H. W. Pätz

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.

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.

Numerical analysis of ESR spectra from atmospheric samples

Improvements of the matrix isolation/electron spin resonance technique for the measurement of NO2, NO3, and RO2 radicals in the atmosphere are described. The use of D2O instead of H2O as the matrix yields a better spectral resolution and, as a consequence, larger a signal-to-noise ratio as well as better identification of the different species. Reference spectra of the different radicals in H2O and D2O matrices are compared. While a large degree of correlation exists amongst the spectra of the different (unsubstituted and substituted) alkylperoxy radicals, the spectra of HO2, CH3C(O)O2, and NO3 show significant differences that allow their distinction in atmospheric samples.A numerical procedure for the analysis of the composite ESR spectra obtained from atmospheric samples was developed. Subtraction of the dominant NO2 signal is performed by matching a reference NO2 spectrum with respect to amplitude, spectral position, and line width to the sample spectrum. The manipulations are performed with the virtually noise-free reference spectrum and are based on physical information. The residual spectrum is then analyzed for RO2 (and/or NO3) by simultaneously fitting up to six different reference spectra.The method was applied to laboratory samples as well as to atmospheric samples in order to demonstrate the ability of retrieving small amounts of HO2 in the presence of large amounts of NO2 and other peroxy radicals. The new algorithm allowed, for the first time, the identification of the HO2 and CH3C(O)O2 radical in tropospheric air at concentrations ranging up to 40 ppt.

Air pollution during the 2003 European heat wave as seen by MOZAIC airliners

This study presents an analysis of both MOZAIC profiles above Frankfurt and Lagrangian dispersion model simulations for the 2003 European heat wave. The comparison of MOZAIC measurements in summer 2003 with the 11-year MOZAIC climatology reflects strong temperature anomalies (exceeding 4 C) throughout the lower troposphere. Higher positive anomalies of temperature and negative anomalies of both wind speed and relative humidity are found for the period defined here as the heat wave (2–14 August 2003), compared to the periods before (16–31 July 2003) and after (16–31 August 2003) the heat wave. In addition, Lagrangian model simulations in backward mode indicate the suppressed long-range transport in the midto lower troposphere and the enhanced southern origin of air masses for all tropospheric levels during the heat wave. Ozone and carbon monoxide also present strong anomalies (both ∼+40 ppbv) during the heat wave, with a maximum vertical extension reaching 6 km altitude around 11 August 2003. Pollution in the planetary boundary layer (PBL) is enhanced during the day, with ozone mixing ratios two times higher than climatological values. This is due to a combination of factors, such as high temperature and radiation, stagnation of air masses and weak dry deposition, which favour the accumulation of ozone precursors and the build-up of ozone. A negligible role of a stratospheric-origin ozone tracer has been found for the lower troposphere in this study. From 29 July to 15 August 2003 forest fires burnt around 0.3×106 ha in Portugal and added to atmospheric pollution in Europe. Layers with enhanced CO and NO y mixing ratios, advected from Portugal, were crossed by the MOZAIC aircraft in the free troposphere over Frankfurt. A series of forward and backward Lagrangian model simulations have been performed to investigate the origin of anomalies durCorrespondence to: M. Tressol (marc.tressol@aero.obs-mip.fr) ing the whole heat wave. European anthropogenic emissions present the strongest contribution to the measured CO levels in the lower troposphere (near 30%). This source is followed by Portuguese forest fires which affect the lower troposphere after 6 August 2003 and even the PBL around 10 August 2003. The averaged biomass burning contribution reaches 35% during the affected period. Anthropogenic CO of North American origin only marginally influences CO levels over Europe during that period.

Measurements of trace gases and photolysis frequencies during SLOPE96 and a coarse estimate of the local OH concentration from HNO3 formation

The Schauinsland Ozone Precursor Experiment (SLOPE96) campaign was conducted in June 1996 to study the physicochemical transformation of pollutants and the production of photooxidants during transport from the city of Freiburg to the Schauinsland mountain. For this purpose, chemical surface measurements were made at the entrance of the valley Groses Tal, and close to the Schauinsland summit, at 1200 m altitude on a saddle at the end of the valley. In addition, measurements of ozone, NO2, and meteorological parameters were made on two tethered balloons and aboard a small aircraft. This paper describes the experimental setup and the measurements of ozone, odd-nitrogen compounds, carbonyl compounds, CO, and photolysis frequencies made during SLOPE96. The various instruments used on the different platforms were harmonized on the basis of intercomparison experiments in order to achieve a consistent picture. Large precursor concentrations from the nearby city of Freiburg are transported to Schauinsland in a valley wind system during stagnant high-pressure conditions. These conditions occurred only on 2 days of the campaign, and only 1 day (June 5) was predictable enough to allow for deployment of the aircraft and the balloons. An OH concentration of 8 - 10× 06 cm−3 is derived from the rate of change of HNO3 and NO2 on June 5 at Schauinsland (1.5–3 ppb NO2; JO1D ≈ 2 × 10−5 s−1), assuming quasi steady state and a homogeneous air mass. The ozone production rate as determined from the photostationary state of NOx at Schauinsland reached maximum daily values between 15 and 60 ppb h 1 around noontime. On average, P(O3) comprised about 30% of the photolysis rate of NO2. Similar to earlier observations at Schauinsland, a good correlation between P(O3) and the product of UV radiation and precursor concentrations was found.

Intercomparison of NO, NO2, NO y , O3, and RO x measurements during the Oxidizing Capacity of the Tropospheric Atmosphere (OCTA) campaign 1993 at Izaña

An informal comparison of NO, NO2, NOy, O3, and ROx measurements obtained by different instruments and techniques at Izana in 1993 during the European Oxidizing Capacity of the Tropospheric Atmosphere (OCTA) campaign was performed. For O3, two UV instruments agree within 7% (95% cl.) limited by a difference in response of 7.0%±0.2% (95% cl.) which likely was caused by O3 losses in one of the inlet lines. The NO mixing ratios obtained by two NO/O3 Chemiluminescence (CL) instruments range between 0–200 parts per trillion by volume (pptv), except for short periods influenced by traffic pollution. The response of the two CL detectors agrees within 3%±10% (95% cl.). The NOy data, ranging between 100 pptv and several ppbv in plumes, were obtained using two different gold-CO-converters and inlet designs with subsequent CL detection of NO. A systematic difference in the slope between the two data series of 1.44±0.05 (95% cl.) was likely caused by NOy, losses in the inlet line of one of the instruments. Three different NO2 data sets were obtained using Tunable Diode Laser Absorption Spectroscopy (TDLAS), a photolytic converter/CL technique (PLC/CL), and the Matrix Isolation Electron Spin Resonance (MIESR) technique. The linear slopes between the data sets of the three methods are consistent with unity at a 95% confidence level, 1.13±0.30 (TDL versus PLC/CL), 0.90±0.47 (TDL versus MIESR), and 1.04+0.34 (PLC/CL versus MIESR). ROx measurements were performed by three different chemical amplifier (CA) designs and the MIESR technique. Using 30-min averaged values between 13–65 pptv, two CA instruments agree within 25% (95% cl.) with the mean of MIESR (1.01+0.20 and 0.98±0.24, 95% cl.), while the third CA responded low (0.65±0.32, 95% cl.).

Climatology of NO y in the troposphere and UT/LS from measurements made in MOZAIC

In December 2000, a fully automatic NOy instrument was installed on one of the five Airbus A340 aircraft used in the MOZAIC project (Measurement of Ozone and Water Vapour by Airbus in-service Aircraft) for measurements of O3 and H2O since 1994. This long-range aircraft was operated by Lufthansa, mainly out of Frankfurt and Munich. After an initial testing period, regular data collection started in May 2001. Until May 2005, 1533 flights have been recorded, corresponding to 8500 flight hours of NOy measurements. Concurrent data of NOy and O3 are available from 1433 flights and concurrent data for CO, O3 and NOy exist from 1125 flights since 2002. The paper describes the data availability in terms of geographical, vertical and seasonal distribution and discusses the quality and limitations of the data, including interference by HCN. The vast majority of vertical profiles were measured over Frankfurt, followed by Munich and North American airports. While most of the data were collected in the upper troposphere and lower stratosphere over the North Atlantic, significant data sets exist also from flights to Far and Middle East, whereas data from the tropics and the Southern Hemisphere are relatively sparse.

The IAGOS NO x Instrument – Design, Operation and First Results from Deployment aboard Passenger Aircraft

We describe the nitrogen oxides instrument designed for the autonomous operation on board of passenger aircraft in the framework of the European Research Infrastructure IAGOS (In-service Aircraft for a Global 10 Observing System, www.iagos.org). We demonstrate the performance of the instrument using data from two deployment periods aboard an A340-300 aircraft of Deutsche Lufthansa. The well-established chemiluminescence detection method is used to measure nitrogen monoxide (NO) and nitrogen oxides (NOx). NOx is measured using a photolytic converter, and nitrogen dioxide (NO2) is determined from the difference between NOx and NO. This technique allows to measure at high time resolution (4 s) and high precision in the 15 low ppt range (NO: 2 = 24 pptv and NOx: 2 = 35 pptv) over different ambient temperature and ambient pressure altitude ranges (from surface pressure down to 190 hPa). The IAGOS NOx instrument is characterized for (1) calibration stability and total uncertainty (2) humidity and chemical interferences (e.g. ozone, HONO, PAN) and (3) inter-instrumental precision. We demonstrate that the IAGOS NOx instrument is a robust, fully automated, and long-term stable instrument suitable for unattended operation on airborne platforms, which 20 provides useful measurements for future air quality studies and emission estimates.

Transport and Transformation of Ozone

Continuous measurements of ozone, NMHC and PAN as well as CO, NO, NO2 and J(NO2) have been performed continuously (NMHC episodically) in the free troposphere at the Spanish GAW station Izana, Canary Islands (28° N, 16° W, 2370 m a.s.l.). As a consequence of the frequent influx of air from latitudes north of 45° N and higher altitudes (around 600 hPa), the yearly cycle of ozone shows a broad maximum between April and July. The cycle is comparable to those of continental European stations. Using isentropic trajectories, it is possible to link the ozone concentration to ozone-sonde data over Europe and Canada. It seems that the direct influence of stratospheric ozone from latitudes south of 45° N to that altitude is very limited. In dust laden air from the Sahara, the ozone is destroyed and concentrations can drop to below 20 ppb. PAN can be detected throughout the year, with a broad maximum occuring in late spring. Concentrations can exceed 200 ppt during low temperatures in winter and spring until May.