) annual average concentrations from the AMoN sites in comparison with the one month of data from HMI. Are the authors confident that the differences are not, in part, caused by a difference in averaging period? In many locations, ammonia measurements are much higher in late spring than in the middle of summer. Why not average the AMoN data just for the time period that overlaps with the OWLETS-2 campaign?

Section 3.3 – in principle, the sum of gas phase ammonia and particle phase ammonium should be a better conserved tracer reflecting high emissions of ammonia. It appears that, in general, gas phase ammonia is present in much higher levels and particle phase ammonium, so the peaks identified in Figure 5 would also represent the main episodes of high total ammonia, but can the authors confirm?

Lines 227-235 I found the example provided about the stability classes a bit too narrow and specific to apply generally to the dataset. I believe that the authors are talking specifically about the impact of dispersion on the surface concentration enhancement? It might be good to clarify.

Detailed comments

I suggest including the dates of the measurement campaign in the abstract.

Line 162 – Figure XX needs to be corrected to the actual number