Alain Marenco

Data composites of airborne observations of tropospheric ozone and its precursors

Tropospheric data from a number of aircraft campaigns have been gridded onto global maps, forming “data composites” of chemical species important in ozone photochemistry. Although these are not climatologies in the sense of a long temporal average, these data summaries are useful for providing a picture of the global distributions of these species and are a start to creating observations-based climatologies. Using aircraft measurements from a number of campaigns, we have averaged observations of O3, CO, NO, NOx, HNO3, PAN, H2O2, CH3OOH, HCHO, CH3COCH3, C2H6, and C3H8 onto a 5° latitude by 5° longitude horizontal grid with a 1-km vertical resolution. These maps provide information about the distributions at various altitudes, but also clearly show that direct observations of the global troposphere are still very limited. A set of regions with 10°–20° horizontal extent has also been chosen wherein there is sufficient data to study vertical profiles. These profiles are particularly valuable for comparison with model results, especially when a full suite of chemical species can be compared simultaneously. The O3 and NO climatologies generated from measurements obtained during commercial aircraft flights associated writh the MOZAIC and NOXAR programs are incorporated with the data composites at 10–11 km. As an example of the utility of these data composites, observations are compared to results from two global chemical transport models, MOZART and IMAGES, to help identify incorrect emission sources, incorrect strength of convection, and missing chemistry in the models. These comparisons suggest that in MOZART the NOx biomass burning emissions may be too low and convection too weak and that the transport of ozone from the stratosphere in IMAGES is too great. The ozone profiles from the data composites are compared with ozonesonde climatologies and show that in some cases the aircraft data agree with the long-term averages, but in others, such as in the western Pacific during PEM-Tropics-A, agreement is lacking. Finally, the data composites provide temporal and spatial information, which can help identify the locations and seasons where new measurements would be most valuable. All of the data composites presented here are available via the Internet (http://aoss.engin.umich.edu/SASSarchive/).

Comparisons of ozone measurements from the MOZAIC airborne program and the ozone sounding network at eight locations

Automatic ozone measuring devices have been operating continuously on board the five long-range aircraft of the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) program since September 1994. This paper presents the main characteristics of the ozone system and the procedures followed to ensure its accurate calibration over long durations. Measurement accuracy was estimated at ±[2 ppbv + 2%], but much better in-flight levels were in fact observed: average discrepancy (between different devices) ranging from 1 ppbv at tropospheric concentrations to a few ppbv at stratospheric concentrations. This demonstrates the ability of the MOZAIC ozone data to produce accurate and reliable ozone climatologies. A 2-year ozone climatology (1994–1996) generated from MOZAIC data collected at between O and 12 km altitude was compared to longer and older measurements made at eight stations of the Ozone Sounding Network (OSN): Hohenpeissenberg, Wallops Island, Tateno, Palestine, Pretoria, Goose Bay, Biscarosse, and Poona. Despite the different nature of the programs (techniques, platforms, sampling frequencies, spatial distribution, and operation periods), the OSN and MOZAIC climatologies were found to show a reasonably high level of agreement. Mean concentrations derived from ozone sondes are about 3 to 13% higher than those obtained by the MOZAIC program in the free troposphere, in a similar geographic location. These differences are within the range of uncertainty of the two techniques. Larger discrepancies observed in the boundary layer and in upper layers are explained by the influence of local pollution and the distance between measurements, amongst other factors, limiting the reliability of comparisons. A comparison of OSD and MOZAIC data at Hohenpeissenberg/Frankfurt and Wallops Island/New York, over an overlapping period (1994–1995), shows good agreement in the free troposphere (800–300 hPa), no detectable bias for Hohenpeissenberg/Frankfurt, when taking into consideration the various causes of discrepancies (Dobson normalization, ozone geographical variations). Indeed, the results of this analysis support the hypothesis that it is not advantageous to scale the ozone sonde data to the overhead ozone column; the scaling appears to cause overestimation of the tropospheric O3 concentrations, by about 3–6% at Hohenpeissenberg, and to cause more scatter in the sonde-MOZAIC differences. The correspondence between the OSN and MOZAIC climatologies obtained in very different conditions demonstrates that they are representative of the atmosphere and that, being complementary while each retains its own advantages, they are therefore both useful for validation studies.

Ubiquity of quasi-horizontal layers in the troposphere

Fine laminar structures in the atmosphere have been described previously, but their characterization has been limited. The modern global coverage of aircraft flights offers an opportunity to provide such a characterization, and examine the ubiquity of such structures, in space and time. Research aircraft measuring vertical profiles of atmospheric chemical constituents frequently discern quasi-horizontal atmospheric layers with mean thicknesses of the order of 1 km and mean altitudes between 5 and 7 km (refs 10,11,12). These layers can be characterized and categorized by various combinations of ozone, water vapour, carbon monoxide and methane deviations from background profiles. Five commercial aircraft have been recently equipped to measure water vapour and ozone concentrations, and automatically collect vertical profile information on landing and take-off (refs 13,14,15). Here we synthesize measurements from both research and commercial flights and demonstrate the ubiquity in space and time of four layer types (as categorized by their chemical signatures). Up to one-fifth of the lowest 12 km of the atmosphere is occupied by such layers. We suggest that this universality reflects basic characteristics of the atmosphere hitherto unexplored, with potential implications for present understanding of a wide variety of dynamic and chemical atmospheric processes.

General characteristics of tropospheric trace constituent layers observed in the MOZAIC program

We present a statistical study on tropospheric layers as allowed by the most extensive ozone and water vapor database currently available. Considering O 3 and H 2 O deviations from an automatically calculated background, we define four types of layers. These tropospheric layers are a common feature, with the percentage of the troposphere occupied by such layers varying from 7% to 33% depending on the region and the season. Most of the layers are found between 4 and 8 km altitude, and the median thickness is about 500 m. At northern midlatitudes we find 4 times more layers in summer than in winter, while in tropical Asia we observe a spring maximum in the occurrence of the layers. The most abundant layer type everywhere is O 3 +H 2 O- and corresponds to the signature of stratospheric intrusions or continental pollution. This suggests that stratosphere-troposphere exchanges or at least their influence are not negligible in summer at midlatitudes or in the tropics. A complete understanding of the layers could lead to a better empirical assessment of the different tropospheric ozone sources and to an assessment of the potential vorticity fluxes in the troposphere.

Redistribution by deep convection and long‐range transport of CO and CH4 emissions from the Amazon basin, as observed by the airborne campaign TROPOZ II during the wet season

Flights performed over South America in the Austral summer, during the airborne campaign Tropospheric Ozone Experiment (TROPOZ II, January 1991), have shown an upper tropospheric maximum (UTM) of carbon monoxide, methane, and relative humidity above 7 km altitude and between 30°S and 5°N. The study of chemical characteristics and convective/transport processes associated with this UTM shows that the air came from the Amazon basin boundary layer, was lifted by convective processes to upper levels, and was then redistributed over South America, and even across the South Atlantic toward the African coast, by the upper level anticyclone present during the wet season over tropical South America. The chemical composition of this UTM (high CO, CH 4 , and relative humidity; medium NOy, NO, and alkanes; low ozone and acetylene) agrees with the observations from the Atmospheric Boundary Layer Expedition ABLE 2B over the Amazon basin during the wet season and supports a biogenic origin for methane and carbon monoxide. The slow (or even absent) photochemical activity associated with this UTM can be explained by the low levels of active hydrocarbons and reactive nitrogen species. The TROPOZ II results complete and extend observations from previous expeditions by demonstrating that large biogenic emissions of ozone precursors from the Amazon basin can enter the upper general circulation during the Austral summer (December-January-February) and, though they do not contribute significantly to the ozone budget over South America, on being exported to the rest of the global circulation they will certainly become involved in the global ozone budget.

Wintertime climatology of MOZAIC ozone based on the potential vorticity and ozone analogy

A climatology is compiled of wintertime ozone measured by MOZAIC aircraft. It spans the winters of 1994/1995 to 1997/1998. It is generated employing and in the process confirming the analogy between potential vorticity and ozone. The methodology consists of interpolating equivalent latitude derived from analyzed potential vorticity onto flight tracks of the aircraft. In a second step the measurements are cast into a gridded form; this generates seasonal synopses of ozone in the coordinate space spanned by time, potential temperature, and equivalent latitude. At low equivalent latitudes the irregular appearance of high-ozone transients in combination with the relatively small measurement density renders a determination of seasonal-mean ozone problematic. However, in middle and high equivalent latitudes of the Northern Hemisphere ozone mixing ratios are well summarized by stating seasonal means and seasonal increases only. The results derived for the four winters exhibit some marked interannual variability. The method allows the determination of “expected” ozone along the flight tracks from the gridded ozone. A comparison with measured ozone for two selected periods of time suggests that discrepancies between expected and measured ozone hint at the presence of filamentary structure in the ozone field and associated stratosphere-troposphere exchange. In the four-winter mean such “outliers” of measured ozone occur most frequently in the vicinity of the North Atlantic storm track, corroborating earlier studies on filamentation activity.

Isentropic scaling analysis of ozone in the upper troposphere and lower stratosphere

We examine ozone concentrations recorded by 7630 commercial flights from August 1994 to December 1997 for spatial scaling properties. The large amount of data allows an approximately isentropic analysis of ozone variability in the upper troposphere and lower stratosphere. Since ozone is a good passive tracer at cruise altitudes, the results provide a strong diagnostic for scalar advection theories and models. Calculations of structure functions and increment probability distribution functions show that ozone variability scales anomalously from ∼2 to ∼2000 km, although not continuously in this interval. We find no evidence for the simple scaling predicted for smooth advection/diffusion, even at the large scales. At mesoscales the upper tropospheric ozone field is rougher and more intermittent than in the lower stratosphere. Within the troposphere the equatorial ozone field is rougher than at higher latitudes, and the intermittency decreases with increasing latitude. In the stratosphere the intermittency and roughness are greater at high latitudes and over land than at midlatitudes and over the ocean.