(CASAC) Peer Review of EPA's Integrated Science Assessment

Dr. Christian SeigneurComments on SOx ISA – 2 nd DraftCharge Question 2. Have these revisions to Chapter 2 improved its assessment of thecurrently available scientific knowledge on atmospheric sciences and exposure and itsrelevance to the evaluation of human health effects presented in later chapters?The second version of the ISA for the health criteria of sulfur oxides shows significantimprovements compared to the first draft version. Comments made last year on thediscussion of the physico-chemical processes that govern the evolution of sulfur oxides inthe atmosphere have been addressed in this revised version. Therefore, the followingcomments on the second version tend to address points of detail or clarification ratherthan fundamental issues.Chapter 2. Source to tissue doseSection 2.2, p. 2-3: “Because the saturation vapor pressure of H 2 SO 4 is extremely low, itwill be removed rapidly by transfer to the aqueous phase of aerosol particles and clouddrops”. This is true only if aqueous particles or cloud drops (or fog droplets) are present.For example, under dry conditions, gaseous sulfuric acid may be transferred to particlesto form dry particulate ammonium sulfate.p. 2-20: The SO 2 and sulfate concentration maps shown in Figures 2-8 and 2-9 originatefrom the CASTNET data using some spatial interpolation methodology. Note that EPA’sClean Air Market Division (CAMD) has developed some data fusion approach thatcombines the CASTNET data with a CMAQ simulation to produced concentration mapsthat are improved compared to the simple CASTNET interpolation maps (CAMDcontact: Melissa Rury).Annex BCTMs are sometimes referred to as chemistry-transport models (e.g., p. B-15, line 1),sometimes as chemical-transport models (e.g., Title of Section B-5). It may be preferableto use a single term throughout the document.p. B-7, lines 12-13: The statement that transportation related sources only have a minorcontribution to SO 2 ambient levels suggests a strong east-coast bias. In California, wherepoint sources have low SO 2 emissions compared to the eastern U.S., ships can be a majorcontributing source to SO 2 concentrations. As a matter of fact, it would be appropriate tohighlight the recent State of California regulation on sulfur content authorized for dieselfuel of ships that are within the proximity of the Californian coast.p. B-17, line 6: The Weather Research & Forecast (WRF) model is now the preferredmeteorological model; add it in the parentheses before RAMS.42