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.

Tropospheric ozone plays a major role in Earth's atmospheric chemistry processes and also acts as an air pollutant and greenhouse gas. Due to its short lifetime, and dependence on sunlight and precursor emissions from natural and anthropogenic sources, tropospheric ozone's abundance is highly variable in space and time on seasonal, interannual and decadal time-scales. Recent, and sometimes rapid, changes in observed ozone mixing ratios and ozone precursor emissions inspired us to produce this up-to-date overview of tropospheric ozone's global distribution and trends. Much of the text is a synthesis of in situ and remotely sensed ozone observations reported in the peer-reviewed literature, but we also include some new and extended analyses using well-known and referenced datasets to draw connections between ozone trends and distributions in different regions of the world. In addition, we provide a brief evaluation of the accuracy of rural or remote surface ozone trends calculated by three state-of-the-science chemistry-climate models, the tools used by scientists to fill the gaps in our knowledge of global tropospheric ozone distribution and trends.

1. Scope -is the work directly or implicitly related to atmospheric composition? 2. Novelty -does the work provide a) a general and/or broader relevance (e.g. not a pure local study), b) new results or methods, and c) does it add significantly to the knowledge of atmospheric composition and its impacts?3. Quality -does the work contain high quality a) atmospheric observations, b) process studies, c) modeling exercises or d) data analysis?Will your paper be within the scope of Atmospheric Environment?We try to be flexible with novel scientific articles on issues of atmospheric composition even, if they are not directly related to atmospheric measurements (e.g. wind tunnel studies, dynamometer studies, remote sensing retrieval, etc). However, we are still cautious of purely mathematical derivations, preliminary results or insignificant case and local studies. The authors should make sure that the articles contain substantial contributions to the science of atmospheric composition before sending them for review.

The concentrations of ozone, nitrogen oxides, and nonmethane hydrocarbons measured near the surface in a variety of urban, suburban, rural, and remote locations are analyzed and compared in order to elucidate the relationships between ozone, its photochemical precursors, and the sources of these precursors. While a large gradient is found among remote, rural, and urban/suburban nitrogen oxide concentrations, the total hydrocarbon reactivity in all continental locations is found to be comparable. Apportionment of the observed hydrocarbon species to mobile and stationary anthropogenic sources and biogenic sources suggests that present-day emission inventories for the United States underestimate the size of mobile emissions. The analysis also suggests a significant role for biogenic hydrocarbon emissions in many urban/suburban locations and a dominant role for these sources in rural areas of the eastern United States. As one moves from remote locations to rural locations and then from rural to urban/suburban locations, ozone and nitrogen oxide concentrations tend to increase in a consistent manner while total hydrocarbon reactivity does not. hydrocarbon concentrations in four chemically distinct regimes of the atmospheric boundary layer, each having a distinct mix of anthropogenic and natural hydrocarbon and NOx emissions. These regimes are: I, the urban/suburban atmosphere, which is the regime most strongly impacted by anthropogenic emissions; II, the rural atmosphere, which is somewhat less impacted by anthropogenic emissions and more impacted by natural emissions than that of the urban atmosphere; III, the atmosphere over the remote, tropical forest which is essentially free of anthropogenic volatile organic compounds (VOC) and NOx emissions and strongly influenced by natural emissions; and IV, the remote, marine atmosphere, which is not only free of anthropogenic emissions but is also characterized by relatively small biogenic sources of VOC and NO x. Because we are most interested in the conditions that foster ozone episodes, our analysis concentrates on observations made during the daylight hours of the summer months.In the sections below we first briefly summarize the fundamentals of the photochemical smog mechanism and the nonlinearities inherent in this system and then discuss the concentrations of 03, NOx, and hydrocarbons typically observed in the four regimes listed above. PHOTOCHEMICAL SMOGWhile uncertainties remain in our understanding of tropospheric photochemistry, the basic set of reactions that lead to 03 production have been identified. These reactions, commonly referred to in the aggregate as the "photochemical smog" mechanism, involve the oxidation of hydrocarbons and other volatile organic compounds in the presence of nitrogen oxides (NOx) and sunlight [Haagen-Srnit, 1952; $einfeld, 1988]. Typical of this mechanism are reactions (R1) through (R7),RH + OH--> R + H20 (R2) R + 02 + M--> RO2 + M (R3) RO 2 + NO--> RO + NO 2 (R4) RO + 0 2 --> HO 2 + RCHO 6037

In the lowermost layer of the atmosphere-the troposphere-ozone is an important source of the hydroxyl radical, an oxidant that breaks down most pollutants and some greenhouse gases. High concentrations of tropospheric ozone are toxic, however, and have a detrimental effect on human health and ecosystem productivity. Moreover, tropospheric ozone itself acts as an effective greenhouse gas. Much of the present tropospheric ozone burden is a consequence of anthropogenic emissions of ozone precursors resulting in widespread increases in ozone concentrations since the late 1800s. At present, east Asia has the fastest-growing ozone precursor emissions. Much of the springtime east Asian pollution is exported eastwards towards western North America. Despite evidence that the exported Asian pollution produces ozone, no previous study has found a significant increase in free tropospheric ozone concentrations above the western USA since measurements began in the late 1970s. Here we compile springtime ozone measurements from many different platforms across western North America. We show a strong increase in springtime ozone mixing ratios during 1995-2008 and we have some additional evidence that a similar rate of increase in ozone mixing ratio has occurred since 1984. We find that the rate of increase in ozone mixing ratio is greatest when measurements are more heavily influenced by direct transport from Asia. Our result agrees with previous modelling studies, which indicate that global ozone concentrations should be increasing during the early part of the twenty-first century as a result of increasing precursor emissions, especially at northern mid-latitudes, with western North America being particularly sensitive to rising Asian emissions. We suggest that the observed increase in springtime background ozone mixing ratio may hinder the USA's compliance with its ozone air quality standard.