O3 Precursors Research Articles

Three‐dimensional model studies of exchange processes of ozone in the troposphere over Europe

A three‐dimensional Mesoscale Chemistry Transport (MCT) model driven by meteorological data from a Numerical Weather Prediction (NWP) model is used to calculate ozone and other chemical species over Europe over a 10‐day period. The meteorological model uses cloud water as one of the prognostic variables and has an advanced treatment of stratiform and convective cloud and precipitation processes. In this way an improved resolution in time and space of cumulus cloud episodes is obtained with a better simulation of the convective transport as a consequence. It should also improve the computations of the photolysis rates which are highly dependent on cloud cover and cloud optical depth. The 10‐day period (July 1–10, 1991) is characterized by warm weather and frequent occurrence of cumulus convection. The model results are compared to ground‐based ozone measurements and ozone profiles. The results presented suggest that physical processes, especially convection, may dominate in the vertical distribution of ozone in the free troposphere, that sinking air which compensates for convective updrafts is important for the tropospheric ozone budget, and that transport of O3 precursors from the boundary layer into the free troposphere by convection enhances the rate of O3 production significantly. However, considerable uncertainty of the absolute magnitude of the convective mixing exists.

Interannual variability over the eastern North Atlantic Ocean: Chemical and meteorological evidence for tropical influence on regional‐scale transport in the extratropics

Observed boreal fall season (September–November) 1991 surface CO data from Mace Head, Ireland, are characterized by particularly high mixing ratios, as evidenced by high means, medians, and maxima for those months, relative to the same dato for boreal fall 1992. Air parcels characterized by elevated CO during fall 1991 are attributed to European sources on the basis of isentropic back trajectory analysis. A histogram of the 1991 data shows a bimodal distribution representing two discrete source regions, North Atlantic and European, while the 1992 data show only one mode, representing primarily zonal westerly flow over the North Atlantic Ocean. A similar distinction exists in O3 data between the two years. Considerable interannual variability has important implications for observationalists and modelers alike; an inherent uncertainty is introduced by basing any determination of trend from only a few years of data. An isentropic flow climatology for Mace Head illustrates significant differences in the regional‐scale flow patterns to Mace Head between the 1991 and the 1992 fall seasons. These differences have been examined in terms of general dynamical principles and tropical/extratropical coupling. There is evidence of the existence of Rossby wave links with the tropical upper troposphere over South America and the central Pacific Ocean which are responsible for the climatic forcing giving rise to the observed interannual variation in large‐scale flow patterns and influencing the chemical character of air parcels reaching Mace Head. Using CO as a tracer for short‐lived continental anthropogenic O3 precursors and calculating ΔO3/ΔCO for air parcel trajectories following anticyclonic paths over western Europe during the late summer and fall season of 1991, we estimate O3 production over western Europe at about 66 (40–96) billion moles of O3 per summer (∼3 Tg O3 per summer), based on 1985 CO emission inventory dam (37 Tg CO yr−1 for western Europe).

Convective transport over the central United States and its role in regional CO and ozone budgets

We have constructed a regional budget for boundary layer carbon monoxide over the central United States (32.5°–50°N, 90°–105°W), emphasizing a detailed evaluation of deep convective vertical fluxes appropriate for the month of June. Deep convective venting of the boundary layer (upward) dominates other components of the CO budget, e.g., downward convective transport, loss of CO by oxidation, anthropogenic emissions, and CO produced from oxidation of methane, isoprene, and anthropogenic nonmethane hydrocarbons (NMHCs). Calculations of deep convective venting are based on the method of Pickering et al. [1992a] which uses a satellite‐derived deep convective cloud climatology along with transport statistics from convective cloud model simulations of observed prototype squall line events. This study uses analyses of convective episodes in 1985 and 1989 and CO measurements taken during several midwestern field campaigns. Deep convective venting of the boundary layer over this moderately polluted region provides a net (upward minus downward) flux of 18.1×108kg CO month−1 to the free troposphere during early summer, assuming the June statistics are typical. Shallow cumulus and synoptic‐scale weather systems together make a comparable contribution (total net flux 16.2×108 kg CO month−1). Boundary layer venting of CO with other O3 precursors leads to efficient free tropospheric O3 formation. We estimate that deep convective transport of CO and other precursors over the central United States in early summer leads to a gross production of 0.66–1.1 Gmol O3 d−1 in good agreement with estimates of O3 production from boundary layer venting in a continental‐scale model [Jacob et al., 1993a, b]. In this respect the central U.S. region acts as a “chimney” for the country, and presumably this O3 contributes to high background levels of O3 in the eastern United States and O3 export to the North Atlantic.

Tropospheric chemistry over the lower Great Plains of the United States. 1. Meteorology

Convective clouds and thunderstorms inject planetary boundary layer air with high concentrations of ozone (O3) and O3 precursors into the free troposphere. By altering the chemistry and radiative balance of the upper troposphere, these local actions can have global consequences. In order to extrapolate from individual observations to general trends we have devised a method to identify weather patterns conducive to convection in the southern or lower Great Plains in early summer and applied this method to meteorological and chemical data from a series of research flights carried out in June 1985. Previous studies have noted that weather patterns in the lower Great Plains in this season are characterized by alternating pulses of polar and maritime air masses and very frequent episodes of violent convection, usually in maritime conditions in advance of a surface cold front or along the frontal zone itself. In this study, a set of selection criteria is applied to surface and upper air meteorological data from central Oklahoma to distinguish the two meridional phases characteristic of this region: maritime and polar. A deep layer of moist, southerly flow and convective instability is encountered in the maritime regime, while the polar phase is convectively stable throughout the midtroposphere and much less conducive to convection. For the period 1980–1985 both maritime and polar regimes occur with a frequency of about 35% in May, while the maritime phase becomes dominant in June (53% maritime versus 20% polar). Within the maritime regime, conditions conducive to the development of severe storms are characterized by the presence of a dry, low‐level inversion that tends to inhibit scattered midday convection over a wide region while simultaneously enhancing the probability of larger thunderstorms and mesoscale convective systems. This “modified maritime” category accounts for approximately 25% of the total cases in May and in June. The instances of frontal passage account for approximately 15% of the total cases. Profiles, from the surface to 200 mbar, made on 18 flights of the NCAR Sabreliner in the 1985 PRESTORM project, are categorized according to the selection criteria. Part 2 of this study presents representative concentrations of ozone, carbon monoxide, and reactive nitrogen compounds as a function of altitude (0–12 km) for each category and discusses the implications of these findings.

Free tropospheric ozone production following entrainment of urban plumes into deep convection

It is shown that rapid vertical transport of air from urban plumes through deep convective clouds can cause substantial enhancement of the rate of O3 production in the free troposphere. Simulation of convective redistribution and subsequent photochemistry of an urban plume from Oklahoma City during the 1985 PRESTORM campaign shows enhancement of O3 production in the free tropospheric cloud outflow layer by a factor of almost 4. In contrast, simulation of convective transport of an urban plume from Manaus, Brazil, into a pristine free troposphere during GTE/ABLE 2B (1987), followed by a photochemical simulation, showed enhancement of O3 production by a factor of 35. The reasons for the different enhancements are (1) intensity of cloud vertical motion; (2) initial boundary layer O3 precursor concentrations; and (3) initial amount of background free tropospheric NOx. Convective transport of ozone precursors to the middle and upper troposphere allows the resulting O3 to spread over large geographic regions, rather than being confined to the lower troposphere where loss processes are much more rapid. Conversely, as air with lower NO descends and replaces more polluted air, there is greater O3 production efficiency per molecule of NO in the boundary layer following convective transport. As a result, over 30% more ozone could be produced in the entire tropospheric column in the first 24 hours following convective transport of urban plumes.

Ozone Control Strategies Based on the Ratio of Volatile Organic Compounds to Nitrogen Oxides

The 1990 Clean Air Act Amendments require states with O3 nonattainment areas to adopt regulations to enforce reasonable available control technologies (RACT) for NOX stationary sources by November 1992. However, if the states can demonstrate that such measures will have an adverse effect on air quality, NOX requirements may be waived. To assist the states in making this decision, the U.S. EPA is attempting to develop guidelines for the states to use in deciding whether NOX reductions will have a positive or negative impact on O3 air quality. Although NOX is a precursor of O3, at low VOC/NOX ratios, the reduction of NOX can result in increased peak O3. EPA is examining existing information on VOC/NOX ratios to develop “rules of thumb” to guide the states in their decision-making process. An examination of 6 a.m. to 9 a.m. VOC/NOX ratios at a number of sites in the eastern U.S. indicates that the ratio is highly variable from day-to-day and there is no apparent relationship between ratios measured at different sites within the same area. In addition, statistical analysis failed to identify significant relationships between the 6 a.m. to 9 a.m. VOC/NOX ratio and the maximum 1-hr. O3 within a given area. Since we know from smog chamber and modeling studies that such a relationship exists, this further invalidates the assumption that a ratio measured at a single site is representative of the ratio for the entire region. Based on this Information, we conclude that having the 6 a.m. to 9 a.m. ambient VOC/NOX ratio for a given area is insufficient information, by itself, to decide whether a VOC-alone, a NOx-alone, or a combined VOC-NOX reduction strategy is a viable or optimum O3-reduction strategy.

The oxidizing capacity of the earth's atmosphere: probable past and future changes.

The principal oxidants in the lower atmosphere are ozone (O3) and two by-products of O3 photodissociation, the hydroxyl radical (OH) and hydrogen peroxide (H2O2). A number of critical atmospheric chemical problems depend on the earth's "oxidizing capacity," which is essentially the global burden of these oxidants. There is limited direct evidence for changes in the earth's oxidizing capacity since recent preindustrial times when, because of industrial and population growth, increasing amounts of O3 precursor trace gases (carbon monoxide, nitrogen oxides, and hydrocarbons) have been released into the atmosphere. The concentrations of O3 and possibly H2O2 have increased over large regions. Models predict that tropospheric O3 will increase approximately 0.3 to 1% per year over the next 50 years with both positive and negative trends possible for OH and H2O2. Models and the observational network for oxidants are improving, but validation of global models is still at an early stage.

Effects of tropical deforestation on global and regional atmospheric chemistry

A major portion of tropospheric photochemistry occurs in the tropics. Deforestation, colonization, and development of tropical rain forest areas could provoke significant changes in emissions of radiatively and photochemically active trace gases. A brief review of studies on trace-gas emissions in pristine and disturbed tropical habitats is followed by an effort to model regional tropospheric chemistry under undisturbed and polluted conditions. Model results suggest that changing emissions could stimulate photochemistry leading to enhanced ozone production and greater mineral acidity in rainfall in colonized agricultural regions. Model results agree with measurements made during the NASA ABLE missions. Under agricultural/pastoral development scenarios, tropical rain forest regions could export greater levels of N2O, CH4, CO, and photochemical precursors of NOy and O3 to the global atmosphere with implications for climatic warming.

A regional model study of the ozone budget in the eastern United States

In order to better understand the photochemical and meteorological processes controlling regional scale air quality problems such as ozone formation, we have developed a three‐dimensional Eulerian model and applied this model to a high‐pressure period (July 4 to July 7, 1986) over the eastern United States. Meteorological and physical variables from a three‐dimensional primitive‐equation model are used to drive the transport parameters over a grid with 60×60 km2 horizontal resolution, and 15 unequally spaced vertical layers extending from the ground to roughly 15 km. The treatment and incorporation of the dynamic model, the transport model, surface deposition, emission of anthropogenic and natural O3 precursors, the chemical mechanism for 35 individual species, solar radiation and the numerical methods are discussed in detail. Model performance is tested by comparing model predicted O3 concentrations with observations from the U.S. Environmental Protection Agency ozone‐monitoring network. Although a significant correlation between model and observed O3 is found, systematic discrepancies also exist and are discussed in relation to the basic model formulation, and variability in the observed O3. Additionally, a comparison of time‐averaged NOx and anthropogenic nonmethane hydrocarbon (NMHC) concentrations to relatively long term observations provides a qualitative assessment of the model's ability to simulate certain aspects of these O3 precursors from the few available observations. The model is used as a diagnostic tool to analyze various aspects of regional scale O3 formation and the budgets of the primary O3 precursors. Ozone formation over much of the continental model domain is shown to be NOx. limited. On the other hand, for midday NOx levels greater than about 4 or 5 ppbv, O3 formation is generally suppressed because of the low NMHC to NOx ratios (1 to 7) that are characteristic of the emissions inventory. Regional‐scale budget analyses show that very Hide NOx or NMHC is transported to the free troposphere for the high‐pressure conditions of this study and in the absence of a significant subgrid‐scale vertical mixing process (i.e., efficient cumulus transport). We calculate a net turnover time of about 1.5 days for continental O3 below 1800 m with in situ photochemical formation being balanced by photochemical loss and transport off the American continent. The results of this work are intended to serve as a baseline for further model development.

Model calculations of tropospheric ozone production potential following observed convective events

Photochemical modeling and analysis of field data have been used to evaluate the effects of convective clouds on tropospheric trace gas chemistry. Observations were made during a 1985 field campaign over the rural south‐central United States. Meteorological data and measurements of CO, NO, NOy, O3, and hydrocarbons were collected in air surrounding and inside clouds during and immediately following cloud convection. A one‐dimensional photochemical model has been used to calculate O3 production potential before and after cloud redistribution of O3 precursor gases. Four distinct types of convective events have been analyzed. Fair weather cumulus clouds increase O3 production in a layer immediately above the boundary layer (to 4 km in the case studied). Outflow from deeper convection can cause enhanced O3 production in the upper troposphere hundreds of kilometers downstream from the clouds. A comparison of trace gas profiles measured in and around a large cumulonimbus during dissipation shows O3 production in the upper troposphere may be increased fourfold by convection relative to undisturbed air. Convective enhancement of O3 production for the entire tropospheric column is about 50%. Compared to nonurban continental regions with no convection, the rate of O3 production potential in air processed by convection is up to 3–4; times greater. Catalysis of O3 production becomes more efficient when NO becomes more dilute after being transported from the boundary layer to the free troposphere. Free tropospheric NO may also be enhanced by lightning, adding to O3 production, particularly when sufficient hydrocarbons are transported to such locations.

On the nonlinearity of the tropospheric ozone production

The relationship of photochemical ozone production versus photochemical loss of an ozone precursor, that is, either NOx or nonmethane hydrocarbons (NMHCs), is studied by using a box model with particular emphasis on the nonlinearity problem of the relationship with respect to the concentration of the precursor. Model calculations indicate that the composition of NMHCs, the ratio of NMHCs to NOx, and the background concentrations of natural hydrocarbons, CO, and CH4 all play important roles in determining the nonlinearity of O3 production with respect to the loss of NOx. In addition, influences on the nonlinearity due to radical loss via reactions of HO2 with RO2, exchanges between PAN and NO2, and inclusion of nighttime NOx loss processes are also investigated. Mechanisms that contribute to the nonlinearity are discussed. The nonlinear property of O3 production versus loss of hydrocarbons and CO is different from that of NOx. When the sum of CO and all hydrocarbons, including CH4, natural NMHCs, and anthropogenic NMHCs, is used as the reference O3 precursor, the nonlinearity is much less pronounced for ambient conditions usually found in rural air.

The distribution of acid deposition in Germany

The distribution of acid deposition by atmospheric precipitation in the Federal Republic of Germany is discussed, based on investigations of the wet H+-deposition during the five years 1980–1984, using a network of 16 automated samplers of our own construction located in various categories of ecosystems. Analytical problems of sampling and the electrometric determination of pH in rainwater are briefly discussed. Results for the average amounts of precipitation, the average H+-concentrations and average H+-depositions in the 16 typical regions of the Federal Republic of Germany are compared and the influences of meteorological parameters are discussed. An increase of the H+-concentration and H+-deposition values has been observed from 1980 onwards with a maximum in 1981 and a slow decrease in the next two years. The comparison of the values found for rural regions with those for more significantly polluted regions shows that in the latter regions the removal of H+-ions by wash-out is more effective. Whereas in the Ruhr region the pH is shifted to more acid values, due to the wash-out of acid particles and aerosols, in regions with metallurgical industry the pH is shifted to more alkaline values due to the wash-out of alkaline particles. In general the free acid in rain and snow is rather uniformly distributed over the whole area as a result of mesoscalic transport of the acid precursors SO2 and NOx and the concomitant formation of acid in the cloud droplets leading to acid deposition by rain-out. The composition of rainwater and the possibility of determining the proportion of the acid anions in rain which are of anthropogenic origin is briefly discussed.