GAW Report No. 205 - IGAC Project

CHAPTER 5 – NORTH AMERICAand small scales to determine local O 3 concentrations. The relative contribution from each scale,from local to inter-regional, can vary widely between episodes. This understanding has led tocoordinated multi-state abatement strategies within the US.Figure 19 - Conceptual diagram illustrating the multiple, concurrent scales of transport winds typically observed duringhigh ozone events in the Northeast US [after Blumenthal et al., 1997]The proximate location of urban areas in the northeast urban corridor has made theimportance of transport processes particularly obvious in that region, but even in the relativelysparsely populated western US, regional transport makes significant contributions to the urbanozone concentrations. For example, recent studies in Texas demonstrate that the transport ofozone from upwind regions can predominate over in situ production within the Houston urban area,even during O 3 exceedance conditions [see Parrish et al., 2009b, and references cited therein].Indeed, the transported ozone in eastern Texas, which represents the minimum ozoneconcentration that is likely achievable through only local controls, can approach or exceed thecurrent NAAQS.Pollutant transport also is important on intercontinental scales. A large fraction of NorthAmerican emissions, as well as the resulting ozone and aerosol produced over the continent, istransported beyond the national borders, primarily to the Gulf of Mexico and North Atlantic regionsand potentially on to Europe. Significant recent research efforts have been directed towardquantifying the importance of this pollutant export from the Northeastern US [see Fehsenfeld et al.,2006 and references cited therein.] The pollution plume from Mexico City also can be observedseveral hundreds of kilometres downwind. Figure 20 compares the increase in O 3 relative to CO inpollution plumes exported from these two regions. Air masses close to urban areas exhibit shallowslopes that steepen during downwind transport. Interestingly, quite similar slopes are seen in theaged plumes transported from Mexico City (0.35) and New York City (0.38). Aircraft-basedmeasurements show ongoing production of SOA [Kleinman et al., 2008; DeCarlo et al., 2008] aswell as ozone for several days downwind, with active photochemistry sustained by aldehydes [Tieet al., 2009] and nitrogen oxides from the thermal decomposition of peroxyacyl nitrates andphotolysis and OH oxidation of nitric acid [Neuman et al., 2006].Intercontinental transport into North America gives an added complexity to Los Angeles airquality issues. There is strong evidence that the background O 3 concentration transported intoCalifornia is increasing [Parrish et al., 2009c], possibly in response to increasing Asian emissionsof O 3 precursors. This background O 3 increase may be occurring throughout the northern midlatitudetroposphere [e.g. Parrish et al., 2009c], and this increase may be negating many of thebenefits from local pollution control measures in California [Jacob et al., 1999; Lin et al., 2008].The impact of transport of background ozone on surface air quality is a matter of considerable186