I think that the report presents significant effort and progress.  Their continued effort should be supported where needed to help them make their efforts more productive.  I think additional analytical support would greatly help in understanding and the maximizing the benefits of this new facility.

I think that measurement of HONO is critical.   Their HPLC can be used to implement the DNPH method of HONO measurement of Zhou et al[1]. Given their emphasis and dependence on HONO for their theory of their chamber wall and background reactivity model, and the demonstrated high-background-reactivity of their test chambers, it is worth the effort.  The calibration is non-trivial, but I think, worth the effort. The HONO method of analysis is simple to implement and run.  Even time-relative response data of HONO would be very useful (if calibration is not performed).

Given the conclusions (pg 62)[*] that the chamber radical sources involve light-induced heterogeneous processes producing HONO from NO2, it is arguable that, since it appears that their chamber will probably demonstrate significant background reactivity (which needs to be modeled), this is additional evidence for the need of a serious effort including analytical and theoretical surface chemists to develop an explicit wall mechanism including these heterogeneous processes.

In addition, I think that radical measurements should also be performed.  This would not have to be done for all experiments.  This could be done by collaborative assistance.

I think that given the attention given to temperature consistency and light uniformity, that attention should also be given to uniformity of temperature within and around both chambers, throughout a long period of time (typical experiment).  Multiple sensors should be used on both chambers continuously throughout every experiment.

Concerning the design of the chamber, I am concerned about the possible need for sealants. I am concerned about the plan for a collapsible chamber, which will increase the need for sealants as the group is (pg 67). Will light uniformity tests be performed for a full chamber experiment period to check for effects from the variable surfaces caused by a collapsible chamber? Have they considered a design, which uses a more constant geometry, and utilizes an interior bag (which is filled with clean air as needed), or movable diaphragm, to allow for replacement of the sample air?

I think that no compromises in performance should be made, in accepting the filter used with their light source.  I think that a chamber, which will be used to address atmospheric reactivity issues, should be able to reproduce the correct relative magnitude of the photolysis rates observed in the atmosphere.

The chamber background reactivity of their test chambers, seems high in terms of producing ozone in "pure-air".  Hopefully, the new air generator system which should lower the background hydrocarbons will help.  I am concerned with not only the possible use of sealants, but the plan to continue to use mixing fans and valves (page 21) which have some amount of "material" (unspecified) which may contribute to reactivity.  They were not able to detect any reactivity when used in an earlier chamber, but if that chamber was as reactive as their test bags, that might have been the reason.  These may be unsuitable for a chamber with stricter performance requirements.

Concerning the "low NOx"-level for study, and the need for a next generation chamber to study these systems, I think that it is important to consider the "performance requirements" such as "how much ozone is formed in a clean chamber", rather than, how much background NOx is found in the chamber. I think that the "congressional mandate" should be considered for at least the minimum performance standards (concerning level of maximum chamber background reactivity) and in this context, which atmospheres of are of interest (rural or urban) may be also considered.  I think that the criteria statement of "what is experimentally feasible" (page 8 of report) should have some qualifying phrase that is connected by some minimum performance standards, such as might be determined by some modeling sensitivity requirements.  In this regard, I think the recommendation, of the chamber design and program being peer-reviewed and assisted by the advisory group, is a good one, and will benefit their efforts.

It appears that determining photolysis rates results in uncertainty (page 33).  This should be checked by modeling of chamber experiments with compounds of  "better-understood" chemistry and photolysis rates.

The proposed research plan includes replicating some aerosol experiments conducted at CalTech, including experiments with a-pinene.  I'm concerned that certain aerosol experiments may leave the chambers with a much higher background reactivity compared to that observed after a typical "gas" chamber experiment.  This brings up the issue of disposable chambers and determining when to replace the chambers.

References

 1.  Zhou X., Qiao H., Deng G. and Civerolo K., A Method for the Measurement of Atmospheric HONO Based on DNPH Derivatization and HPLC Analysis . Environmental Science and Technology, 33, 3672-3679 (1999).

* Note: The above coments refer to a draft and the page numbers of report as distributed may be slightly different than indicated above.. However, the revisions were primarily editorial and the comments above are still applicable. - Bill Carter