Comment on “The 2003 North American electrical blackout: An accidental experiment in atmospheric chemistry” by L. T. Marufu et al.

[1] In order to assess the air-quality impact of the 14 Aug. 2003 blackout in the United States and Canada, Marufu et al. [2004] compare air-quality measurements over Selinsgrove, Pennsylvania on 15 Aug. 2003 to concentrations measured over the same site during reportedly similar synoptic weather conditions on 4 Aug. 2002. From these comparisons, Marufu et al. found that the SO2 and O3 mixing ratios and particle light scattering near the surface at Selinsgrove were lower in 2003 than in 2002 by >90%, 50% and 70%, respectively. The authors report measurements of CO and particle light absorption, provide results of back trajectory models, and estimate the influence of reductions in power plants emissions on air quality at Selinsgrove. Ultimately, the authors conclude that the decreased concentrations are ‘‘predominantly due to reductions in power-plant emissions hundreds of km upwind of the study area’’ and, through forward-trajectory analysis, conclude that ‘‘these improvements in air quality benefited much of the eastern U.S.’’ [2] Clearly, the decrease in power-plant emissions lowered concentrations of directly emitted species, i.e., SO2 and NOx, from levels that would have been experienced otherwise. It may be reasonable to expect regional improvements in O3 and light extinction from levels observed during seemingly similar meteorological regimes in the past as a result. However, a quantitative estimate of the effect of the 2003 blackout – including the impact on SO2, NOx, secondary species such as O3 and sulfate, and visibility – cannot be estimated solely from the data presented in the manuscript. Even in a qualitative sense, a review of the manuscript indicates that there are problems with the representativeness of the aircraft data that should be taken into account before interpreting the Selinsgrove data and inferring the impact of the blackout on air quality. [3] The most important limitation of the manuscript is the failure to consider the variability associated with concentrations of atmospheric species. Marufu et al. [2004] state that synoptic weather patterns were fairly similar in the Selinsgrove region during the measurement flight in 2002 and the blackout measurement flight of 2003. However, this does not necessarily mean that concentrations of atmospheric species should be similar. For example, a comparison of observed O3 maps from EPA’s AirNow archives shows that O3 levels during 4 Aug. 2002 were markedly higher over Pennsylvania, Maryland and New Jersey than during 14 Aug. 2003, i.e., the day of the onset of the blackout – a day that was also synoptically similar to 4 Aug. 2002 and whose peak O3 concentrations would have been unaffected by the blackout. Moreover, local meteorology could have influenced the differences in air quality at Selinsgrove between 4 Aug. 2002 and 15 Aug. 2003. The dissimilarity in the backward trajectories shown for Selinsgrove, which differ by 30 to 80 depending on the start height of the trajectory, demonstrates that local dissimilarities in atmospheric conditions can be large enough to result in a large variability in SO2 and O3 mixing ratios. [4] Another aspect of the variability associated with the concentrations of atmospheric species is the regional representativeness of a single location: a relative change in concentration at one location may not be representative of relative changes over a broad geographical region. This can be checked by comparing 24-hour PM2.5 data from EPA’s Air Quality System (AQS) (http://www.epa.gov/ttn/airs/ airsaqs) for the day of the onset of the blackout to similar measurements on the day after; since 14 Aug. 2003 and 15 Aug. 2003 had synoptically similar meteorology (a slowmoving high pressure system), ‘‘persistence’’ would have dominated PM2.5 levels in the region. (Although some power generation units tripped in Pennsylvania during the blackout’s cascade, the transmission grid isolated these outages and electricity distribution within the state was fairly unaffected, except for Erie, PA. Therefore, measurement data from EPA’s AQS monitors are available.) In addition to addressing the issue of the representativeness of a single location, this analysis also provides insight into the potential impact of the blackout on PM2.5. For 15 stations with valid data for both days within the state of Pennsylvania, PM2.5 concentrations are found to decrease by an average of 8%, ranging from a decrease of 31% to an increase of 37% (Table S1). As shown in Figure 1, sites with moderate PM2.5 levels exhibited little change in particle concentrations, whereas those sites with higher PM2.5 levels exhibited greater variability in the day-to-day comparisons. Considering the 10 sites within 200 km of