GAW Report No. 205 - IGAC Project

CHAPTER 8 – KEY ISSUES AND OUTLOOKThe sources of organic aerosol (OA) are currently the subject of vigorous debate in thescientific community. The consensus is that most OA in rural areas is not directly emitted, butrather formed in the atmosphere from condensation of oxidized VOCs (secondary organic aerosolor SOA) [Zhang et al., 2007]. Radiocarbon dating of OA typically shows a very high fraction ofmodern carbon, suggesting that most SOA is formed from biogenic VOCs [Bench et al., 2007]. Onthe other hand, SOA typically correlates very well with the oxidation products of anthropogenicVOCs, suggesting that urban emissions play an important role in its formation [de Gouw et al.,2005]. Several studies indicated that there is much more SOA in urban air than models canaccount for [de Gouw et al., 2005; Volkamer et al., 2006]. Figure 6 summarizes some of the recentresearch aimed at explaining the observed levels of SOA using detailed chemical models. Thegraph shows that models underestimate SOA by 1-2 orders of magnitude over a wide range of thedegree of photochemical processing of an air mass. Given the relatively poor understanding of thesources of OA, it is very difficult to predict how a reduction in the emissions of precursors will affectthe levels of OA in the atmosphere. Since OA accounts for such an important contribution to totalPM levels, understanding OA sources is a high priority for establishing effective PM controlstrategies.Figure 6 - The ratio between measured and modelled secondary organic aerosol (SOA) as a function of photochemical age,i.e. the degree of photochemical processing, of a polluted air mass [Volkamer et al., 2006]It is important to note that currently developing megacities can benefit from the experiencesof megacities that developed earlier. An excellent example is provided by comparison of theevolution of maximum ozone and PM concentrations in three similarly sized megacities (Figure 7).Ozone concentrations peaked in Los Angeles about 1970 and have decreased over the followingfour decades. PM concentration measurements started later, but seem to have followed a similartrajectory. In Mexico City, a later developing megacity, the ozone concentrations apparently neverreached the peak concentrations observed in Los Angeles and dropped more rapidly, approachingLos Angeles concentrations. PM concentrations in Mexico City have approximately paralleled theozone concentrations. Evidently Mexico City avoided some to the most severe air pollutionproblems experienced in Los Angeles through implementation of emission controls beforeproblems became so severe. In contrast, pollutant concentrations in Beijing may be following adifferent trajectory; PM concentrations are decreasing, but ozone concentrations, which have beenlow, are evidently rising. Increased attention is being paid to air quality concerns in Beijing, and itwill be very enlightening to see how ozone concentrations evolve there in the future.Correlations between ambient concentrations of air pollutants can yield importantinformation regarding source emissions. For example, Parrish et al. [2009a] report ambientmeasurements of hydrocarbons, carbon monoxide, and nitrogen oxides from three megacities(Beijing, Mexico City, and Tokyo) and compare them with similar measurements from US cities inthe mid-1980s and the early 2000s (Figure 8). The common hydrocarbon pattern seen in all datasets indicates that emissions associated with gasoline-fuelled vehicles dominate in all of thesecities. This commonality suggests that vehicular emission controls are important to begin as soon289