Philippe Demoulin

Long‐term trends of inorganic chlorine from ground‐based infrared solar spectra: Past increases and evidence for stabilization

Long-term time series of hydrogen chloride (HCl) and chlorine nitrate (ClONO2) total column abundances has been retrieved from high spectral resolution ground-based solar absorption spectra recorded with infrared Fourier transform spectrometers at nine NDSC (Network for the Detection of Stratospheric Change) sites in both Northern and Southern Hemispheres. The data sets span up to 24 years and most extend until the end of 2001. The time series of Cl-y (defined here as the sum of the HCl and ClONO2 columns) from the three locations with the longest time-span records show rapid increases until the early 1990s superimposed on marked day-to-day, seasonal and inter-annual variability. Subsequently, the buildup in Cl-y slows and reaches a broad plateau after 1996, also characterized by variability. A similar time evolution is also found in the total chlorine concentration at 55 km altitude derived from Halogen Occultation Experiment (HALOE) global observations since 1991. The stabilization of inorganic chlorine observed in both the total columns and at 55 km altitude indicates that the near-global 1993 organic chlorine (CCly) peak at the Earths surface has now propagated over a broad altitude range in the upper atmosphere, though the time lag is difficult to quantify precisely from the current data sets, due to variability. We compare the three longest measured time series with two-dimensional model calculations extending from 1977 to 2010, based on a halocarbon scenario that assumes past measured trends and a realistic extrapolation into the future. The model predicts broad Cl-y maxima consistent with the long-term observations, followed by a slow Cl-y decline reaching 12-14% relative to the peak by 2010. The data reported here confirm the effectiveness of the Montreal Protocol and its Amendments and Adjustments in progressively phasing out the major man-related perturbations of the stratospheric ozone layer, in particular, the anthropogenic chlorine-bearing source gases. (Less)

Comparison of modeled and empirical approaches for retrieving columnar water vapor from solar transmittance measurements in the 0.94‐μm region

Four atmospheric transmittance models, LOWTRAN 7, MODTRAN 3, FASCOD3P, and the Thomason model, are investigated to quantify the relationship between water vapor transmittance as function of water vapor amount, Tw(U), for an instrument specific band pass in the 0.94-μm region. In a second step an empirical Tw(U) function is established using long term measurements with our high-precision Sun photometer (SPM) in Bern, Switzerland along with 1300 simultaneous and collocated water vapor retrievals performed with a dual-channel microwave radiometer (MWR). In order to avoid a possible bias in the empirical Tw(U) function, the MWR data set is prescreened by comparing retrievals coincident with radiosonde ascents. Over a 2½-year period of common observations, radiosondes and SPM agreed to within 0.19 cm (13%) of columnar water vapor (CWV) using the empirical Tw(U) relationship. Completely independent comparisons with an additional MWR and two Fourier transform spectrometers yielded agreement within 13% and 9%, respectively. Comparing empirical and modeled results, we found that with respect to the experimental data, LOWTRAN 7, MODTRAN 3, and FASCOD3P reported higher water vapor transmittances over almost the entire range of realistic absorber amounts. By relating these differences to differences in retrieved CWV for the case of two standard atmospheres, we found that using Tw(U) predicted by LOWTRAN 7, MODTRAN 3, and FASCOD3P leads to an overestimate of CWV by about 18–30%, 7–20%, and 2–18%, respectively. The Thomason model yields good agreement with respect to the experimental data up to medium absorber amounts, whereas at slant path amounts larger than 10 cm, errors up to 60% in retrieved CWV occurred. We also show in this work that a misinterpretation of the LOWTRAN 7 water vapor output counterbalances incorrectly predicted Tw, leading to results that agree well with experimental ones.

Free tropospheric CO, C2H6, and HCN above central Europe: Recent measurements from the Jungfraujoch station including the detection of elevated columns during 1998

Time series of free tropospheric carbon monoxide (CO), ethane (C 2 H 6 ), and hydrogen cyanide (HCN) column abundances have been derived from observations at the International Scientific Station of the Jungfraujoch (ISSJ) at 3.58-km altitude in the Swiss Alps (latitude 46.55°N, 7.98°E longitude). The free troposphere was assumed to extend from 3.58 to 11 km altitude, and the related columns were derived for all three molecules from high spectral resolution infrared solar spectra recorded between January 1995 and October 1999. The three molecules show distinct seasonal cycles with maxima during winter for CO and C 2 H 6 , and during spring for HCN. These seasonal changes are superimposed on interannual variations. The tropospheric columns of all three molecules were elevated during 1998. Increases were most pronounced for HCN with enhanced values throughout the year, up to a factor of 2 in January 1998 when compared to averages of the other years. The increased tropospheric columns coincide with the period of widespread wildfires during the strong El Nino warm phase of 1997-1998. The emission enhancements above ISSJ are less pronounced, and they peaked after the increases measured above Mauna Loa (19.55°N, 155.6°W). Tropospheric trends for CO, C 2 H 6 , and HCN of (2.40 ± 0.49), (0.47 ± 0.64), and (7.00 ± 1.61)% yr 1 (1 sigma) were derived for January 1995 to October 1999. However, if 1998 measurements are excluded from the fit, CO and HCN trends that are not statistically significant, and a statistically significant decrease in the C 2 H 6 tropospheric column, are inferred. Comparisons of the infrared CO columns with CO in situ surface measurements suggest that the CO free tropospheric vertical volume mixing ratio profile generally decreases with altitude throughout the year.

Observed trends in total vertical column abundances of atmospheric gases from IR solar spectra recorded at the Jungfraujoch

Since 1984, about 15000 high quality infrared solar spectra have beenrecorded with state-of-the-art grating and Fourier transform spectrometersat the International Scientific Station of the Jungfraujoch, Switzerland.Nonlinear least squares spectral curve fitting of selected microwindowscontaining isolated and well characterized lines of 20 telluric gases haveallowed to retrieve their total vertical column abundances above thestation, leading to observational data bases essential to derive long- andshort-term changes experienced by these species during the last 12 years. Inthis paper, we focus on atmospheric gases of particular interest within thecontext of the EUROTRAC/TOR (Tropospheric Ozone Research) project; secularevolution as well as seasonal cycles of the minor constituentsCH4, CO and of the trace gasesC2H6, OCS, C2H2, HCNand H2CO are reported and discussed. The long-livedN2O is included as a tracer of the dynamic activity of theatmosphere.

Ground-based infrared measurements of HNO3 total column abundances: Long-term trend and variability

The long-term trend and variability of the total column amount of atmospheric nitric acid (HNO3) have been investigated based on time series of infrared solar absorption spectra recorded at two remote high-altitude sites, the International Scientific Station of the Jungfraujoch (ISSJ) in the Swiss Alps (altitude 3.6 km, latitude 46.5°N, longitude 8.0°E) and the National Solar Observatory McMath solar telescope facility on Kitt Peak (altitude 2.1 km, latitude 31.9°N, longitude 111.6°W), southwest of Tucson, Arizona. The HNO3 ν5 band Q branch at 879.1 cm−1 and three P branch manifolds near 869 cm−1 were analyzed using a nonlinear least squares spectral fitting technique and a consistent set of spectroscopic line parameters. The ISSJ measurements evaluated in the present work consist of two solar spectra recorded with a grating spectrometer in June 1951 and a set of observations obtained with a high-resolution Fourier transform spectrometer between June 1986 and June 1990. The modern ISSJ measurements show a ∼20% peak-to-peak amplitude seasonal cycle with a winter maximum superimposed on significant variability and a summer minimum; the June results from 1986 to 1990 are both higher and lower than the two retrieved June 1951 HNO3 total column amounts. The fitted trend, (−0.16±0.50)% yr−1, 2σ, indicates that there has been no detectable change in the HNO3 total column over the last 4 decades. The Kitt Peak measurements, recorded with a high-resolution Fourier transform spectrometer between December 1980 and June 1990, also show marked variability in the HNO3 total column, but in contrast to the ISSJ measurements, no obvious seasonal cycle is observed. The deduced trend in the total column above Kitt Peak, (−0.8±1.6)% yr−1, 2σ, is consistent with the ISSJ time series of measurements, in that no significant HNO3 long-term trend has been found. The sets of measurements from the two sites are compared with each other and with previously published results, with emphasis on the reported variability of HNO3 and the changes in the HNO3 total column with season and latitude.

Modeled and empirical approaches for retrieving columnar water vapor from solar transmittance measurements in the 0.72, 0.82, and 0.94 μm absorption bands

A Sun photometer (18 channels between 300 and 1024 nm) has been used for measuring the columnar content of atmospheric water vapor (CWV) by solar transmittance measurements in absorption bands with channels centered at 719, 817, and 946 nm. The observable is the band-weighted transmittance function defined by the spectral absorption of water vapor and the spectral features of solar irradiance and system response. The transmittance function is approximated by a three-parameter model. Its parameters are determined from MODTRAN and LBLRTM simulations or empirical approaches using CWV data of a dual-channel microwave radiometer (MWR) or a Fourier transform spectrometer (FTS). Data acquired over a 2-year period during 1996-1998 at two different sites in Switzerland, Bern (560 m above sea level (asl)) and Jungfraujoch (3580 m asl) were compared to MWR, radiosonde (RS), and FTS retrievals. At the low-altitude station with an average CWV amount of 15 mm the LBLRTM approach (based on recently corrected line intensities) leads to negligible biases at 719 and 946 nm if compared to an average of MWR, RS, and GPS retrievals. However, at 817 nm an overestimate of 2.7 to 4.3 mm (18-29%) remains. At the high-altitude station with an average CWV amount of 1.4 mm the LBLRTM approaches overestimate the CWV by 1.0, 1.4. and 0.1 mm (58, 76, and 3%) at 719, 817, and 946 nm, compared to the ITS instrument. At the low-altitude station, CWV estimates, based on empirical approaches, agree with the MWR within 0.4 mm (2.5% of the mean); at the high-altitude site with a factor of 10 less water vapor the agreement of the sun photometers (SPM) with the ITS is 0.0 to 0.2 mm (1 to 9% of the mean CWV there). Sensitivity analyses show that for the conditions met at the two stations with CWV ranging from 0.2 to 30 mm, the retrieval errors are smallest if the 946 nm channel is used.

Ground-Based Infrared Measurements of Carbonyl Sulfide Total Column Abundances' Long-Term Trends and Variability

Total vertical column abundances of carbonyl sulfide (OCS) have been derived from time series of high-resolution infrared solar absorption spectra recorded at the National Solar Observatory McMath solar telescope facility on Kitt Peak (altitude 2.09 km, latitude 31.9°N, longitude 111.6°W), southwest of Tucson, Arizona, and at the International Scientific Station of the Jungfraujoch (altitude 3.58 km, latitude 46.5°N, longitude 8.0°E), in the Swiss Alps. The analysis of both data sets is based on nonlinear least squares spectral fittings of narrow intervals centered on lines of the intense ν3 band of OCS, the P(37) transition at 2045.5788 cm−1 and the P(15) transition at 2055.8609 cm−1, with a consistent set of spectroscopic line parameters. The Kitt Peak measurements, recorded on 30 different days between May 1977 and March 1991, show a 10% peak-to-peak seasonal cycle with a summer maximum and a winter minimum and a trend in the total column abundance equal to (0.1 ± 0.2)% yr−1, 2σ. Jungfraujoch solar spectra recorded on 67 different days between October 1984 and April 1991 have been analyzed. The fitted trend in the Jungfraujoch total columns, (−0.1 ± 0.5)% yr−1, 2σ, is consistent with the Kitt Peak trend results within the errors. The Jungfraujoch total columns show a more complex seasonal variation than noted in the Kitt Peak data. The mean of the daily averaged total columns, 8.44 × 1015 molecules cm−2 above Kitt Peak and 6.41 × 1015 molecules cm−2 above the Jungfraujoch station, correspond respectively to mean tropospheric mixing ratios of 0.54 ± 0.04 and 0.52 ± 0.04 parts per billion by volume; these values are consistent with previously reported remote and in situ measurements. Taken together, the results from the two sites indicate that there has been no significant change in the OCS total column abundance at northern mid-latitudes over the last decade.

Vertical column abundances of HCN deduced from ground-based infrared solar spectra: long-term trend and variability

A set of high-resolution IR solar spectra recorded at the International Scientific Station of the Jungfraujoch, Switzerland, from 84/06 to 93/06, and at the National Solar Observatory McMath-Pierce solar telescope facility on Kitt Peak, Arizona, U.S.A. from 78/05 to 92/07 have been analyzed to determine the vertical column abundances of hydrogen cyanide, HCN, above the two stations. The analysis was based on least-squares curve fitting of calculated spectra to the observations encompassing the P4 and the P8 lines of HCN respectively located at 3299.5273 and 3287.2483 cm−1. The results obtained for the two stations indicate that no significant long-term trend affects either of the two databases; however, this analysis reveals variable increases during springtime of up to a factor of 2 in the HCN total column above the Jungfraujoch and even up to 3 above Kitt Peak. The calculated mean vertical column abundances, excluding the spring observations, are equal to (2.55±0.30)×1015 molec./cm2 (S.D.) and (2.75±0.30)×1015 molec./cm2 respectively above the Jungfraujoch and the Kitt Peak observatories. Based on a realistic volume mixing ratio profile, these columns translate into mean volume mixing ratios equal to 190×10−12 ppv at the respective altitudes of the stations.

Ground-based infrared solar spectroscopic measurements of carbon monoxide during 1994 Measurement of Air Pollution From Space flights

Results of the comparison of carbon monoxide ground-based infrared solar spectroscopic measurements with data obtained during 1994 Measurement of Air Pollution From Space (MAPS) flights are presented. Spectroscopic measurements were performed correlatively with April and October MAPS flights by nine research groups from Belgium, Canada, Germany, Japan, New Zealand, Russia, and the United States. Characterization of the techniques and error analysis were performed. The role of the CO a priori profile used in the retrieval was estimated. In most cases an agreement between spectroscopic and MAPS data is within estimated MAPS accuracy of _+ 10%.

Quantitative evaluation of the post-Mount Pinatubo NO2 reduction and recovery, based on 10 years of Fourier transform infrared and UV-visible spectroscopic measurements at Jungfraujoch

The colocation of two technically different instruments for ground-based remote sensing of NO2 total column amounts at the primary Network for the Detection of Stratospheric Change Alpine station of the Jungfraujoch (46.5°N, 8.0°E) has been exploited for mutual validation of the long-term NO2 time series from both instruments and for a quantitative evaluation of the impact of the Mount Pinatubo eruption on the NO2 abundance above this northern midlatitude observatory. The two techniques are high-resolution Fourier transform infrared solar absorption spectrometry and zenith-sky differential optical absorption spectroscopy in the UV visible. The diurnal variation of NO2 has been simulated by a simple photochemical model that allows a comparison between the data from the two techniques. This model is shown to reproduce the observed morning to evening ratios to 2.3%, on average, which is fully adequate for the needs of this study. From the 1985–1996 combined time series of NO2 morning and evening abundances, it has been concluded that the enhanced aerosol load injected into the stratosphere by Mount Pinatubo caused a maximum NO2 reduction above the Jungfraujoch by 45% in early January 1992 that died out quasi-exponentially to zero by the beginning of 1995.