This breakout session covered a wide range of topics related possible future field campaigns. The issues are given in the following order: (1) transport, (2) aerosols, (3) oxidants, (4) interactions of aerosols, oxidants and transport, and (5) other.
1. Transport
Although there are diurnal variations in almost all emissions (e.g., daytime maxima in biogenic emissions, twice-daily maxima of emissions associated with automotive exhaust), we usually expect little variation from one day to the next. Yet oxidant levels vary dramatically. This raises the question: what aspects of the meteorology are most significant is taking these relatively constant emissions to produce large changes in ozone from one day to the next? The main suspects are wind speed, mixing layer heights, possible venting from the boundary layer, variations in solar intensity, and temperature. Although these are not independent variables, it is not clear what their relative ranking would be for contributing to a specific air quality episode.
2. Aerosols
2.1 Key Characteristics of Urban Aerosol Plumes
Aerosols are increasingly implicated in matters of health via their direct impact through respiration and through their secondary impact on oxidant levels. In order to evaluate their significance in these areas, an improved knowledge is needed of aerosol composition relative to emissions. How does composition change downwind of a source? One would expect evolution in size, composition and number density, but quantitative results are very limited. Aerosol characterization is further hindered by our poor understanding of formation processes that contribute to the relative abundance of primary aerosols, which are formed via mechanical or combustion processes, and secondary aerosols, which areformed via gas-phase condensation.
2.2 Role of Aerosols in Photochemistry
Aerosols have been implicated in the concentration, life-time, and chemical behavior of sulfur, nitrogen and carbon compounds. They are also thought to provide reaction sites and to serve as carriers for many atmospheric trace gas species. Yet much work remains to define the gaseous species that lead to aerosol formation, and to identifying key surface reactions under various conditions, e.g., polluted urban air versus relatively clean marine air.
3. Oxidants
3.1 Systematic Features that Characterize Regions where Ozone Production Rates are Limited by NOx Versus VOCs
A key question facing regulators is the likelihood of success if restrictions are placed on emissions of NOx (NO + NO2) and volatile organic compounds (VOCs). Modeling studies and so-called "indicator species" have been used to evaluate the efficacy of these proposed emission reductions in various parts of the country. Yet the uncertainty level is still large. It now appears that specific regions are sometimes NOx-limited in their ability to produce ozone and VOC-limited at other times. Identifying which regions and when they have these limitations is recommended as one of the goals future ACP field campaign.
3.2 Role of biogenic emissions in Affecting Oxidant Levels
Reductions in anthropogenic hydrocarbon emissions are part of the strategy to lower ozone levels over broad regions of the United States. However, if biogenic hydrocarbons dominate the production of ozone, then it is likely that costly reductions in anthropogenic VOC emissions might have negligible impact on reductions. A key biogenic emission is isoprene, which has a very high reactivity with hydroxyl and appears to be relatively abundant throughout much of the eastern United States. Understanding the role of isoprene and related biogenic hydrocarbons is identified as a critical step in the development of any control strategy. And while the oxidation rate of isoprene is relatively well known, and the associated mechanisms understood, the same cannot be said about many other organics (e.g., terpenes). Further, the relation of many primary biogenic emissions to other organics (e.g., peroxides) is not clear.
Related to an improved understanding of biogenic chemistry are relatively simple observations related to their distribution and emission rates. Observations of their concentrations over a range of altitudes and geographical regions would provide very useful information for models used in both scientific and policy analyses.
4. Interactions of Transport, Oxidants and Aerosols
4.1 Influence of the Vertical Structure of the Boundary Layer On Ozone Production
Variations in mixing and insolation results in pronounced differences in chemical processes between night and day. During the day, insolation warms the surface of the earth, resulting in active mixing throughout a depth of a few kilometers. This well mixed system usually has ample solar radiation to initiate a range of photochemical chains that can result in the high ozone levels. After sunset, radiative cooling can result in thermal stratification over broad geographic areas, with the consequence that surface based emissions are trapped in a very shallow layer. This decoupling of the surface with the free atmosphere also brings about the termination of mixing aloft, with the consequence that the chemical structure of the lower atmosphere can become highly stratified. Rather than being viewed as a well mixed reactor, the lower atmosphere becomes a series of layered chemical systems, each having a unique ozone destruction rate. While the theories of daytime (photolytic) chemistry in a daytime (convective) boundary layers are relatively well understood, the processes at night are generally known only through empirical analysis of surface observations. Quantifying any of these processes in future ACP field campaigns is recognized as an important contribution to our understanding of air chemistry.
4.2 Variation of Ozone Production with Altitude and Time of Day
Topics related to the broad question of variations in ozone production with altitude and time of day include the role of early morning entrainment on ozone production, the extent of vertical mixing of NOx and VOCs emitted at the surface, and the representativeness of surface observations to ambient air quality. A related question is whether an enhanced understanding of vertical boundary mixing allow time-shifting of emissions be used in ozone control strategies.
4.3 Role of Multi-day Stagnation Events on O3 Episodes
Surface air quality stations frequently report ozone mixing ratios of 60 ppb on one day followed by values well over 100 ppb on the next. Frequently there is little change in the meteorology between the such days, which leads to the question of whether the accumulation of substances in the lower atmosphere has a dominant role in generating episodes. If so, which species need to be carried over from the preceding day, e.g., NOx, PAN, other radical sources? If not, then what meteorological and chemical conditions allow an ozone episode to be created from the processing of a single day?
5. Other Issues
5.1 Interpretation of Field Data with Models
As in past meetings, there is a request from the modelers to have more input in the planning of field campaigns and to participate in the analysis of the resulting data. Field scientists respond by issuing a general invitation to all modelers to do both. The only criteria for participation in the studies is a scientific question at least partly amenable to resolution by the available field measurement capabilities. Linking up with a field scientist with a common interest is suggested as the most efficient way to bring modelers into the field campaigns.
Ease of accessibility to the data sets is also identified as a limitation to modeler participation. At present, the data usually resides on the workstation of the scientists who made the observations. It is recommended that ACP provide a community posting of data previously reviewed for accuracy and that the investigator feels can be made accessible to the ACP modelers.
What kind of measurements do the modelers need? Repetitive measurements are considered highly desirable because a set of observations from one site, taken once, are very difficult to interpret. Modelers also express a strong interest in having field campaigns at revisited sites, with vertical profiles used to characterize the ambient chemistry and meteorology, in contrast to aircraft sampling over longer horizontal domains.
5.2 Ancillary Measurements for Field Campaigns
ACP field campaigns need to provide simultaneously state-of-the-art measurements from research aircraft, complementary observations at the surface, and measurements of conditions between the surface and the minimum sampling altitude of the aircraft. Leveraging ACP efforts with collaborators from other programs is thought to be the most efficient way to meet this goal.