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

Using the Atlanta metropolitan area as a case study, we examine the effects of urbanization and its associated heat island on urban ozone concentrations. Air quality data from Atlanta suggest that urban ozone concentrations are enhanced by increases in ambient temperature. Model calculations suggest that this enhancement is caused by the effect of temperature on the atmospheric chemistry of peroxyacetyl nitrate (PAN), as well as the temperature dependence of natural and anthropogenic hydrocarbon emissions. A comparison of summertime temperatures in Atlanta and a nearby rural station, suggests that Atlanta's temperature over the past 15 years has increased by about 2øC due to urbanization and its concomitant intensification of the urban heat island. Numerical simulations using conditions of a typical summertime day in Atlanta suggest that this rise in temperature could have, (1.) resulted in a significant increase in the net emissions of natural hydrocarbons in the area in spite of the loss of about 20% of the area's forests over the same period, and (2.) negated the beneficial effects on summertime ozone concentrations that would have been obtained from a 50% reduction in anthropogenic hydrocarbon emissions. Because a NOx-based ozone abatement strategy appears to be less sensitive to temperature increases than does a hydrocarbon-based strategy, a NO x strategy may prove to be more effective in the future if temperatures continue to rise as a result of urbanization and the "greenhouse effect". 13,971 13,972 Cardelino and Chameides: Urbanization and Ozone L• 6 54-u_ 5 15 -18 23 • _ 9 _ LO 4 88 --48 n-88 27Arbor, Mich., 1981.

An Observation-Based Model (OBM) is described, which uses in-situ atmospheric observations to determine the sensitivity of ozone concentrations in an urban atmosphere to changes in the emissions of ozone precursors (i.e., volatile organic compounds and nitrogen oxides). The model is formulated following the concept of Relative Incremental Reactivity (RIR) developed by Carter and Atkinson. In the OBM, however, observed concentrations rather than emission inventories are used to drive the photochemical simulations and thereby ensure that the calculations are carried out for the proper mix of nitrogen oxides and volatile organic compounds. From these calculations, a series of sensitivity factors, or RIRs, are inferred that can be used to (1) determine whether reducing emissions of nitrogen oxide or emissions of hydrocarbons would be most effective in abating ozone in a given urban area, and (2) identify the most critical subset of hydrocarbons present in an urban atmosphere causing ozone exceedances. Because the OBM is relatively easy and inexpensive to operate and makes use of data that are increasingly available, it can be used to analyze a wide array of ozone episodes and, thus, could prove to be a relatively cost-effective tool for the analysis of ozone precursor relationships in an urban atmosphere. On the other hand, because the OBM is diagnostic rather than prognostic, it cannot be used in a predictive mode to estimate exactly how much emission reduction is needed to reduce ozone concentrations. For this reason, the OBM should be viewed as a complement to, rather than a substitute for, more sophisticated gridded, emission-based models.To illustrate the characteristics of the OBM and to demonstrate its applicability, we first compare the results of the OBM IMPLICATIONSThe proposed implementation of a network of Photochemical Assessment Monitoring Stations (PAMS) promises to create a rich database of continuous ozone, hydrocarbon, and NO measurements in ozone nonattainment areas throughout the United States. The Observation-Based Model (OBM) described in this work represents a relatively simple and cost-effective tool for analyzing these data, determining the adequacy of the data, and gaining useful insights into the identity of the chemical precursors that limit the photochemical production of urban ozone pollution.to those obtained from a series of simulations of the Atlanta metropolitan area using the Urban Airshed Model (UAM), a three-dimensional Eulerian grid model. The OBM is then used to analyze a dataset obtained from the 1990 Atlanta Ozone Study, an EPA field sampling program conducted during the summer of 1990. Because of limitations and potential flaws in the 1990 Atlanta dataset, the results of this OBM analysis are largely illustrative rather than definitive. Nevertheless, a few important issues are elucidated by the analysis. These include (1) the importance of accounting for biogenic hydrocarbons produced from urban vegetation; (2) the potential flaw in using early-morning VOC-to-NOx rat...

Observations of the chlorofluorocarbon CFCl3 obtained several times daily over the period July 1978 to June 1981 at Adrigole, Ireland (52°N, 10°W); Ragged Point, Barbados (13°N, 59°W); Point Matatula, American Samoa (14°S, 171°W); and Cape Grim, Tasmania (41°S, 145°E) are reported. In addition, observations at Cape Meares, Oregon (45°N, 124°W) are given for the period January 1980 to June 1981. On January 1, 1980, the average mixing ratio of CFCl3 in the lower troposphere is estimated to have been 168 pptv, and this is calculated to have been increasing 5.7% annually. An optimal estimation procedure for deriving the atmospheric lifetime of CFCl3 by using a nine‐box two‐dimensional model of the atmosphere is described. In this procedure, model parameters are estimated based upon minimizing the differences between the temporal trends observed and those calculated in the two‐dimensional model. Assuming that the only destruction of CFCl3 occurs in the stratosphere, the lifetime, on January 1, 1980, estimated by the trend technique is years; the lifetime estimated from the global inventory of CFCl3 is years. The maximum likelihood current lifetime estimate obtained by combining the estimates from both analysis techniques is 78 years.

Abstract. Data obtained in Hong Kong during the HongKong and the Pearl River Delta (PRD) Pilot Air Monitoring Study in autumn 2002 are analyzed to unravel the relationship between ground-level ozone (O 3 ), pollution precursors, and cross-border transport. Ten ozone episodes, during which the hourly O 3 concentration exceeded 100 ppbv in 9 cases and 90 ppbv in one case, are subject to detailed analysis, including one case with hourly O 3 of 203 ppbv, which is the highest concentration on record to date in Hong Kong. Combined with high-resolution back trajectories, dCO/dNO y (the ratio of enhancement of CO concentration above background to that of NO y ) is used to define whether O 3 is locally or regionally produced. Five out of the ten Hong Kong O 3 -episodes studied show a "pollution signature" that is indicative of impact from Guangdong Province. Examination of speciated volatile organic compounds (VOCs) shows that the reactivity of VOCs is dominated by anthropogenic VOCs, of which the reactive aromatics dominate, in particular xylenes and toluene. Calculations using a photochemical box model indicate that between 50-100% of the O 3 increase observed in Hong Kong during the O 3 episodes can be explained by photochemical generation within the Hong Kong area, provided that nitrous acid (HONO) is present at the concentrations derived from this study. An Observation-Based Model (OBM) is used to calculate the sensitivity of the O 3 production to changes in the concentrations of the precursor compounds. Generally the production of O 3 throughout much of the Hong Kong area is limited by VOCs, while high nitric oxide (NO) concentrations suppress O 3 concentration.