VOL. 82, NO. 37 JOURNAL OF GEOPHYSICAL RESEARCH DECEMBER20, 1977 e TroposphericOzone Variations Along a Meridional Band ROBERT CHATFIELD 1 AND HALSTEAD HARRISON University of Washington, Department of Atmospheric Sciences, Seattle, Washington 98195 We present summaries of 703 previously unanalyzed ozonesonde launches at six stations distributed along a meridional band near75øWbetween 9ø and 53øN.Altitudeand seasonal variations areexamined to assist understanding the patterns and relative roles of injection, transport, loss, and synthesis of ozone in the middle troposphere. By comparison with other North American stations we conclude that a longitudinal gradient of 10-20%is likely, increasing eastward. The data are generally consistent with conventional understanding of tropospheric ozonein termsof increased injection from the stratosphere during spring at high latitudes, southward transport with molecular lifetimes of several months, and a principal sink at the ground, but details of the altitude and seasonal behavior can be interpreted ambiguously asevidence either for shorter-scale transport or for ozone synthesis withinthe troposphere. INTRODUCTION In the precedingpaper, part 1 of this series, we discussed evidence for a systematic gain difference separating reported ozonesonde flightsemploying Regener-type chemiluminescent detectors, when comparedwith the Mast-type electrochemical sondes. We further examined the variabilities of the two in- struments and concluded that for the former their increased potential for finer vertical resolution was more than offsetby gain variationsfrom flight to flight. In the present paper we wish to attempt comparisonbetween flights at different sta- tions,with the hopeof resolving meaningful patterns of spatial and seasonal variations. Consequently, the lessvariable elec- trochemical sondes seemed preferable, and we have restricted our attention to analysisof a little-known and, we believe, previously unreported extension of Hering andBorden's [1964, 1965a,b, 1967]North American Ozonesonde Network, which employed the electrochemical sondes in 703 launches between 1966 and 1969 at six stationslocated roughly along a merid- ional band near 75øW between 9ø and 53øN. We report monthlymeanmixing fractions and variances at these stations, with 1 km vertical resolution. By comparison with earlier summaries of data from chemiluminescent sondes taken at other stations over the North American continent we infer the likely presence of a longitudinalgradient increasing over the continental width by 10-20% from west to east. For the six stations with electrochemical sondes we present com- plementaryplots to explore variationswith altitude, latitude, and season. We describe our somewhat limited understanding of the patterns and compare interpretations in terms of models whichalternatively emphasize transport of relatively longlived ozone, or shorter-scale synthesis and decay. TI-IE DATA Tropospheric ozone over the six stations is summarized by Tables 1-67 For each month and altitudeband the upperentry is the meanmixingfraction in micrograms per gram of air, and the lower entry is the standard deviation in the same units, both computed in the ordinary way for the n flights listedat the foot of each table. Three qualifications should be empha- sized immediately.First, the electrochemical sondes respond to other oxidants than ozone alone, and in particular to NO•., whose unmeasured mixing fractions in the middle troposphere abovethese stations probablylie near to or below0.005 ug/g. Thus in the worst case, if the smaller table entries were associ- ated with the larger NO•. fractions, the implied ozone fractions could be high by 20%; however,for most of the table entries and with more reasonable values for NO•. the bias is closer to 5%. The readershouldappreciate this qualification throughout this paper, especially in comparing differences between sta- tions. Second, sampling fluctuations from weather variations within a single season, and climate variations or trends over the short spanof four sampling years,limit the resolution of temporal comparisons to be considerably grosserthan that implied by our choosing monthly entries to these tables.In the figureswhich we derive from thesetables we have employed various smoothing filters. Readers wishing to analyze these data for other purposes may wish to do this also. Third, the listed standard deviations may mislead for at least two rea- sons: the small number of flights in each table entry make inappropriatean estimateof confidence intervalsbasedon a Gaussian probability distributionwith a large numberof de- grees of freedom,and internal correlations between soundings (many of which were launched on closely following days (or hours)into the same synoptic weather patterns) further reduce the equivalent numberof truly independent observations. To- gether thesereductions of net degrees of freedom affect the listedstandard deviations by making them equivalent to large- setstandard deviations whichwould be greater by variableand unknown amounts which could easilybe as large as 100%.For our present purposes, however,we shall wish to explore sea- sonal and spatial variations of relative standard deviations rather than their absolute magnitudes. With these precautions, then, let usdisplay the data in various ways to helpussee what they reveal. • Presentaddress: Department of AtmosphericScience, Colorado State University, Fort Collins, Colorado 80523. •' Tables 1-6 are available with entire article on microfiche. Order from AmericanGeophysical Union, 1909K Street,N. W., Washing- ton, D.C. 20006. Document J77-008;$1.00. Paymentmust accompany order. Copyright ¸ 1977by the American Geophysical Union. LONGITUDINAL VARIATIONS Since zonal circulations and mixing along the linesof lati- tude are, in general, more rapid and efficient than meridional circulations and mixingalong the longitude circles, it is plau- sible to expect an approximatezonal symmetryfor tropo- spheric ozone, providing that its lifetime in the troposphere is longer thanor comparable to thezonal circulation time of about Paper number 7C0490. 5969