Review of Zeider et al, “Differences in aerosol and cloud properties along the central California coast when winds change from northerly to southerly,” ACP

SUMMARY

This study utilizes ground-based and airborne in situ measurements and two different numerical models (one a global, coarse-gridded aerosol model with prescribed dynamics, the other a mesoscale dynamical model with interactive aerosols) to more deeply examine the assertion that coastally-trapped disturbances (CTDs)/southerly surges result in enhancements of aerosol and cloud droplet loading in near-coastal regions. Satellite measurements (the primary source of observational data used in previous studies on this topic) are also utilized for broader context. The airborne measurements are from a number of local summertime field experiments spanning about a decade, but the rarity of CTDs and the fact that none of the campaigns considered were explicitly focused on sampling them means there are relatively few cases analyzed (17 out of 114 research flights across 6 field campaigns). This is even more so the case for the ground-based measurements due to the fact that sampling was only performed every 3 days.

The authors find a rather variable relationship between CTD occurrence and aerosol loading compared to Juliano et al. (2019), who examined satellite retrievals and reanalysis data and specifically selected “strong” CTD cases. Perhaps this should not come as a surprise given that the region sampled by the aircraft (typically in the vicinity of Monterey Bay) is shown by Juliano et al. (2019) to have a more muted microphysical response than locales farther offshore and to the north (see their Fig. 5). Despite the more variable CTD/non-CTD relationship, the authors do generally find enhanced aerosol loading (especially in the accumulation mode) and cloud drop number concentration, with further supporting evidence from cloud water composition measurements that show enhanced concentrations of anthropogenic combustion tracers (e.g., NO3-, V, elemental carbon). A few case studies seek to explore the observed relationships in greater detail using HYSPLIT back trajectories and aircraft measurements. I did not find these case studies to add much to the discussion, and in fact they may raise more questions for readers (see Major Points below).

Overall, the study is worthy of publication in ACP after the authors address the comments below. I look forward to seeing the next iteration of the paper.

Mikael Witte

MAJOR POINTS

Case studies:

The NiCE case study appears to have been a relatively weak (or perhaps incipient?) CTD based on the back trajectories shown in Fig. 6. While it is obvious that Na is considerably greater during the westbound segment of the flight, it’s hard to buy the argument that this was due to airmasses spending a significant amount of time in a busy shipping lane. The back trajectory in Fig 6c shows the airmass spending a decent amount of time offshore of Big Sur, but to my knowledge this is not exactly a major shipping lane. I also find it hard to believe that pollution from LA/Long Beach is making it this far north. So what other sources of pollution could be impacting this area? My guess is that the NAM may not be capturing smaller-scale/intermittent offshore flow events along the Oregon and Northern California coasts, but I don’t see an easy way to prove this.

The CSM case, on the other hand, does exhibit substantial southerly flow both within and above the boundary layer, but from Fig. 3, the smoke source is apparently to the north and east! So are the extremely high smoke/aerosol concentrations a consequence of advection in southerly flow or more of a “ping-ponging” effect in which smoke is advected over the area with the (climatological) northerlies at an earlier time and then essentially held in place/re-circulated over your sampling region with the reversal to southerly winds?

The fact that these two cases are rather complicated points to a much more nuanced picture regarding how aerosols arrive at the marine boundary layer than is given by Juliano et al. (2019), and that was the main point I came away with from the case studies (and really, the paper as a whole). I think it would be helpful if your summary explicitly recognized this fact – it’s not like the wind flips to southerly and there’s an instantaneous and unambiguous increase in Na/Nd. The timing of a reversal, its strength and duration (which determine the southward fetch and impacts the probability of, e.g., SoCal-sourced particulate), and the availability of aerosol (from smoke, shipping, etc.) are interconnected factors that determine how much “extra” aerosol makes it into the boundary layer, and this was more or less glossed over in Juliano’s BAMS paper. If one of the points of your paper is to motivate further research on this topic, acknowledging how much the details matter in determining the unfolding of individual events would make for a stronger argument.

SoCal aerosol sources: I don’t see any clear evidence for an aerosol source from Southern California. Unless you can point to something(s) in the observations that would support this idea, I suggest you significantly scale back your discussion of this point and remove it from the conclusions. It seems just as likely that “local” emissions from the Bay Area (or more broadly, coastal Northern California) could be the source of the combustion tracers in the NiCE case study, and it’s obvious that wildfire smoke dominates the CSM case; perhaps you can find a clearer example of a back trajectory that passes over SoCal from another case?

COAMPS: I only see COAMPS data in one figure – did you analyze COAMPS output for any other days than the CSM case study? Do you have a general sense for whether NAAPS (as a proxy for “coarse-gridded models in general”) can produce proper southerlies vs. weaker northerlies? Given the very minor role COAMPS simulations play in the current version of the paper, I question whether the conclusions drawn from a single case study add much to the manuscript.

MINOR POINTS

(all refer to specific line numbers XX-YY, abbreviated LXX-YY)

L174: Is there any sensitivity of the results to your chosen LWC threshold?

L760-761: re: the number of outliers in southerly flow in Fig. 9d – do you have sufficient sampling to say with certainty whether these are “outliers” or are you undersampling the distribution of Na3:Na10?

TYPOGRAPHICAL POINTS

L90: “an important inventory…is leveraged…”

L91: “increased statistics” – unclear wording. I suggest “improved sampling” or perhaps “increased sampling density.” I noticed this wording elsewhere as well – doesn’t make sense to increase statistics themselves. What we really want is more data points.

L118: “spaceborne” vs “space-borne”

L174: why not “>” instead of “needing to exceed”? similarly, use “<” instead of text “less than”

L179-180: Suggest rewording to “The mode wind direction was calculated for…”

L752: “by increase N_a>10nm above the region…” (remove “of” from quoted phrase)

L783: suggest “different methods” vs “different ways”

L831: “Buoy” vs “Bouy”