Andreas Volz-Thomas

An improved fast-response vacuum-UV resonance fluorescence CO instrument

The fast-response resonance fluorescence instrument for the airborne measurement of carbon monoxide described by Gerbig et al. [1996] was modified by implementing an improved optical filter with more efficient optics and an optimized resonance lamp. Besides reductions in size and weight, the new instrument achieves a sensitivity 10 times higher, a lower background (65 ppb compared with 250 ppb), and a faster time response (<0.1 s) than the original instrument. The precision is ±1.5 ppb at an atmospheric mixing ratio of 100 ppb CO, and the detection limit is 3 ppb (2σ) for an integration time of 1 s. First results from the North Atlantic Regional Aerosol Characterization Experiment (ACE-2) campaign during July 1997, when the new instrument was deployed aboard the U.K. Meteorological Office C-130 aircraft, are used to demonstrate the performance of the new instrument.

Trends in surface ozone concentrations at Arosa (Switzerland)

Abstract During the years 1989–1991, ozone was measured at four sites around Arosa (Switzerland). One of these sites was identical with that, where surface ozone was measured in the 1950s (Gotz and Volz, 1951; Perl, 1965). Comparison of both old and recent data indicates that surface ozone concentrations at Arosa have increased by a factor of approximately 2.2. The increase shows a seasonal variation with a relative increase of more than a factor of three in December and January. The results are discussed in the context of measurements made at other times, locations and altitudes. The comparison indicates that the increase in ozone levels at Arosa has most likely occured between the fifties and today. The measurements additionally suggest that photochemical ozone production in the free troposphere has significantly contributed to the observed ozone trends in winter.

Airborne intercomparison of vacuum ultraviolet fluorescence and tunable diode laser absorption measurements of tropospheric carbon monoxide

During the fall 1997 North Atlantic Regional Experiment (NARE 97), two separate intercomparisons of aircraft-based carbon monoxide measurement instrumentation were conducted. On September 2, CO measurements were simultaneously made aboard the National Oceanic and Atmospheric Administration (NOAA) WP-3 by vacuum ultraviolet (VUV) fluorescence and by tunable diode laser absorption spectroscopy (TDLAS). On September 18, an intercomparison flight was conducted between two separate instruments, both employing the VUV fluorescence method, on the NOAA WP-3 and the U.K. Meteorological Office C-130 Hercules. The results indicate that both of the VUV fluorescence instruments and the TDLAS system are capable of measuring ambient CO accurately and precisely with no apparent interferences in 5 s. The accuracy of the measurements, based upon three independent calibration systems, is indicated by the agreement to within 11% with systematic offsets of less than 1 ppbv. In addition, one of the groups participated in the Measurement of Air Pollution From Satellite (MAPS) intercomparison [Novelli et al., 1998] with a different measurement technique but very similar calibration system, and agreed with the accepted analysis to within 5%. The precision of the measurements is indicated by the variability of the ratio of simultaneous measurements from the separate instruments. This variability is consistent with the estimated precisions of 1.5 ppbv and 2.2 ppbv for the 5 s average results of the C-130 and the WP-3 instruments, respectively, and indicates a precision of approximately 3.6% for the TDLAS instrument. The excellent agreement of the instruments in both intercomparisons demonstrates that significant interferences in the measurements are absent in air masses that ranged from 7 km in the midtroposphere to boundary layer conditions including subtropical marine air and continental outflow with embedded urban plumes. The intercomparison of the two VUV instruments that differed widely in their design indicates that the VUV fluorescence technique for CO measurements is not particularly sensitive to the details of its implementation. These intercomparisons help to establish the reliability of ambient CO measurements by the VUV fluorescence technique.

A photoelectric detector for the measurement of photolysis frequencies of ozone and other atmospheric molecules

Photoelectric detectors for the measurement of photolysis frequencies of different trace gases in the atmosphere are described. They exhibit uniform response characteristics over one hemisphere (2π sr) and wavelength characteristics closely matched to those of the photolysis frequencies JO1D, JNO2, and JNO3, respectively. Absolute calibration of the JO1D detector was performed by chemical actinometry with an accuracy of ±16 percent. Simultaneous measurements of JNO2 and JO1D are presented.

Simultaneous measurements of peroxy and nitrate radicals at Schauinsland

We present simultaneous field measurements of NO3 and peroxy radicals made at night in a forested area (Schauinsland, Black Forest, 48° N, 8° N, 1150 ASL), together with measurements of CO, O3, NOx, NOy, and hydrocarbons, as well as meteorological parameters. NO2, NO3, HO2, and ∑(RO2) radicals are detected with matrix isolation/electron spin resonance (MIESR). NO3 and HO2 were found to be present in the range of 0–10 ppt, whilst organic peroxy radicals reached concentrations of 40 ppt. NO3, RO2, and HO2 exhibited strong variations, in contrast to the almost constant values of the longer lived trace gases. The data suggest anticorrelation between NO3 and RO2 radical concentrations at night.The measured trace gas set allows the calculation of NO3 and peroxy radical concentrations, using a chemical box model. From these simulations, it is concluded that the observed anthropogenic hydrocarbons are not sufficient to explain the observed RO2 concentrations. The chemical budget of both NO3 and RO2 radicals can be understood if emissions of monoterpenes are included. The measured HO2 can only be explained by the model, when NO concentrations at night of around 5 ppt are assumed to be present. The presence of HO2 radicals implies the presence of hydroxyl radicals at night in concentrations of up to 105 cm−3.

Climatologies of NOx and NOy: A comparison of data and models

Abstract Climatologies of tropospheric NOx (NO + NO2) and NOy (total reactive nitrogen: NOx + N03 + 2 × N2O5 + HNO2 + HNO3 + HNO4 + ClONO2 + PAN (peroxyacetylnitrate) + other organic ni trates) have been compiled from data previously published and, in most cases, publicly archived. Emphasis has been on non-urban measurements, including rural and remote ground sites, as well as aircraft data. Although the distribution of data is sparse, a compilation in this manner can begin to provide an understanding of the spatial and temporal distributions of these reactive nitrogen species. The cleanest measurements in the boundary layer are in Alaska, northern Canada and the eastern Pacific, with median NO mixing ratios below 10 pptv, NOx below 50 pptv, and NOy below 300 pptv. The highest NO values (greater than 1 ppbv) were found in eastern North America and Europe, with correspondingly high NOy (∼ 5 ppbv). A significantly narrower range of concentrations is seen in the free troposphere, particularly at 3–6 km, with NO typically about 10 pptv in the boreal summer. NO increases with altitude to ∼ 100 pptv at 9–12 km, whereas NOy does not show a trend with altitude, but varies between 100 and 1000 pptv. Decreasing mixing ratios eastward of the Asian and North American continents are seen in all three species at all altitudes. Model-generated climatologies of NOx and NOy from six chemical transport models are also presented and are compared with observations in the boundary layer and the middle troposphere for summer and winter. These comparisons test our understanding of the chemical and transport processes responsible for these species distributions. Although the model results show differences between them, and disagreement with observations, none are systematically different for all seasons and altitudes. Some of the differences between the observations and model results may likely be attributed to the specific meteorological conditions at the time that measurements were made differing from the model meteorology, which is either climatological flow from GCMs or actual meteorology for an arbitrary year. Differences in emission inventories, and convection and washout schemes in the models will also affect the calculated NOα and NOy distributions.

Trends in stratospheric and free tropospheric ozone

Current understanding of the long-term ozone trends is described. Of particular concern is an assessment of the quality of the available measurements, both ground and satellite based. Trends in total ozone have been calculated for the ground-based network and the combined data set from the solar backscatter ultraviolet (SBUV) instruments on Nimbus 7 and NOAA 11. At midlatitudes in the northern hemisphere the trends from 1979 to 1994 are significantly negative in all seasons and are larger in winter/spring (up to 7%/decade) than in summer/fall (about 3%/decade). Trends in the southern midlatitudes are also significantly negative in all seasons (3 to 6%/decade), but there is a smaller seasonal variation. In the tropics, trends are slightly negative and at the edge of being significant at the 95% confidence level: these tropical trends are sensitive to the low ozone amounts observed near the end of the record and allowance must also be made for the suspected drift in the satellite calibration. The bulk of the midlatitude loss in the ozone column has taken place at altitudes between 15 and 25 km. There is disagreement on the magnitude of the reduction, with the SAGE I/II record showing trends as large as -20 ± 8%/decade at 16-17 km and the ozonesondes indicating an average trend of -7 ± 3%/decade in the northern hemisphere. (All uncertainties given in this paper are two standard errors or 95% confidence limits unless stated otherwise). Recent ozone measurements are described for both Antarctica and the rest of the globe. The sulphate aerosol resulting from the eruption of Mount Pinatubo in 1991 and dynamic phenomena seem to have affected ozone levels, particularly at northern midlatitudes and in the Antarctic vortex. However, the record low values observed were partly caused by the long-term trends and the effect on the calculated trends was less than 1.5%/decade.

Chemical air mass differences near fronts

Two case studies are presented of aircraft measurements (ozone, NOy, CO, and meteorological parameters) in the vicinity of fronts located over the eastern side of the North Atlantic Ocean during spring 1994. The aim of these studies was twofold: (1) to investigate whether frontal circulations can transport ozone from the boundary layer to the free troposphere in well-defined layer; and (2) to ascertain whether or not conveyor belts associated with extratropical cyclones exhibit well-defined chemical signatures. The first case study (March 2, 1994) sampled a well-defined ozone-enhanced layer within the free troposphere. It is demonstrated that this air was transferred from the boundary layer to the free troposphere during the development of a baroclinic wave. Two warm conveyor belts sampled within this flight (one associated with the developing baroclinic wave and the other with a mature low-pressure system) displayed clear and contrasting chemical signatures, a consequence of their geographically different origins. During the second case study (April 25, 1994), both the dry intrusion and the warm conveyor belt of a mature, occluded low-pressure system were sampled. Their chemical signatures (in particular, that of the dry intrusion) showed that interleaving of the two airstreams had occurred, probably in the vicinity of the occluded front. It is thus demonstrated that chemical measurements within conveyor belts provide valuable information on the nature, history, and extent of these coherent flows.

Measurements of alkyl nitrates in rural and polluted air masses

Abstract Gas chromatographic measurements of organic nitrates (C1C8) were made at Julich (52°N, 7°E) and at Schauinsland/Black Forest (48°N, 6°E), beginning in autumn 1988. Samples were collected cryogenically by passing 3l of ambient air through a sampling loop, filled with glass beads, at 77 K. Separation was performed by capillary gas chromatography. An optimized chemiluminescence NO-analyser operating at a pressure of 1 mbar in combination with a catalytic converter was used as a specific detector for NO-containing molecules with a detection limit on the order of 1 ppt for a 3l sample. The measurements show the presence of C1C5 nitrates at concentration levels between 1 and 230 ppt. Nitrates greater than C5 were only observed at Julich and at very low mixing ratios. Nitrates > C 8 were not observed. The combined concentrations of organic nitrates account, on the average, for 1–2% of the NOy. On a few occasions, the fraction could be as high as 7%. Therefore, organic nitrates are not likely to explain the missing fraction of NOy, first reported by Fahey et al. (1986). From the correlation between the concentrations of organic nitrates and ozone and from the absence of nitrates > C5 at Schauinsland it is concluded that ozone formation proceeds mainly via the smaller alkylperoxy radicals, HO2 and CH3C(O)O2.

Airborne measurements of the photolysis frequency of NO2

A set of photoelectric detectors for airborne measurements of the photolysis frequency of NO2, i.e., JNO2, was developed and integrated aboard the research aircraft Hercules C-130 operated by the U.K. Meteorological Office. The instrument consists of two separate sensors, each of which provides an isotropic response over a solid angle of 2π steradian (sr). The sensors are mounted on top and below the aircraft, respectively, to obtain a field of view of 4π sr, and permit the discrimination of the upwelling and downwelling components of the actinic flux. From experimental tests and model calculations it is demonstrated that small differences between the spectral sensitivity of the sensors and the spectral response of JNO2 can lead to significant errors in the determination of JNO2, especially under cloudy conditions. We present correction factors for clear sky conditions and suggest the use of a new filter combination in the sensors which requires only small corrections and provides acceptable accuracy, even under cloudy conditions. A climatology of JNO2 values is presented from a series of flights made in 1993 at latitudes of 36°–59°N. For clear sky conditions and solar zenith angles of 33°–35°, JNO2 was 8.3 × 10−3 s−1 at sea level and increased with altitude to values of 13 × 10−3 s−1 at 7.5 km altitude. Above clouds, JNO2 reached maximum values of 24 × 10−3 s−1, and peak values of 29 × 10−3 s−1 were observed for very short periods in the uppermost layers of clouds. Enhancement of the actinic flux due to light scattered from clouds was also observed at altitudes below 0.5 km. Comparison of the clear sky data with predictions from different radiative transfer models reveals the best agreement for models of higher angular resolution. The Delta Eddington method underpredicts the measurements significantly, whereas the JNO2 values predicted by the discrete ordinate method and multidirectional model are only about 5% smaller than our measurements, a difference that is within the experimental uncertainties.