Lawrence I. Kleinman

Ozone formation at a rural site in the southeastern United States

Trace gas measurements pertinent to understanding the transport and photochemical formation of O3 were made at a surface site in rural Georgia as part of the Southern Oxidant Study during the summer of 1991. It was found that there was a strong correlation between O3 and the oxidation products of NOx: O3(ppb) = 27 + 11.4 (NOy(ppb) − NOx(ppb)), r2 = 0.78. This fit is similar to that observed at other rural sites in eastern North America and indicates a nominal background O3 level of 27 ppb; values higher than 27 ppb are due to photochemical production in the recent past, which varied from near zero to ≈50 ppb. The origin of the O3 above background was investigated by using a free radical budget equation to calculate an in situ O3 production rate in terms of measured concentrations of NO and free radical precursors (O3, HCHO, peroxides, and other carbonyls). A comparison of observed and predicted diurnal trends in O3 indicates significant O3 production in the afternoon at a time when O3 concentration is either steady or decreasing. The afternoon near-surface layer is thereby a source region for O3 which can be exported. In situ production accounts for approximately one half of the morning increase in O3 concentration on days with high O3; the remainder is due to entrainment of dirty air aloft by the growing convective boundary layer. Additional evidence for the role of vertical transport in controlling the hour-to-hour changes in O3 is found in the diurnal cycles of SO2 and HNO3 which also have rapid increases in the morning. The day-to-day variability of O3 was investigated using a back trajectory model. NOy concentration at the measurement site could be reasonably accounted for by considering NOx emission sources located within 1-day transport distance. In as much as there is a strong correlation between O3 and NOy, the coincidence between trajectory location and NOx emission sources appears to be an important factor influencing midday O3 concentration. Hydrocarbon measurements are consistent with NOx being the limiting factor for formation of O3.

Seasonal dependence of boundary layer peroxide concentration: The low and high NO x regimes

Concentrations of H2O2, organic peroxides, and other photochemically generated species were calculated using a zero-dimensional box model. The range of emission rates considered centered on values that are representative of conditions in a moderately polluted region such as the eastern United States. Solar intensity, temperature, humidity, and mixing layer height were varied to simulate changing conditions over a yearly cycle. Predicted concentrations of H2O2 have a pronounced seasonal dependence similar to that observed. Evidence is presented that the seasonal behavior of peroxide concentration is due to a transition between two qualitatively different photochemical states of the atmosphere. These states are called the low and high NOx regimes according to whether radical production is greater or less than the NOx emission rate. In the low NOx regime, more radicals are formed than can be accommodated by reaction with NOx, and the “excess” radicals are removed by bimolecular radical-radical reactions leading in large part to peroxides. In this regime, peroxide formation is nearly proportional to the difference between radical source strength and the NOx emission rate and peroxide concentration scales with the factors affecting radical source strength (e.g., water vapor and photolysis rate constants). Radical production in the high NOx regime is insufficient to react with and remove all NOx. Consequences include the suppression of peroxide formation, a reduction in the oxidizing capacity of the atmosphere, and high concentrations of primary pollutants.

Dependence of ozone production on NO and hydrocarbons in the troposphere

An expression for the production rate of 03, P(O 3), is derived based on a radical budget equation applicable to low and high NOx conditions. Differentiation of this equation with respect to NO or hydrocarbons (HC) gives an approximate analytic formula in which the relative sensitivity of P(O3) to changes in NO or HC depends only on the fraction of radicals which are removed by reactions with NOx. This formula is tested by comparison with results from a photochemical calculation driven by trace gas observations from the 1995 Southern Oxidants Study (SOS) campaign in Nashville, Tennessee.

Low and high NOx tropospheric photochemistry

Emission inputs to the troposphere are processed in two fundamentally different ways, leading to two atmospheric states with greatly different properties. We define these states in terms of rates of NOx emissions and radical production. In the low NOx state, trace gas inputs to the atmosphere are removed primarily by oxidation reactions. In the high NOx state the oxidative potential of the atmosphere is greatly diminished and primary pollutants build up, as alternate removal processes are slow. We show here that this dual behavior is due to a titrationlike reaction between NOx and free radicals in which either reactant can be in excess.

Chemical and physical properties of plumes of anthropogenic pollutants transported over the North Atlantic during the North Atlantic Regional Experiment

Plumes of photochemical pollutants transported from the industrialized regions of the northeast United States and Canada were sampled over the North Atlantic Ocean at distances up to 1000 km from the coast. The plumes were found in well defined layers up to 1 km thick and were usually isolated from the surface by a low altitude inversion. Plume composition was consistent with the occurrence of extensive photochemical processing during transit from source regions as indicated by high 03 concentrations (03 maximum -150 parts per billion by volume (ppbv)), generally high fractional conversion (>85%) of NOx to its oxidation products, and high peroxide concentrations (median 3.6 ppbv; maximum 11 ppbv). These observations are in accord with processing times estimated from back trajectory analysis. CO and 03 concentrations were well correlated (r 2 = 0.64) with a slope (0.26) similar to previous measurements in photochemically aged air. Good correlations were also observed between CO and accumulation mode particle number densities (r 2 = 0.64), and CO and NOy (r 2 = 0.67). 03 was found to depend nonlinearly on the NOx oxidation product concentration. At low values of (NOy-NOx), the slope (14) was within the range of values measured previously in photochemically aged air masses, at higher concentrations the slope was much lower (4.6). The low slope at high concentrations is attributed to minimization of losses of NOx oxidation products in spatially well-defined plumes during transport. A strong linear correlation (r 2 = 0.73) was found between 03, and the concentration of radical sink species as represented by the quantity ((NOy-NOx) + 2H202).

Ozone production in the New York City urban plume

In the summer of 1996 the Department of Energy G-1 aircraft was deployed in the New York City metropolitan area as part of the North American Research Strategy for Tropospheric Ozone-Northeast effort to determine the causes of elevated O3 levels in the northeastern United States. Measurements of O3, O3 precursors, and other photochemically active trace gases were made upwind and downwind of New York City with the objective of characterizing the O3 formation process and its dependence on ambient levels of NOx and volatile organic compounds (VOCs). Four flights are discussed in detail. On two of these flights, winds were from the W-SW, which is the typical direction for an O3 episode. On the other two flights, winds were from the NW, which puts a cleaner area upwind of the city. The data presented include plume and background values of O3, CO, NOx, and NOy concentration and VOC reactivity. On the W-SW flow days O3 reached 110 ppb. According to surface observations the G-1 intercepted the plume close to the region where maximum O3 occurred. At this point the ratio NOx/NOy was 20–30%, indicating an aged plume. Plume values of CO/NOy agree to within 20% with emission estimates from the core of the New York City metropolitan area. Steady state photochemical calculations were performed using observed or estimated trace gas concentrations as constraints. According to these calculations the local rate of O3 production P(O3) in all four plumes is VOC sensitive, sometimes strongly so. The local sensitivity calculations show that a specified fractional decrease in VOC concentration yields a similar magnitude fractional decrease in P(O3). Imposing a decrease in NOx, however, causes P(O3) to increase. The question of primary interest from a regulatory point of view is the sensitivity of O3 concentration to changes in emissions of NOx and VOCs. A qualitative argument is given that suggests that the total O3 formed in the plume, which depends on the entire time evolution of the plume, is also VOC sensitive. Indicator ratios O3/NOz and H2O2/NOz mainly support the conclusion that plume O3 is VOC sensitive.

Peroxy radical concentration and ozone formation rate at a rural site in the southeastern United States

As part of the Southern Oxidants Study, Brookhaven National Laboratory operated an intensive measurement site near Metter, Georgia, during parts of the summers of 1991 and 1992. Measurements were made of photochemically active trace gases and meteorological parameters relevant to determining causes for elevated ambient ozone concentration. The 1992 data set was used to calculate peroxy radical concentration and ozone formation rate based on determining the departure from the photostationary state (PSS) and based on a radical budget equation, such as applied previously to the 1991 data set. Averaged over the 28-day experimental period, we find maximum radical production occurring near noon at 2.5 ppb h−1, maximum peroxy radical concentration also occurring near noon at 80 ppt, and maximum ozone production of 8 ppb h−1 occurring near 1000 EST. Ozone photolysis accounts for 55% of radical production, HCHO and other carbonyl compounds about 40%. The radical budget and PSS methods depend in different ways on atmospheric photochemistry and a comparison between them affords a test of our understanding of the photochemical production of O3. We find that these methods agree to the extent expected based on uncertainty estimates. For the data set as a whole, the median estimate for fractional error in hourly average peroxy radical concentration determined from the radical budget method is approximately 30% and from the PSS method, 50%. Error estimates for the PSS method are highly variable, becoming infinite as peroxy radical concentration approaches zero. This behavior can be traced back to the difference form of the PSS equations. To conduct a meaningful comparison between the methods, the data set was segregated into subsets based on PSS uncertainty estimates. For the low-uncertainty subset, consisting of a third of the whole data set, we find that the ratio of peroxy radical concentration predicted from the PSS method to that predicted from the radical budget method to be 1.22±32%.

Model correlations for ozone, reactive nitrogen, and peroxides for Nashville in comparison with measurements : Implications for O3-NOx-hydrocarbon chemistry

We present an analysis of correlations between O 3 , NO x reaction products (NO 2 ), and peroxides in photochemical models for polluted environments in comparison with measurements in the vicinity of Nashville, Tennessee. This analysis is associated with the use of O 3 /NO z , H 2 O 2 /NO z , and similar ratios as indicators for the relative impact of NO x and hydrocarbons (volatile organic compounds, VOC) on ozone formation. The measurements are used both to evaluate the NO x -VOC indicator theory and to identify NO x -VOC chemistry in Nashville. Results show that a linear correlation exists between O 3 and the sum 2H 2 O 2 +NO z , consistent between models and measurements. The ratio O 3 /2H 2 O 2 +NO z has a near-constant value in both the Nashville urban plume and surrounding rural area. A similar correlation is found with total peroxides (O 3 versus 2peroxides+NO z ) but with greater scatter. The correlations between O 3 , NO z , and peroxides are consistent with an assumed dry deposition rate of 5 cm s -1 for H 2 O 2 and HNO 3 . Changes in dry deposition and RO 2 reaction rates cause minor adjustments in the NO x -VOC-indicator analysis for H 2 O 2 /NO z . Measured indicator ratios for Nashville are close to the NO x -VOC transition predicted by models and form an intermediate point between previous measurements for Atlanta (NO x sensitive) and Los Angeles (VOC sensitive). The model ozone production efficiency (P(O 3 )/L(NO x )) is 3-4, significantly lower than would be derived from the O 3 -NO z slope (5-8).

Ozone process insights from field experiments- Part II: Observation-based analysis for ozone production

Abstract A complete characterization of O3 photochemistry from a regulatory point of view includes knowing the production rate for O3, the sensitivity of this rate to NOx and VOCs, and the effects of emission controls on O3 concentration. Observation-based analysis techniques have been developed to determine these quantities based on observed concentrations of O3 and other photochemical ingredients. The promise of these methods is that reliable predictions on O3 control measures will be forthcoming from easily made measurements. We review several techniques that have been used in recent field programs. Techniques are divided into two families according to whether predicted quantities pertain to the present state of an air parcel or to its history. The present time frame methods address the question of what is happening now, whereas the past time frame methods are used to determine how the air mass evolved to its present state. Present time frame methods are used to determine O3 production rates and sensitivities. In this category, we discuss the constrained steady state, photostationary state, and radical budget methods. Past time frame techniques are used to address questions on the dependence of O3 on precursor emissions. In this category, we discuss indicator species and the “observation-based model”.

Determination of and evidence for non‐core‐shell structure of particles containing black carbon using the Single‐Particle Soot Photometer (SP2)

[1] The large uncertainty associated with black carbon (BC) direct forcing is due, in part, to the dependence of light absorption of BC-containing particles on the position of the BC within the particle. It is predicted that this absorption will be greatest for an idealized core-shell configuration in which the BC is a sphere at the center of the particle whereas much less absorption should be observed for particles in which the BC is located near or on the surface. Such microphysical information on BC-containing particles has previously been provided only by labor-intensive microscopy techniques, thus often requiring that climate modelers make assumptions about the location of the BC within the particle that are based more on mathematical simplicity than physical reality. The present paper describes a novel analysis method that utilizes the temporal behavior of the scattering and incandescence signals from individual particles containing