Anthropogenic forcing on tropospheric ozone and OH since preindustrial times

A global three-dimensional model of tropospheric chemistry is used to investigate the changes in tropospheric O3 and OH since preindustrial times as a result of fuel combustion and industry, biomass burning, and growth in atmospheric CH4. Model results indicate a 63% increase of the global tropospheric O3 burden from preindustrial times to present (80% and 50% in the northern and southern hemispheres, respectively). Anthropogenic emissions of NOx and of CO and hydrocarbons make comparable contributions to the global O3 increase (60% and 40% respectively), even though the local rate of tropospheric O3 production is generally NOχ limited. The rise in O3 production parallels closely the rise in the emissions of CO and hydrocarbon because the O3 yield per mole of CO or hydrocarbon oxidized has remained constant at 0.7–0.8 mol/mol since preindustrial times. In contrast, the O3 production efficiency per mole of NOχ emitted has decreased globally by a factor of 2. We find a 9% decrease in the global mean OH concentration (mass-weighted) since preindustrial times. A linear relationship is found in the model between the global mean OH concentration and the SN/SC3/2 ratio, where SN and SC are the sources of NOχ and of CO and hydrocarbons, respectively. The relative constancy of the global mean OH concentration since preindustrial times reflects the conservation of the SN/SC3/2 ratio despite large increases in both SN and SC. Comparisons of model results with reconstructed nineteenth century observations of O3 at continental sites indicate a systematic overestimate of about 5 ppbv. Correcting this overestimate would require either a large missing chemical sink for O3 or a downward revision of the natural NOχ source from lightning (3 Tg N yr−1 in our model). The nineteenth century observations of O3 over France show no vertical gradient between the boundary layer and the free troposphere, which is inconsistent with our current understanding of tropospheric O3. The model underestimates preindustrial CO concentrations derived from polar ice cores; these measurements are difficult to reconcile with any reasonable CO emission inventories.

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title: Evaluation of the Montsouris series of ozone measurements made in the nineteenth century
abstract: There is growing evidence that ozone levels in the lower troposphere over the continents of the Northern Hemisphere have been increasing during the past decades1,2. Questions regarding pre-industrial or 'background' ozone concentrations have led to the search for data from the early days of ozone monitoring, during the second half of the last century. Unfortunately, most measurements were then made using Schönbein test paper, giving only semi-quantitative information due to poor standardization and the influence of humidity and wind speed on its sensitivity35. We have reinvesti-gated a set of ozone measurements gathered at the Observatoire de Montsouris, located on the outskirts of Paris, where a quantitative method was established in 18766 and used continuously for 34 years. The evaluation of the technique, together with the analysis of nearly 3,000 of the original daily measurements that previously remained unnoticed in a statistical bulletin of the City of Paris7, provides conclusive evidence that ozone levels in central Europe 100 years ago averaged 10 p.p.b. and exhibited a seasonal variation, with a maximum during the spring months. Comparisons with modern data show that ozone levels in rural areas have more than doubled over the past century and that the tropospheric ozone budget is now strongly influenced by photochemical production due to increased levels of NOx.

title: Tropospheric ozone in the nineteenth century: The Moncalieri series
abstract: A 26-year (1868–1893) data series of daily ozone readings performed at Moncalieri, northern Italy, by the Schonbein test paper technique has been analyzed. The availability of a series of simultaneous readings by the Schonbein and a quantitative technique (Levy, 1877) and the conversion chart for humidity by Linvill et al. (1980) allowed us to develop a procedure to convert the Moncalieri data into parts per billion by volume values. The results seem to indicate that in comparison to one century ago, the ozone level in Europe has increased by more than twice not only at the surface but also in the free troposphere.

title: Surface ozone levels at the end of the nineteenth century in South America
abstract: Records of surface ozone data collected twice a day by means of the Schonbein method at Villa Colon, Montevideo, Uruguay, during the years 1883–1885 and at Cordoba, Argentina, during the years 1886–1892 have been converted to present-day concentrations. Most of the uncertainties suffered from the original technique are removed by a two-step procedure, in particular a conversion algorithm for humidity and a linear regression constructed by simultaneous readings of Schonbein and a quantitative method. The relative error of this procedure, previously applied to the Moncalieri data series, is 33%. Within these limitations we can state that one century ago the surface ozone levels at that latitude of the southern hemisphere were comparable to those of Moncalieri and Montsouris. Furthermore, the recent levels observed in remote areas at that latitude are approximately twice as large as a century ago.

title: Evidence of a long‐term increase in tropospheric ozone from Pic du Midi data series: Consequences: Positive radiative forcing
abstract: The rate at which ozone is increasing in the troposphere is uncertain due to the lack of accurate long-term measurements. Old ozone measurements obtained at the Pic du Midi Observatory (3000 m high, southwestern France) were recently rediscovered. Four sets of data available at this station are presented herein: (1) 1874–1881 and (2) 1881–1909 by the Schonbein method and (3) 1982–1984 and (4) 1990–1993 by UV absorption analyzers. The results show an increase in ozone by a factor of 5 since the beginning of the twentieth century, corresponding to an exponential increase of 1.6% per year, although this trend is probably higher (2.4% per year) for the last few decades. A stable 10 ppb ozone mixing ratio is observed during the first 20 years of the series, which is representative of the preindustrial era ozone level. The increase is seen to start around 1895. Other data, obtained at various European high-altitude stations between 1920 and 1980, tie in closely with the Pic du Midi observations. A tentative evaluation of the impact of tropospheric ozone on radiative forcing confirms that ozone is currently the second most significant greenhouse gas, responsible for 22% and 13% of radiative forcing changes since 1800 in the northern and southern hemispheres, respectively. If these rates were to be maintained in the future, ozone would continue to evolve differently in the two hemispheres (maximum level in the northern hemisphere) and could make an even more significant contribution to the radiative forcing of the northern hemisphere.

title: The Oxidizing Capacity of the Earth's Atmosphere: Probable Past and Future Changes
abstract: 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 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 ∼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.

title: The changing photochemistry of the troposphere
abstract: The chemistry of the troposphere is substantially influenced by a wide range of chemical processes which are primarily driven by the action of solar ultraviolet radiation of wavelengths shorter than about 310 nm on ozone and water vapour. This leads to the formation of hydroxyl (OH) radicals which, despite very low tropospheric concentrations, remove most gases that are emitted into the atmosphere by natural and anthropogenic processes. Although tropospheric ozone only makes up about 10% of all atmospheric ozone, through the formation of OH, it determines the oxidation efficiency of the atmosphere and it is, therefore, of the utmost importance for maintaining the chemical composition of the atmosphere. Due to a variety of human activities, leading especially to increasing emissions of CH 4 , CO and NO x , it is conceivable that the concentrations of ozone are increasing in polluted and those of hydroxyl decreasing in clean tropospheric environments. The result is most likely an overall decrease in the oxidation efficiency of the atmosphere and consequently a buildup of several longlived trace gases that are primarily removed by reaction with OH. Here we discuss the most important processes that determine the oxidation efficiency of the atmosphere and give some examples of changes in O 3 , CO, and OH concentration distributions that may have occurred as a result of human activities.

title: The atmospheric CH 4 increase since the Last Glacial Maximum
abstract: An estimate of the distribution of wetland area and associated CH 4 emission is presented for the Last Glacial Maximum (LGM, 18 kyr BP, kiloyear Before Present) and the Pre-Industrial Holocene (PIH, 9000–200 years BP). The wetland source, combined with estimates of the other biogenic sources and sink, yields total source strengths of 120 and 180 Tg CH 4 /yr for LGM and PIH respectively. These source strengths are shown to be consistent with source estimates inferred from a photochemical model, and point to changes in wetland CH 4 source as a major factor driving the atmospheric CH 4 increase from LGM to PIH.

title: The chemical composition of ancient atmospheres: A model study constrained by ice core data
abstract: A coupled chemistry radiation transport two-dimensional model of the lower and middle atmosphere was adapted to study the chemical composition of the atmosphere at preindustrial time and last glacial maximum (LGM). The model was constrained by trace gas concentrations (CO2, CH4, and N2O) inferred from polar ice core records. The formulation of tropospheric dynamics and chemistry was improved in order to more accurately simulate the transport and the oxidation processes below the tropopause. Our objectives are to infer the changes in middle-atmosphere temperature, ozone layer, and oxidation capacity of the atmosphere (e.g., methane lifetime) over the last 18,000 years. A middle-atmosphere cooling was obtained between LGM and preindustrial Holocene (PIH) as well as between PIH and present time. This is mainly due to changes in the CO2 and chlorofluorocarbon (CFC) concentrations, respectively. CFCs are also the main contributors to the middle-atmosphere ozone decrease since PIH. Between LGM and PIH the compensating effects of CO2 and N2O lead to little variation in stratospheric ozone. A 17% decrease in tropospheric OH was obtained between LGM and PIH, whereas the model provides a 6% OH increase since PIH. The corresponding changes in the methane sink are too small to have played a dominant role in the past methane concentration changes. Our model derived methane emissions for LGM, PIH, and present time are in good agreement with methane sources evaluated during these three periods.

title: Ozone chemistry changes in the troposphere and consequent radiative forcing of climate.
abstract: A global three-dimensional transport/chemistry model of the troposphere has been applied to simulate changes in the chemical composition of the troposphere since pre-industrial times. Calculated increases of tropospheric O3 have been evaluated with a radiative transfer model. Our calculations confirm earlier studies that the combined effects of anthropogenic NOx, CO and CH4 emissions cause a strong enhancement of O3 levels throughout the global troposphere, while this blend of emissions does not seem to have affected global average OH concentrations significantly. In fact, anthropogenic NOx induced photochemistry may have compensated for the strong OH reduction which would have resulted from anthropogenic CO and CH4 emissions alone. Our model simulations suggest that during the past 1.5 Century net photochemical 03 formation in the global troposphere has changed sign, from -87 Tg/yr to 427 Tg/yr, caused mostly by anthropogenic emissions in the mid- latitude northern hemisphere and to a lesser extent by biomass burning emissions in the tropics. We calculate that the global O3 burden in the troposphere has increased by a factor of 1.7, the CO burden by a factor of 2.5 and that of H2O2 by a factor of 2. Preliminary calculations suggest that the global mean radiative forcing of climate by increasing tropospheric O3 is 0.55 W/m2 for the month July, maximizing in the mid-latitude northern hemisphere, and 0.44 W/m2 for January; the annual average, about 0.5 W/m2, is approximately one third of the radiative forcing by increasing CO2.

title: Three‐dimensional view of the large‐scale tropospheric ozone distribution over the North Atlantic Ocean during summer
abstract: A global chemical transport model is used to study the three-dimensional structure of the tropospheric ozone (O3) distribution over the North Atlantic Ocean during summer. A simplified representation of summertime O3 photochemistry appropriate for northern hemisphere midlatitudes is included in the model. The model is evaluated by comparing simulated O3 mixing ratios to summertime O3 measurements taken in and near the North Atlantic Ocean basin. The model successfully reproduces (1) the means and standard deviations of ozonesonde measurements over North America at 500 mbar; (2) the statistical characteristics of surface O3 data at Sable Island off the coast of North America and at Bermuda in the western North Atlantic; and (3) the mean midtropospheric O3 measured at Bermuda and also at the Azores in the eastern North Atlantic. The model underestimates surface O3 in the eastern North Atlantic, overestimates O3 in the lower free troposphere over the western North Atlantic, and also has difficulty simulating the upper tropospheric ozonesonde measurements over North America. An examination of the mean summertime O3 distribution simulated by the model shows a significant continental influence on boundary layer and free-tropospheric O3 over the western North Atlantic. The model has also been exercised using a preindustrial NOx emission scenario. By comparing the present-day and preindustrial simulations, we conclude that anthropogenic NOx emissions have significantly perturbed tropospheric O3 levels over most of the North Atlantic. We estimate that present-day O3 levels in the lower troposphere over the North Atlantic are at least twice as high as corresponding preindustrial O3 levels. We find that the anthropogenic impact is substantial even in the midtroposphere, where modeled present-day O3 mixing ratios are at least 1.5 times higher than preindustrial O3 levels.

title: The global impact of human activity on tropospheric ozone
abstract: Within a conceptual framework of stratospheric injection, CO-CH4 background tropospheric chemistry, parameterized pollution production in the continental boundary layer and surface deposition, we use an 11 level GCTM to simulate global distributions of present and pre-industrial tropospheric O3. The chemistry is driven by previously simulated present and pre-industrial NOx fields, while prescribed fields of CO, CH4 and H2O are held constant. An evaluation with measurements from 12 surface sites, 21 ozonesonde sites and 1 aircraft campaign finds agreement within ±25% for 73% of the observations while identifying systematic errors in the wintertime high-latitude Northern Hemisphere (NH), the Southern Hemisphere (SH) tropics during biomass burning, and the remote SH. We predict that human activity has increased the annual integral of tropospheric ozone by 39% with 3/4's of that increase in the free troposphere, though the boundary layer [BL] annual integral has increased by 66%. The 2 largest components of the global O3 budget are stratospheric injection at 696 TgO3/yr, and loss through dry deposition, which increases from 459 TgO3/yr to a present level of 825 TgO3/yr. While tropospheric chemistry's net contribution is relatively small, changing from a pre-industrial destruction of −236 TgO3/yr to a present production of +128 TgO3/yr, it is a balance between two much larger terms, −558 TgO3/yr of destruction in the background troposphere and +686 TgO3/yr of production in the polluted boundary layer. Human impact on O3 predominates in the summertime extratropical NH and in the tropics during their biomass burning seasons [increases of 50%–100% or more]. Conversely, there has been little increase in most of the upper troposphere [<20%], where ozone's influence on tropospheric climate is strongest.

title: Efficient Three-Dimensional Global Models for Climate Studies: Models I and II
abstract: A global atmospheric model is developed with a computational efficiency which allows long-range climate experiments. The model solves the simultaneous equations for conservation of mass, energy and momentum, and the equation of state on a grid. Differencing schemes for the dynamics are based on work of Arakawa; the schemes do not need any viscosity for numerical stability, and can thus yield good results with coarse resolution. Radiation is computed with a semi-implicit spectral integration, including all significant atmospheric gases, aerosols and cloud particles. Cloud cover and vertical distribution are computed. Convection mixes moisture, heat and momentum, with buoyant air allowed to penetrate to a height determined by its buoyancy. Ground temperature calculations include diurnal variation and seasonal heat storage. Ground hydrology incorporates a water-holding capacity appropriate for the root zone of local vegetation. Snow depth is computed. Snow albedo includes effects of snow age and mas...

title: GLOBAL EMISSIONS OF NITROGEN AND SULFUR OXIDES FROM 1860 TO 1980
abstract: Statistical models have been developed that relate the rate of emissions of a pollutant to the rate of fuel consumption. These relations may be used to estimate emissions in other regions, or at other times, if fuel consumption data are available. This approach has been used to estimate global emissions of nitrogen and sulfur oxides in fossil fuel combustion at ten year intervals from 1860 to 1980. Emissions from each of the populated continents, i.e., North America, South America, Asia, Europe, Africa and Oceania from 1930 to 1980 are also presented. When averaged globally over the 1860 to 1980 period, sulfur emissions increased at the rate of 2.9 percent per year and the nitrogen emissions at the rate of 3.4 percent per year. The ratio of global sulfur emissions to nitrogen emissions has declined steadily; it was nearly 5 in the 19th century and became 3 by 1980. After the second world war, the most rapid increases in emissions have been registered in Asia, South America, and Africa.

title: High‐resolution ammonium ice core record covering a complete glacial‐interglacial cycle
abstract: High-resolution ammonium measurements were performed along the Greenland Ice Core Program (GRIP) deep ice core, covering a complete climatic cycle. No overall anthropogenic increase is observed over the last 300 years; however, springtime concentrations have roughly doubled since 1950. Biomass burning is estimated to be a major source for ammonia emissions for preindustrial times. It contributes between 10% to 40% to the total ammonium deposited on the central Greenland ice sheet during the Holocene. No correlation is found between the ammonium summer concentrations recorded over the last 100 years and the area burned in northern North America, which is considered to be the main source area for ammonium deposited on the central Greenland ice sheet. This suggests that the meteorological factor is predominant for the pattern of ammonium spikes observed in the ice core. If unchanged meteorological conditions are assumed for the Holocene, as indicated by the δ18O ice record, a decreasing biomass burning activity toward present time can be derived from the ammonium ice record. Soil and vegetation emissions are responsible for the ammonium background concentrations in the ice. The record therefore may be used to trace back the biomass history of the North American continent. A pronounced decreasing trend in background ammonium is found during the Holocene, reflecting decreasing temperature and therefore lower NH3 emissions in the source region. Variations in the ammonium concentration during the glacial age are discussed in terms of changes in transport and deposition mechanisms and changes in source strength, which can be related to the extent of the Laurentide ice sheet. The data suggest that the Laurentide ice sheet was built up immediately after the last interglacial and went through several large fluctuations during the last ice age.

title: Ozone in Michigan's Environment 1876–1880
abstract: Atmospheric ozone was monitored in Michigan during the late 1880's using Schoenbein's test paper. A conversion chart was constructed to relate the Schoenbein ozone scale at various relative humidifies to ozone levels indicated by a Dasibi ozone monitor. Average monthly ozone values were highest during the spring months and lowest during the winter season. Highest recorded ozone levels accompanied southwest wind flow into Michigan. Ozone level increased with time as an air mass passed over Michigan and dropped immediately after frontal passage. These patterns are similar to patterns observed in today's data. These results strongly suggest that anthropogenic nitrogen emitted by actively growing green plants and soils is the major source of photochemical ozone precursors in earth's boundary layer.

title: Surface Ozone During the Second Half of the Nineteenth Century
abstract: During the last few years, increases in tropospheric ozone concentration have been detected and the need for more study has been recognized. There is very little knowledge about surface ozone background concentrations prior to the advent of widespread gasoline combustion. Therefore, the aim of this study was to uncover any useful information from the nineteenth century, and to explore the feasibility of converting it to obtain an approximate level of ozone concentration. After discovering ozone, Schonbein promulgated a simple method using iodized starch paper for qualitatively assessing its amount present in the air. This method was implemented at a few hundred sites, and although vulnerable to influence by humidity and oxidants in the air, continued to be used into the early years of this century. In the search for the best method of converting the Schonbein data, the most useful data sources turned out to be the 31-year series of regular quantitative ozone concentration measurements at Montsour...

title: Time variations of CO and O3 concentrations in a region subject to biomass burning
abstract: Carbon monoxide (CO) and ozone (O3) concentrations have been observed in the Brazilian Amazon region, at a site strongly affected by biomass burning (Cuiaba, 16°S, 58°W). Time variations are described for the first long-term program of studying the effect of biomass burning on O3 and CO over a complete seasonal cycle, including the seasonal maxima of 1987 and 1988. Concentrations of O3 are measured continuously, and CO samples are collected three times a week. In order to obtain elements for comparison, an identical observational program was maintained at a site totally outside of the direct influence of biomass burning (Natal, 6°S, 35°W). The biomass burning contribution to the Cuiaba concentrations of CO and O3 is very large. Diurnal maximum concentrations exceeded 90 ppbv (parts per billion by volume) O3 in 1987 and 120 ppbv O3 in 1988, in September. For the wet season, the monthly average ozone concentration in March–April is about 10 ppbv. During the month of maxima, September, the O3 concentration average was 41 ppbv for 1987 and 71 ppbv for 1988. The CO concentrations are about 90 ppbv in the wet season. In September, 460 ppbv and 660 ppbv of CO were observed for 1987 and 1988, respectively. At Natal the seasonal variation is of the order of a factor of 2. For the biomass burning site this factor is 4 for 1987 and almost 7 for 1988. In contrast, during the wet season, the concentrations of CO and O3 at both stations are about the same.

title: Simulation of summertime ozone over North America
abstract: The concentrations of O3 and its precursors over North America are simulated for three summer months with a three-dimensional, continental-scale photochemical model using meteorological input from the Goddard Institute for Space Studies (GISS) general circulation model (GCM). The model has 4°×5° grid resolution and represents non linear chemistry in urban and industrial plumes with a subgrid nested scheme. Simulated median afternoon O3 concentrations at rural U.S. sites are within 5 ppb of observations in most cases, except in the south central United States where concentrations are overpredicted by 15–20 ppb. The model captures successfully the development of regional high-O3 episodes over the northeastern United States on the back side of weak, warm, stagnant anticyclones. Simulated concentrations of CO and nonmethane hydrocarbons are generally in good agreement with observations, concentrations of NOx are underpredicted by 10–30%, and concentrations of peroxyacylnitrates (PANs) are overpredicted by a factor of 2 to 3. The overprediction of PANs is attributed to flaws in the photochemical mechanism, including excessive production from oxidation of isoprene, and may also reflect an underestimate of PANs deposition. Subgrid nonlinear chemistry as captured by the nested plumes scheme decreases the net O3 production computed in the United States boundary layer by 8% on average.

title: Ozone levels in Paris one century ago
abstract: Based on a procedure for re-evaluating historical ozone data previously developed by the authors, ozone observations by the Schoenbein method in the Paris air quality network during 1876 were discovered, re-analysed and a yearly ozone level map was derived.

title: Dry deposition parameterization in a chemistry general circulation model and its influence on the distribution of reactive trace gases.
abstract: A dry deposition scheme has been developed for the chemistry general circulation model to improve the description of the removal of chemically reactive trace gases at the earth's surface. The chemistry scheme simulates background CH 4 -CO-NO x -HO x photochemistry and calculates concentrations of, for example, HNO 3 , NO x , and O 3 . A resistance analog is used to parameterize the dry deposition velocity for these gases. The aerodynamic resistance is calculated from the model boundary layer stability, wind speed, and surface roughness, and a quasi-laminar boundary layer resistance is incorporated. The stomatal resistance is explicitly calculated and combined with representative cuticle and mesophyll resistances for each trace gas. The new scheme contributes to internal consistency in the model, in particular with respect to diurnal and seasonal cycles in both the chemistry and the planetary boundary layer processes and surface characteristics that control dry deposition. Evaluation of the model indicates satisfactory agreement between calculated and observed deposition velocities. Comparison of the results with model simulations in which the deposition velocity was kept constant indicates significant relative differences in deposition fluxes and surface layer trace gas concentrations up to about ±35%. Shortcomings are discussed, for example, violation of the constant flux approach for the surface layer, the lacking canopy description, and effects of surface water layers.

title: Regional budgets for nitrogen oxides from continental sources: Variations of rates for oxidation and deposition with season and distance from source regions
abstract: Measurements of nitrogen deposition and concentrations of NO, NO2, NOy (total oxidized N), and O3 have been made at Harvard Forest in central Massachusetts since 1990 to define the atmospheric budget for reactive N near a major source region. Total (wet plus dry) reactive N deposition for the period 1990–1996 averaged 47 mmol m−2 yr−1 (126 μmol m−2 d−1, 6.4 kg N ha−1 yr−1), with 34% contributed by dry deposition. Atmospheric input adds about 12% to the N made available annually by mineralization in the forest soil. The corresponding deposition rate at a distant site, Schefferville, Quebec, was 20 mmol m−2 d−1 during summer 1990. Both heterogeneous and homogeneous reactions efficiently convert NOx to HNO3 in the boundary layer. HNO3 is subsequently removed rapidly by either dry deposition or precipitation. The characteristic (e-folding) time for NOx oxidation ranges from 0.30 days in summer, when OH radical is abundant, to ∼1.5 days in the winter, when heterogeneous reactions are dominant and O3 concentrations are lowest. The characteristic time for removal of NOx oxidation products (defined as NOy minus NOx) from the boundary layer by wet and dry deposition is ∼1 day, except in winter when it decreases to 0.6 day. Biogenic hydrocarbons contribute to N deposition through formation of organic nitrates but are also precursors of reservoir species, such as peroxyacetylnitrate, that may be exported from the region. A simple model assuming pseudo first-order rates for oxidation of NOx, followed by deposition, predicts that 45% of NOx in the northeastern U.S. boundary layer is removed in 1 day during summer and 27% is removed in winter. It takes 3.5 and 5 days for 95% removal in summer and winter, respectively.

title: Summertime tropospheric ozone distributions over central and eastern Canada
abstract: Ozone measurements were obtained between the surface and the 6-km altitude on aircraft flights over central and eastern Canada during the summer 1990 NASA Global Tropospheric Experiment Arctic Boundary Layer Expedition (GTE/ABLE 3B). Tropospheric O3 budgets for these regions were observed to be highly variable and significantly impacted by long-range transport and regional scale air mass modification processes. For example, integrated O3 abundance below 5-km altitude averaged 40% and 30% greater in air masses influenced by anthropogenic sources and biomass burning, respectively, than in background (polar) air. Conversely, aged air transported from subtropical areas of the Pacific at times reduced O3 abundance in this height interval by up to 20%. Though intrusion of anthropogenic air was infrequent during the experiment period, the influence of biomass-burning emissions was particularly notable as two thirds of the flights sampled air influenced by plumes from fires burning in Alaska and western Canada. The impinging pollution, both natural and anthropogenic, not only elevated O3 levels directly but also was a source of reactive nitrogen (and nonmethane hydrocarbons) which generally increases the tropospheric lifetime of O3 via moderation of photochemical destruction rates.

title: Origin of tropospheric ozone at remote high northern latitudes in summer
abstract: Author(s): Mauzerall, DL; Jacob, DJ; Fan, SM; Bradshaw, JD; Gregory, GL; Sachse, GW; Blake, DR |  We quantify the tropospheric ozone budget over remote high northern latitudes in summer using chemical and meteorological measurements between 0 and 6-km made during the summer of 1990 Arctic Boundary Layer Expedition (ABLE 3B). We include all components of the ozone budget, both sinks (in situ photochemical loss and deposition); and sources (in situ photochemical production, advection of pollution ozone into the region, production in biomass wildfire plumes, and downwards transport from the upper troposphere/stratosphere). In situ production and loss of ozone are calculated with a photochemical model. The net influx of pollution ozone from North America and Eurasia is estimated from the average enhancement ratio of ΔO3/ΔC2Cl4 observed in pollution plumes and scaled by the net influx of C2Cl4. The contribution of ozone produced in biomass wildfire plumes is estimated from the average enhancement ratio of ΔO3/ΔCO in aged fire plumes. Regional photochemical production and loss in the 0-6 km column are found to be approximately equal; hence, net photochemical production is near zero. However, when ozone production and loss terms are separated, we find that dispersed in situ photochemical production driven by background NO levels (5-10 pptv) is the largest source term in the ozone budget (62%). Influx of stratospheric ozone is of secondary importance (27%), long-range transport of pollution ozone makes a small contribution (9%), and photochemical production of ozone within biomass wildfire plumes is a relatively negligible term (2%) in the budget. Biomass fires and transport of anthropogenic pollution into the region may however have a major effect on the ozone budget through enhancement of background NOx mixing ratios which increase dispersed photochemical production. Using a I-D time-dependent photochemical model between 0 and 6 km, we obtain good agreement between the observed and model-generated vertical ozone profiles. We find that in situ photochemistry within the 0-6 km column accounts for nearly 90% of the ozone mixing ratio within the boundary layer, while above 5 km it accounts for only about 40%. Although photochemical production of ozone within the 0-6 km column is larger than the other source terms combined, the 1-D model results indicate that influx from above is necessary to account for the observed increase in ozone mixing ratios with altitude.

title: Ice core data of atmospheric carbon monoxide over Antarctica and Greenland during the last 200 years
abstract: The first polar ice core measurements of carbon monoxide compatible with atmospheric data were obtained using an improved experimental protocol. A new CO extraction method has been developed with special care to eliminate any CO contamination. The procedure has been applied to ice core samples originating from Antarctica and Greenland in order to reconstruct past CO variations over the last 200 years. Consistent results in terms of atmospheric concentrations are obtained. We find that CO concentration started to increase over Greenland around 1850 and that, by contrast, CO levels at high southern latitudes remained fairly constant between 1860 and 1916. Based on available data on past CO sources, a scenario is proposed for the CO increase observed in Greenland. In addition, our Greenland CO results suggest that simulations of preindustrial CO distribution could have underestimated CO concentrations mainly in the northern hemisphere leading to an overestimate of the change since preindustrial times, and to an underestimate of the past interhemispheric gradient of carbon monoxide.

title: Secular increase of the total vertical column abundance of carbon monoxide above central Europe since 1950
abstract: The secular increase of the total vertical column abundance of carbon monoxide has been derived from sets of infrared solar spectra recorded from an altitude of 3.58 km at the Jungfraujoch Station, Switzerland, in 1950–1951 and in 1985–1987. The results are based on equivalent width measurements of the R3 line of the 1-0 vibration-rotation band of 12C16O at 2159.30 cm−1. The set of 1985–1987 observations indicates a strong seasonal cycle in the total column abundance of CO, with a ±25% modulation between minimum values in late summer and the maximum values in late winter. Variability on shorter time scales is also present in both the old and recent data sets. The mean cumulative rate of increase of the total column abundance of CO above the Jungfraujoch is found to be (0.85 ± 0.20)%/yr between 1950–1951 and 1985–1987. The present findings are compared with trends reported in earlier studies.

title: Ozone production in the rural troposphere and the implications for regional and global ozone distributions
abstract: The relationship between O3 and NOx (NO + NO2) which was measured during summer and winter periods at Niwot Ridge, Colorado, has been analyzed and compared to model calculations. Both model calculations and observations show that the daily O3 production per unit of NOx is greater for lower NOx. Model calculations without nonmethane hydrocarbons (NMHC) tend to underestimate the O3 production rate at NOx higher than 1.5 parts per billion by volume and show the opposite dependence on NOx. The model calculations with NMHC are consistent with the observed data in this regime and demonstrate the importance of NMHC chemistry in the O3 production. In addition, at eight other rural stations with concurrent O3 and NOx measurements in the central and eastern United States the daily O3 increase in summer also agrees with the O3 and NOx relationship predicted by the model. The consistency of the observed and model-calculated daily summer O3 increase implies that the average O3 production in rural areas can be predicted if NOx is known. The dependence of O3 production rate on NOx deduced in this study provides the basis for a crude estimate of the total O3 production. For the United States an average summer column O3 production of about 1×1012Cm−2S−1 from anthropogenically emitted NOx and NMHC is estimated. This photochemical production is roughly 20 times the average cross-tropopause O3 flux. Production of O3 from NOx that is emitted from natural sources in the United States is estimated to range from 1.9×1011 to 12×1011 cm−2 s−1, which is somewhat smaller than ozone production from anthropogenic NOx sources. Extrapolation to the entire northern hemisphere shows that in the summer, 3 times as much O3 is generated from natural precursors as those of anthropogenic origin. The winter daily O3 production rate was found to be about 10% of the summer value at the same NOx level. However, because of longer NOx lifetime in the winter, the integrated O3 production over the lifetime of NOx may be comparable to the summer value. Moreover, because the natural NOx sources are substantially smaller in the winter, the wintertime O3 budget in the northern hemisphere should be dominated by ozone production from anthropogenic ozone precursors. The photochemical lifetime of O3 in the winter in the mid-latitude is approximately 200 days. We propose that this long lifetime allows anthropogenically produced O3 to accumulate and contribute substantially to the observed spring maximum that is usually attributed to stratospheric intrusion. Furthermore, the anthropogenic O3 may be transported not only zonally but also to lower latitudes. Thus the long-term interannual increase in O3, observed in the winter and spring seasons at Mauna Loa, may be due to the same anthropogenic influences as the similar winter trend observed at Hohenpeissenberg, Germany.

title: Dependence of ozone production on NO and hydrocarbons in the troposphere
abstract: 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.

title: Tropospheric chemistry: A global perspective
abstract: A model for the photochemistry of the global troposphere constrained by observed concentrations of H2O, O3, CO, CH4, NO, NO2, and HNO3 is presented. Data for NO and NO2 are insufficient to define the global distribution of these gases but are nonetheless useful in limiting several of the more uncertain parameters of the model. Concentrations of OH, HO2, H2O2, NO, NO2, NO3, N2O5, HNO2, HO2NO2, CH3O2, CH3OOH, CH2O, and CH3CCl3 are calculated as functions of altitude, latitude, and season. Results imply that the source for nitrogen oxides in the remote troposphere is geographically dispersed and surprisingly small, less than 107 tons N yr−1. Global sources for CO and CH4 are 1.5 × 109 tons C yr−1 and 4.5 × 108 tons C yr−1, respectively. Carbon monoxide is derived from combustion of fossil fuel (15%) and oxidation of atmospheric CH4 (25%), with the balance from burning of vegetation and oxidation of biospheric hydrocarbons. Production of CO in the northern hemisphere exceeds that in the southern hemisphere by about a factor of 2. Industrial and agricultural activities provide approximately half the global source of CO. Oxidation of CO and CH4 provides sources of tropospheric O3 similar in magnitude to loss by in situ photochemistry. Observations of CH3CCl3 could offer an important check of the tropospheric model and results shown here suggest that computed concentrations of OH should be reliable within a factor of 2. A more definitive test requires better definition of release rates for CH3CCl3 and improved measurements for its distribution in the atmosphere.