Showing papers by "University of California, Irvine published in 1997"
TL;DR: The Bayesian classifier is shown to be optimal for learning conjunctions and disjunctions, even though they violate the independence assumption, and will often outperform more powerful classifiers for common training set sizes and numbers of attributes, even if its bias is a priori much less appropriate to the domain.
Abstract: The simple Bayesian classifier is known to be optimal when attributes are independent given the class, but the question of whether other sufficient conditions for its optimality exist has so far not been explored. Empirical results showing that it performs surprisingly well in many domains containing clear attribute dependences suggest that the answer to this question may be positive. This article shows that, although the Bayesian classifier‘s probability estimates are only optimal under quadratic loss if the independence assumption holds, the classifier itself can be optimal under zero-one loss (misclassification rate) even when this assumption is violated by a wide margin. The region of quadratic-loss optimality of the Bayesian classifier is in fact a second-order infinitesimal fraction of the region of zero-one optimality. This implies that the Bayesian classifier has a much greater range of applicability than previously thought. For example, in this article it is shown to be optimal for learning conjunctions and disjunctions, even though they violate the independence assumption. Further, studies in artificial domains show that it will often outperform more powerful classifiers for common training set sizes and numbers of attributes, even if its bias is a priori much less appropriate to the domain. This article‘s results also imply that detecting attribute dependence is not necessarily the best way to extend the Bayesian classifier, and this is also verified empirically.
3,225 citations
TL;DR: Three kinds of algorithms that learn axis-parallel rectangles to solve the multiple instance problem are described and compared, giving 89% correct predictions on a musk odor prediction task.
2,767 citations
TL;DR: In this paper, the role of buyer search costs in markets with differentiated product offerings is analyzed in the context of an electronic marketplace, and the allocational efficiencies such a reduction can bring to a differentiated market are formalized.
Abstract: Information systems can serve as intermediaries between the buyers and the sellers in a market creating an "electronic marketplace" that lowers the buyers' cost to acquire information about seller prices and product offerings. As a result, electronic marketplaces reduce the inefficiencies caused by buyer search costs, in the process reducing the ability of sellers to extract monopolistic profits while increasing the ability of markets to optimally allocate productive resources. This article models the role of buyer search costs in markets with differentiated product offerings. The impact of reducing these search costs is analyzed in the context of an electronic marketplace, and the allocational efficiencies such a reduction can bring to a differentiated market are formalized. The resulting implications for the incentives of buyers, sellers, and independent intermediaries to invest in electronic marketplaces are explored. Finally, the possibility to separate price information from product attribute information is introduced, and the implications of designing markets promoting competition along each of these dimensions are discussed.
2,366 citations
Book•
01 Jan 1997TL;DR: The second edition of the Fundamentals of Microfabrication as discussed by the authors provides an in-depth coverage of the science of miniaturization, its methods, and materials, from the fundamentals of lithography through bonding and packaging to quantum structures and molecular engineering.
Abstract: MEMS technology and applications have grown at a tremendous pace, while structural dimensions have grown smaller and smaller, reaching down even to the molecular level. With this movement have come new types of applications and rapid advances in the technologies and techniques needed to fabricate the increasingly miniature devices that are literally changing our world.A bestseller in its first edition, Fundamentals of Microfabrication, Second Edition reflects the many developments in methods, materials, and applications that have emerged recently. Renowned author Marc Madou has added exercise sets to each chapter, thus answering the need for a textbook in this field.Fundamentals of Microfabrication, Second Edition offers unique, in-depth coverage of the science of miniaturization, its methods, and materials. From the fundamentals of lithography through bonding and packaging to quantum structures and molecular engineering, it provides the background, tools, and directions you need to confidently choose fabrication methods and materials for a particular miniaturization problem.New in the Second EditionRevised chapters that reflect the many recent advances in the fieldUpdated and enhanced discussions of topics including DNA arrays, microfluidics, micromolding techniques, and nanotechnology In-depth coverage of bio-MEMs, RF-MEMs, high-temperature, and optical MEMs.Many more links to the WebProblem sets in each chapter
2,334 citations
TL;DR: The late fall in DBP after age 60 years, associated with a continual rise in SBP, cannot be explained by "burned out" diastolic hypertension or by "selective survivorship" but is consistent with increased large artery stiffness.
Abstract: Background We attempted to characterize age-related changes in blood pressure in both normotensive and untreated hypertensive subjects in a population-based cohort from the original Framingham Heart Study and to infer underlying hemodynamic mechanisms. Methods and Results A total of 2036 participants were divided into four groups according to their systolic blood pressure (SBP) at biennial examination 10, 11, or 12. After excluding subjects receiving antihypertensive drug therapy, up to 30 years of data on normotensive and untreated hypertensive subjects from biennial examinations 2 through 16 were used. Regressions of blood pressure versus age within individual subjects produced slope and curvature estimates that were compared with the use of ANOVA among the four SBP groups. There was a linear rise in SBP from age 30 through 84 years and concurrent increases in diastolic blood pressure (DBP) and mean arterial pressure (MAP); after age 50 to 60 years, DBP declined, pulse pressure (PP) rose steeply, and MA...
2,176 citations
TL;DR: In this paper, the authors describe four approaches to the employee-organization relationship, as defined from the employer's perspective, and find that employees performed better on core tasks, demonstrated more citizenship behavior, and expressed a higher level of affective commitment to an employer when they worked in an overinvestment (by the employer) or mutual investment relationship than when they working in a quasi-spot-contract or underinvestment relationship.
Abstract: This article describes four approaches to the employee-organization relationship, as defined from the employer's perspective. An empirical study of employees from ten companies found support for the basic hypothesis that employee responses differ under the four types of relationship. In general, employees performed better on core tasks, demonstrated more citizenship behavior, and expressed a higher level of affective commitment to an employer when they worked in an overinvestment (by the employer) or mutual investment relationship than when they worked in a quasi-spot-contract or underinvestment relationship. These results were obtained even after we controlled for several other variables that could affect employee performance and attitudes.
1,708 citations
TL;DR: The use of a naive Bayesian classifier is described, and it is demonstrated that it can incrementally learn profiles from user feedback on the interestingness of Web sites and may easily be extended to revise user provided profiles.
Abstract: We discuss algorithms for learning and revising user profiles that can determine which World Wide Web sites on a given topic would be interesting to a user. We describe the use of a naive Bayesian classifier for this task, and demonstrate that it can incrementally learn profiles from user feedback on the interestingness of Web sites. Furthermore, the Bayesian classifier may easily be extended to revise user provided profiles. In an experimental evaluation we compare the Bayesian classifier to computationally more intensive alternatives, and show that it performs at least as well as these approaches throughout a range of different domains. In addition, we empirically analyze the effects of providing the classifier with background knowledge in form of user defined profiles and examine the use of lexical knowledge for feature selection. We find that both approaches can substantially increase the prediction accuracy.
1,353 citations
TL;DR: In this article, the authors explore the relationship between soil mineralogy and organic carbon along two natural gradients (i.e., soil-age and climate) in volcanic soil environments.
Abstract: A large source of uncertainty in present understanding of the global carbon cycle is the distribution and dynamics of the soil organic carbon reservoir. Most of the organic carbon in soils is degraded to inorganic forms slowly, on timescales from centuries to millennia1. Soil minerals are known to play a stabilizing role, but how spatial and temporal variation in soil mineralogy controls the quantity and turnover of long-residence-time organic carbon is not well known2. Here we use radiocarbon analyses to explore interactions between soil mineralogy and soil organic carbon along two natural gradients—of soil-age and of climate—in volcanic soil environments. During the first ∼150,000 years of soil development, the volcanic parent material weathered to metastable, non-crystalline minerals. Thereafter, the amount of non-crystalline minerals declined, and more stable crystalline minerals accumulated. Soil organic carbon content followed a similar trend, accumulating to a maximum after 150,000 years, and then decreasing by 50% over the next four million years. A positive relationship between non-crystalline minerals and organic carbon was also observed in soils through the climate gradient, indicating that the accumulation and subsequent loss of organic matter were largely driven by changes in the millennial scale cycling of mineral-stabilized carbon, rather than by changes in the amount of fast-cycling organic matter or in net primary productivity. Soil mineralogy is therefore important in determining the quantity of organic carbon stored in soil, its turnover time, and atmosphere–ecosystem carbon fluxes during long-term soil development; this conclusion should be generalizable at least to other humid environments.
1,308 citations
TL;DR: The combination of cockroach allergy and exposure to high levels of this allergen may help explain the frequency of asthma-related health problems in inner-city children.
Abstract: Background It has been hypothesized that asthma-related health problems are most severe among children in inner-city areas who are allergic to a specific allergen and also exposed to high levels of that allergen in bedroom dust. Methods From November 1992 through October 1993, we recruited 476 children with asthma (age, four to nine years) from eight inner-city areas in the United States. Immediate hypersensitivity to cockroach, house-dust-mite, and cat allergens was measured by skin testing. We then measured major allergens of cockroach (Bla g 1), dust mites (Der p 1 and Der f 1), and cat dander (Fel d 1) in household dust using monoclonal-antibody–based enzyme-linked immunosorbent assays. High levels of exposure were defined according to proposed thresholds for causing disease. Data on morbidity due to asthma were collected at base line and over a one-year period. Results Of the children, 36.8 percent were allergic to cockroach allergen, 34.9 percent to dust-mite allergen, and 22.7 percent to cat allerg...
1,245 citations
TL;DR: Data suggest that BDNF protein produced in adult CNS neurons is polarized primarily along axonal processes and is preferentially stored in terminals within the innervation target.
Abstract: A sensitive immunohistochemical technique was used, along with highly specific affinity-purified antibodies to brain-derived neurotrophic factor (BDNF), to generate a detailed mapping of BDNF immunoreactivity (BDNF-ir) throughout the adult rat CNS. A parallel analysis of sites of BDNF synthesis was performed with in situ hybridization techniques using a cRNA probe to the exon encoding mature rat BDNF protein. These combined data revealed (1) groups of cell bodies containing diffuse BDNF-ir throughout the CNS that were strongly correlated with fields of cells containing BDNF mRNA; (2) varying degrees of BDNF-ir outside of cell bodies, in what appeared to be fibers and/or terminals; and (3) many regions containing extremely heavy BDNF-immunoreactive fiber/terminal labeling that lacked BDNF mRNA (e.g., medial habenula, central nucleus of the amygdala, bed nucleus of stria terminalis, lateral septum, and spinal cord). The latter observation suggested that in these regions BDNF was derived from anterograde axonal transport by afferent systems. In the two cases in which this hypothesis was tested by the elimination of select afferents, BDNF immunostaining was completely eliminated. These data, along with the observation that BDNF-ir was rarely found within dendrites or fibers en passage, suggest that BDNF protein produced in adult CNS neurons is polarized primarily along axonal processes and is preferentially stored in terminals within the innervation target.
1,084 citations
TL;DR: OH, NO3, and O3 are shown to play a central role in the formation and fate of airborne toxic chemicals, mutagenic polycyclic aromatic hydrocarbons, and fine particles.
Abstract: Tropospheric air pollution has impacts on scales ranging from local to global. Reactive intermediates in the oxidation of mixtures of volatile organic compounds (VOCs) and oxides of nitrogen (NOx) play central roles: the hydroxyl radical (OH), during the day; the nitrate radical (NO3), at night; and ozone (O3), which contributes during the day and night. Halogen atoms can also play a role during the day. Here the implications of the complex VOC-NOx chemistry for O3 control are discussed. In addition, OH, NO3, and O3 are shown to play a central role in the formation and fate of airborne toxic chemicals, mutagenic polycyclic aromatic hydrocarbons, and fine particles.
Proceedings Article•
08 Dec 1997TL;DR: GreedyDual-Size as discussed by the authors incorporates locality with cost and size concerns in a simple and nonparameterized fashion for high performance, which can potentially improve the performance of main-memory caching of Web documents.
Abstract: Web caches can not only reduce network traffic and downloading latency, but can also affect the distribution of web traffic over the network through cost-aware caching. This paper introduces GreedyDual-Size, which incorporates locality with cost and size concerns in a simple and non-parameterized fashion for high performance. Trace-driven simulations show that with the appropriate cost definition, GreedyDual-Size outperforms existing web cache replacement algorithms in many aspects, including hit ratios, latency reduction and network cost reduction. In addition, GreedyDual-Size can potentially improve the performance of main-memory caching of Web documents.
TL;DR: Using a low-coherence Michelson interferometer, the authors measured two-dimensional images of optical birefringence in bovine tendon as a function of depth, which allowed rapid noncontact investigation of tissue structural properties.
Abstract: Using a low-coherence Michelson interferometer, we measure two-dimensional images of optical birefringence in bovine tendon as a function of depth. Polarization-sensitive detection of the signal formed by interference of backscattered light from the sample and a mirror in the reference arm give the optical phase delay between light that is propagating along the fast and slow axes of the birefringent tendon. Images showing the change in birefringence in response to laser irradiation are presented. The technique permits rapid noncontact investigation of tissue structural properties through two-dimensional imaging of birefringence.
TL;DR: The Aneurysm Detection and Management (ADAM) study is an ongoing randomized clinical trial comparing two strategies for the management of AAA in patients 50 to 79 years of age with asymptomatic AAAs.
Abstract: Background: Independent risk factors for abdominal aortic aneurysm (AAA) have not been clearly defined in multivariable analyses of large patient populations. Objective: To identify factors that ar...
TL;DR: This study is the first to report localized hemispheric structural anomalies in ADHD, which are concordant with theoretical models of abnormal frontal-striatal and parietal function, and with possible differing morphologic substrates of response to stimulant medication.
Abstract: Article abstract-Objective: To test by MRI-based morphometry the a priori hypotheses that developmental anomalies exist in attention-deficit hyperactivity disorder (ADHD) in left caudate and right prefrontal/frontal/ and/or posterior parietal hemispheric regions, in accord with neurochemical, neuronal circuitry and attentional network hypotheses, and prior imaging studies. Design: Case-control study. Setting: Academic medical center. Participants: Fifteen male subjects with ADHD without comorbid diagnoses (aged 12.4 +/- 3.4 years) and 15 male normal controls (aged 14.4 +/- 3.4), group-matched for age, IQ, and handedness. Main outcome measures: Global and hemispheric regional volumes (in cm 3 ) of cerebral hemispheres, cortex, white matter, ventricles, caudate, lenticulate, central gray nuclei, insula, amygdala, and hippocampus. Results: Despite similar hemispheric volumes, ADHD subjects had smaller volumes of (1) left total caudate and caudate head (p NEUROLOGY 1997;48: 589-601
TL;DR: Evidence is presented that the lexicon is part of a temporal-parietalhnedial-temporal declarative memory system and that granlmatical rules are processed by a frontamasal-ganglia procedural system.
Abstract: Language comprises a lexicon for storing words and a grammar for generating rule-governed forms. Evidence is presented that the lexicon is part of a temporal-parietalhnedial-temporal “declarative memory” system and that granlmatical rules are processed by a frontamasal-ganglia “procedural” system. Patients produced past tenses of regular and novel verbs (looked and plagged), which require an -ed-suffixation rule, and irregular verbs (dug), which are retrieved from memory. Word-finding difficulties in posterior aphasia, and the general declarative memory impairment in Alzheimer's disease, led to more errors with irregular than regular and novel verbs. Grammatical difficulties in anterior aphasia, and the general impairment of procedures in Parkinson's disease, led to the opposite pattern. In contrast to the Parkinson's patients, who showed sup pressed motor activity and rule use, Huntington's disease patients showed excess motor activity and rule use, underscoring a role for the basal ganglia in grammatical processing.
TL;DR: In this paper, the authors examined whether the large measured wage differentials for on-the-job computer use reflect a true return to computer skills, or do they just reflect that higher wage workers use computers on their jobs.
Abstract: Are the large measured wage differentials for on-the-job computer use a true return to computer skills, or do they just reflect that higher wage workers use computers on their jobs? We examine this issue with three large cross-sectional surveys from Germany. First, we confirm that the estimated wage differential associated with computer use in Germany is very similar to the U.S. differential. Second, we also measure large differentials for on-the-job use of calculators, telephones, pens or pencils, or for those who work while sitting down. We argue that these findings cast some doubt on the literal interpretation of the computer use wage differential as reflecting true returns to computer use or skill.
TL;DR: The findings of an exploratory study of intermediaries in electronic markets that suggest that markets do not necessarily become disintermediated as they become facilitated by information technology are discussed.
Abstract: It is often argued that as electronic markets lower the cost of market transactions, traditional roles for intermediaries will be eliminated, leading to "disintermediation." We discuss the findings of an exploratory study of intermediaries in electronic markets that suggest that markets do not necessarily become disintermediated as they become facilitated by information technology. We explore thirteen case studies of firms participating in electronic commerce and find evidence of certain new emerging roles for electronic intermediaries, including aggregating, matching suppliers and customers, providing trust, and providing interorganizational market information. Two specific examples are discussed in greater detail to illustrate an unsuccessful strategy for electronic intermediation (BargainFinder) as well as a successful one (Firefly).
TL;DR: The crystal structure of horseradish peroxidase isozyme C (HRPC) has been solved to 2.15 Å resolution and the key residue involved in direct interactions with aromatic donor molecules is identified, predicted to be important for the ability of HRPC to bind aromatic substrates.
Abstract: The crystal structure of horseradish peroxidase isozyme C (HRPC) has been solved to 2.15 A resolution. An important feature unique to the class III peroxidases is a long insertion, 34 residues in HRPC, between helices F and G. This region, which defines part of the substrate access channel, is not present in the core conserved fold typical of peroxidases from classes I and II. Comparison of HRPC and peanut peroxidase (PNP), the only other class III (higher plant) peroxidase for which an X-ray structure has been completed, reveals that the structure in this region is highly variable even within class III. For peroxidases of the HRPC type, characterized by a larger FG insertion (seven residues relative to PNP) and a shorter F′ helix, we have identified the key residue involved in direct interactions with aromatic donor molecules. HRPC is unique in having a ring of three peripheral Phe residues, 142, 68 and 179. These guard the entrance to the exposed haem edge. We predict that this aromatic region is important for the ability of HRPC to bind aromatic substrates.
TL;DR: The feasibility of automatic recognition of recommendations is supported by empirical results and some resources are recommended by more than one person, and these multiconfirmed recommendations appear to be significant resources for the relevant community.
Abstract: The feasibility of automatic recognition of recommendations is supported by empirical results. First, Usenet messages are a significant source of recommendations of Web resources: 23% of Usenet messages mention Web resources, and ?>0% of these mentions are recommendations. Second, recommendation instances can be machine-recognized with nearly 90% accuracy. Third, some resources are recommended by more than one person. These multiconfirmed recommendations appear to be significant resources for the relevant community. Finally, the number of distinct recommenders of a resource is a tallying, and redistributing recom-
TL;DR: Results indicate that this sophisticated technique, which is still early in its development, can achieve precision comparable to that of densitometry and can predict femoral fracture load to within -40% to +60% with 95% confidence.
TL;DR: The techniques of mindfulness meditation, with their emphasis on developing detached observation and awareness of the contents of consciousness, may represent a powerful cognitive behavioral coping strategy for transforming the ways in which the authors respond to life events.
Abstract: Background: This study examined the effects of an 8-week stress reduction program based on training in mindfulness meditation. Previous research efforts suggesting this program may
TL;DR: An elaborate genetic system in the Lyme disease spirochete Borrelia burgdorferi that promotes extensive antigenic variation of a surface-exposed lipoprotein, VlsE, could potentially produce millions of antigenic variants in the mammalian host.
TL;DR: An initial attempt to establish the interrater reliability of the trigger point examination that failed, and a second study by the same examiners that included a training period and that successfully established interRater reliability in the diagnosis of the MTrP are reported.
Abstract: The myofascial trigger point (MTrP) is the hallmark physical finding of the myofascial pain syndrome (MPS). The MTrP itself is characterized by distinctive physical features that include a tender point in a taut band of muscle, a local twitch response (LTR) to mechanical stimulation, a pain referral pattern characteristic of trigger points of specific areas in each muscle, and the reproduction of the patient's usual pain. No prior study has demonstrated that these physical features are reproducible among different examiners, thereby establishing the reliability of the physical examination in the diagnosis of the MPS. This paper reports an initial attempt to establish the interrater reliability of the trigger point examination that failed, and a second study by the same examiners that included a training period and that successfully established interrater reliability in the diagnosis of the MTrP. The study also showed that the interrater reliability of different features varies, the LTR being the most difficult, and that the interrater reliability of the identification of MTrP features among different muscles also varies.
TL;DR: The development of an optical technique for noninvasive imaging of in vivo blood flow dynamics and tissue structures with high spatial resolution (2-10 microm) in biological systems based on optical Doppler tomography (ODT).
Abstract: We report the development of an optical technique for noninvasive imaging of in vivo blood flow dynamics and tissue structures with high spatial resolution (2-10 microm) in biological systems. The technique is based on optical Doppler tomography (ODT), which combines Doppler velocimetry with optical coherence tomography to measure blood flow velocity at discrete spatial locations. The exceptionally high resolution of ODT permits noninvasive in vivo imaging of both blood microcirculation and tissue structures surrounding the vessel, which has significance for biomedical research and clinical applications. Tomographic imaging of in vivo blood flow velocity in the chick chorioallantoic membrane and in rodent skin is demonstrated.
TL;DR: EUS-guided FNA of the pancreas appears to be a safe and effective method that increases both the diagnostic and staging capability of EUS in pancreatic cancer.
TL;DR: Brynjolfsson et al. as discussed by the authors used the CES-translog production function to estimate the role of differences among firms in IT-related output and substitution elasticities.
Abstract: Fueled by its constant technological and price improvements, information technology IT is displacing other inputs in the production of goods and services. By 1994, IT accounts for over 15% of fixed investments by the U.S. private sector, and the ratio of new IT investments to labor costs is approaching 5% 1990 dollar basis. The ability to take advantage of improvements in IT is determined in part by the substitutability of IT for other factors of production.
This paper builds on the empirical framework of Brynjolfsson and Hitt Brynjolfsson, E., L. Hitt. 1995. Information technology as a factor of production the role of differences among firms. Econom. Innovations and New Tech.3 183-199. and extends it to jointly estimate output and substitution elasticities using the CES-translog production function. Our primary source of IT-related data is the IDG/Computerworld annual survey data on IS spending by large U.S. firms, for the period 1988 to 1992, previously analyzed by Brynjolfsson and Hitt [Brynjolfsson, E., L. Hitt. 1995. Information technology as a factor of production the role of differences among firms. Econom. Innovations and New Tech.3 183-199.], [Brynjolfsson, E., L. Hitt. 1996. Paradox lost? Firm-level evidence on the returns to information systems spending. Management Sci.424 541-558.] and Lichtenberg Lichtenberg, F. R. 1995. The output contributions of computer equipment and personnel a firm level analysis. Econom. Innovations and New Tech.3 201-217..
A key result is that IT capital is a net substitute for both ordinary capital and labor, suggesting that the factor share of IT in production will grow to more significant levels over time. We confirm earlier findings of positive returns to IT investment for this data set. Further, we find excess returns on IT investment relative to labor input and some evidence of excess returns relative to ordinary capital. Taken together, these results shed new light on the productivity paradox of IT and on the growth of information intensity across the economy as firms take advantage of the continuing improvements in IT.
TL;DR: Findings provide the first evidence that CREB-mediated transcription is integral to hippocampal-dependent memory consolidation processes.
Abstract: Extensive evidence suggests that long term memory (LTM) formation is dependent on the activation of neuronal second messenger systems and requires protein synthesis. The cAMP response element binding protein (CREB) is a constitutively expressed regulatory transcription factor that couples changes in second messenger levels to changes in cellular transcription. Several recent studies suggest that CREB and related transcription factors regulate gene expression necessary for neuronal plasticity and LTM. However, the role of CREB, within defined mammalian brain structures, in mediating the cellular events underlying LTM formation has not been investigated. We examined whether CREB-mediated transcription within the dorsal hippocampus is critical to LTM consolidation of water maze spatial training, which is known to depend on dorsal hippocampal function. Pretraining infusions of antisense oligodeoxynucleotides (ODN) directed against CREB mRNA were used to disrupt hippocampal CREB protein levels in adult rats. Control groups received pretraining infusions of ODN of the same base composition but in a randomized order (scrambled ODN) or buffer. Task acquisition and memory up to 4 h (i.e., short term memory) were similar in CREB antisense ODN and control groups. In contrast, CREB antisense ODN-infused rats exhibited significantly impaired memory 48 h later (i.e., LTM). Moreover, administration of antisense ODN 1 day after training did not affect subsequent retention performance. These findings provide the first evidence that CREB-mediated transcription is integral to hippocampal-dependent memory consolidation processes.
TL;DR: In this paper, Price et al. studied the role of lightning in maintaining the global distribution of nitrogen oxides (NOx) in the troposphere and derived a global rate for cloud-to-ground (CG) flashes of 20−30 flashes with a mean energy per flash of 6.7×109 J.
Abstract: This paper begins a study on the role of lightning in maintaining the global distribution of nitrogen oxides (NOx) in the troposphere. It presents the first global and seasonal distributions of lightning-produced NOx (LNOx) based on the observed distribution of electrical storms and the physical properties of lightning strokes. We derive a global rate for cloud-to-ground (CG) flashes of 20–30 flashes/s with a mean energy per flash of 6.7×109 J. Intracloud (IC) flashes are more frequent, 50–70 flashes/s but have 10% of the energy of CG strokes and, consequently, produce significantly less NOx. It appears to us that the majority of previous studies have mistakenly assumed that all lightning flashes produce the same amount of NOx, thus overestimating the NOx production by a factor of 3. On the other hand, we feel these same studies have underestimated the energy released in CG flashes, resulting in two negating assumptions. For CG energies we adopt a production rate of 10×1016 molecules NO/J based on the current literature. Using a method to simulate global lightning frequencies from satellite-observed cloud data, we have calculated the LNOx on various spatial (regional, zonal, meridional, and global) and temporal scales (daily, monthly, seasonal, and interannual). Regionally, the production of LNOx is concentrated over tropical continental regions, predominantly in the summer hemisphere. The annual mean production rate is calculated to be 12.2 Tg N/yr, and we believe it extremely unlikely that this number is less than 5 or more than 20 Tg N/yr. Although most of LNOx, is produced in the lowest 5 km by CG lightning, convective mixing in the thunderstorms is likely to deposit large amounts of NOx, in the upper troposphere where it is important in ozone production. On an annual basis, 64% of the LNOx, is produced in the northern hemisphere, implying that the northern hemisphere should have natural ozone levels as much as 2 times greater than the southern hemisphere, even before anthropogenic influences. The amount of O3 produced from this NOx is expected to exceed the stratospheric source by a factor of 1.5, and thus the hemispheric asymmetry in LNOx would lead to a significant excess of northern hemisphere O3 even in the preindustrial troposphere. (The monthly climatologies for LNOx on a 1°×1° latitude-longitude grid can be obtained by e-mail to cprice@flash.tau.ac.il).