Showing papers by "Ulrich Platt published in 2017"

Sensitivity of the radiative forcing by stratospheric sulfur geoengineering to the amount and strategy of the SO 2 injection studied with the LMDZ-S3A model

Abstract: . The enhancement of the stratospheric sulfate aerosol layer has been proposed as a method of geoengineering to abate global warming. Previous modelling studies found that stratospheric aerosol geoengineering (SAG) could effectively compensate for the warming by greenhouse gases on the global scale, but also that the achievable cooling effect per sulfur mass unit, i.e. the forcing efficiency, decreases with increasing injection rate. In this study we use the atmospheric general circulation model LMDZ with the sectional aerosol module S3A to determine how the forcing efficiency depends on the injected amount of SO2 , the injection height, and the spatio-temporal pattern of injection. We find that the forcing efficiency may decrease more drastically for larger SO2 injections than previously estimated. As a result, the net instantaneous radiative forcing does not exceed the limit of –2 W m−2 for continuous equatorial SO2 injections and it decreases (in absolute value) for injection rates larger than 20 Tg S yr−1 . In contrast to other studies, the net radiative forcing in our experiments is fairly constant with injection height (in a range 17 to 23 km ) for a given amount of SO2 injected. Also, spreading the SO2 injections between 30 ∘ S and 30 ∘ N or injecting only seasonally from varying latitudes does not result in a significantly larger (i.e. more negative) radiative forcing. Other key characteristics of our simulations include a consequent stratospheric heating, caused by the absorption of solar and infrared radiation by the aerosol, and changes in stratospheric dynamics, with a collapse of the quasi-biennial oscillation at larger injection rates, which has impacts on the resulting spatial aerosol distribution, size, and optical properties. But it has to be noted that the complexity and uncertainty of stratospheric processes cause considerable disagreement among different modelling studies of stratospheric aerosol geoengineering. This may be addressed through detailed model intercomparison activities, as observations to constrain the simulations of stratospheric aerosol geoengineering are not available and analogues (such as volcanic eruptions) are imperfect.

Observations of bromine monoxide transport in the Arctic sustained on aerosol particles

Abstract: . The return of sunlight in the polar spring leads to the production of reactive halogen species from the surface snowpack, significantly altering the chemical composition of the Arctic near-surface atmosphere and the fate of long-range transported pollutants, including mercury. Recent work has shown the initial production of reactive bromine at the Arctic surface snowpack; however, we have limited knowledge of the vertical extent of this chemistry, as well as the lifetime and possible transport of reactive bromine aloft. Here, we present bromine monoxide (BrO) and aerosol particle measurements obtained during the March 2012 BRomine Ozone Mercury EXperiment (BROMEX) near Utqiaġvik (Barrow), AK. The airborne differential optical absorption spectroscopy (DOAS) measurements provided an unprecedented level of spatial resolution, over 2 orders of magnitude greater than satellite observations and with vertical resolution unable to be achieved by satellite methods, for BrO in the Arctic. This novel method provided quantitative identification of a BrO plume, between 500 m and 1 km aloft, moving at the speed of the air mass. Concurrent aerosol particle measurements suggest that this lofted reactive bromine plume was transported and maintained at elevated levels through heterogeneous reactions on colocated supermicron aerosol particles, independent of surface snowpack bromine chemistry. This chemical transport mechanism explains the large spatial extents often observed for reactive bromine chemistry, which impacts atmospheric composition and pollutant fate across the Arctic region, beyond areas of initial snowpack halogen production. The possibility of BrO enhancements disconnected from the surface potentially contributes to sustaining BrO in the free troposphere and must also be considered in the interpretation of satellite BrO column observations, particularly in the context of the rapidly changing Arctic sea ice and snowpack.

Long open-path measurements of greenhouse gases in air using near-infrared Fourier transform spectroscopy

Abstract: . In complex and urban environments, atmospheric trace gas composition is highly variable in time and space. Point measurement techniques for trace gases with in situ instruments are well established and accurate, but do not provide spatial averaging to compare against developing high-resolution atmospheric models of composition and meteorology with resolutions of the order of a kilometre. Open-path measurement techniques provide path average concentrations and spatial averaging which, if sufficiently accurate, may be better suited to assessment and interpretation with such models. Open-path Fourier transform spectroscopy (FTS) in the mid-infrared region, and differential optical absorption spectroscopy (DOAS) in the UV and visible, have been used for many years for open-path spectroscopic measurements of selected species in both clean air and in polluted environments. Near infrared instrumentation allows measurements over longer paths than mid-infrared FTS for species such as greenhouse gases which are not easily accessible to DOAS. In this pilot study we present the first open-path near-infrared (4000–10 000 cm−1, 1.0–2.5 µm) FTS measurements of CO2, CH4, O2, H2O and HDO over a 1.5 km path in urban Heidelberg, Germany. We describe the construction of the open-path FTS system, the analysis of the collected spectra, several measures of precision and accuracy of the measurements, and the results a four-month trial measurement period in July–November 2014. The open-path measurements are compared to calibrated in situ measurements made at one end of the open path. We observe significant differences of the order of a few ppm for CO2 and a few tens of ppb for CH4 between the open-path and point measurements which are 2 to 4 times the measurement repeatability, but we cannot unequivocally assign the differences to specific local sources or sinks. We conclude that open-path FTS may provide a valuable new tool for investigations of atmospheric trace gas composition in complex, small-scale environments such as cities.

Horizontal and vertical structure of reactive bromine events probed by bromine monoxide MAX-DOAS

Abstract: . Heterogeneous photochemistry converts bromide (Br−) to reactive bromine species (Br atoms and bromine monoxide, BrO) that dominate Arctic springtime chemistry. This phenomenon has many impacts such as boundary-layer ozone depletion, mercury oxidation and deposition, and modification of the fate of hydrocarbon species. To study environmental controls on reactive bromine events, the BRomine, Ozone, and Mercury EXperiment (BROMEX) was carried out from early March to mid-April 2012 near Barrow (Utqiaġvik), Alaska. We measured horizontal and vertical gradients in BrO with multiple-axis differential optical absorption spectroscopy (MAX-DOAS) instrumentation at three sites, two mobile and one fixed. During the campaign, a large crack in the sea ice (an open lead) formed pushing one instrument package ∼ 250 km downwind from Barrow (Utqiaġvik). Convection associated with the open lead converted the BrO vertical structure from a surface-based event to a lofted event downwind of the lead influence. The column abundance of BrO downwind of the re-freezing lead was comparable to upwind amounts, indicating direct reactions on frost flowers or open seawater was not a major reactive bromine source. When these three sites were separated by ∼ 30 km length scales of unbroken sea ice, the BrO amount and vertical distributions were highly correlated for most of the time, indicating the horizontal length scales of BrO events were typically larger than ∼ 30 km in the absence of sea ice features. Although BrO amount and vertical distribution were similar between sites most of the time, rapid changes in BrO with edges significantly smaller than this ∼ 30 km length scale episodically transported between the sites, indicating BrO events were large but with sharp edge contrasts. BrO was often found in shallow layers that recycled reactive bromine via heterogeneous reactions on snowpack. Episodically, these surface-based events propagated aloft when aerosol extinction was higher (> 0.1 km−1); however, the presence of aerosol particles aloft was not sufficient to produce BrO aloft. Highly depleted ozone (

Periodicity in the BrO∕SO 2 molar ratios in the volcanic gas plume of Cotopaxi and its correlation with the Earth tides during the eruption in 2015

Abstract: . We evaluated NOVAC (Network for Observation of Volcanic and Atmospheric Change) gas emission data from the 2015 eruption of the Cotopaxi volcano (Ecuador) for BrO∕SO2 molar ratios. The BrO∕SO2 molar ratios were very small prior to the phreatomagmatic explosions in August 2015, significantly higher after the explosions, and continuously increasing until the end of the unrest period in December 2015. These observations together with similar findings in previous studies at other volcanoes (Mt. Etna, Nevado del Ruiz, Tungurahua) suggest a possible link between a drop in BrO∕SO2 and a future explosion. In addition, the observed relatively high BrO∕SO2 molar ratios after December 2015 imply that bromine degassed predominately after sulfur from the magmatic melt. Furthermore, statistical analysis of the data revealed a conspicuous periodic pattern with a periodicity of about 2 weeks in a 3-month time series. While the time series is too short to rule out a chance recurrence of transient geological or meteorological events as a possible origin for the periodic signal, we nevertheless took this observation as a motivation to examine the influence of natural forcings with periodicities of around 2 weeks on volcanic gas emissions. One strong aspirant with such a periodicity are the Earth tides, which are thus central in this study. We present the BrO∕SO2 data, analyse the reliability of the periodic signal, discuss a possible meteorological or eruption-induced origin of this signal, and compare the signal with the theoretical ground surface displacement pattern caused by the Earth tides. Our central result is the observation of a significant correlation between the BrO∕SO2 molar ratios with the north–south and vertical components of the calculated tide-induced surface displacement with correlation coefficients of 47 and 36 %, respectively. From all other investigated parameters, only the correlation between the BrO∕SO2 molar ratios and the relative humidity in the local atmosphere resulted in a comparable correlation coefficient of about 33 %.

Remote measurement of high preeruptive water vapor emissions at Sabancaya volcano by passive differential optical absorption spectroscopy

Abstract: Water (H2O) is by far the most abundant volcanic volatile species and plays a predominant role in driving volcanic eruptions. However, numerous difficulties associated with making accurate measurements of water vapor in volcanic plumes have limited their use as a diagnostic tool. Here we present the first detection of water vapor in a volcanic plume using passive visible-light differential optical absorption spectroscopy (DOAS). Ultraviolet and visible-light DOAS measurements were made on 21 May 2016 at Sabancaya Volcano, Peru. We find that Sabancaya's plume contained an exceptionally high relative water vapor abundance 6 months prior to its November 2016 eruption. Our measurements yielded average sulfur dioxide (SO2) emission rates of 800–900 t/d, H2O emission rates of around 250,000 t/d, and an H2O/SO2 molecular ratio of 1000 which is about an order of magnitude larger than typically found in high-temperature volcanic gases. We attribute the high water vapor emissions to a boiling-off of Sabancaya's hydrothermal system caused by intrusion of magma to shallow depths. This hypothesis is supported by a significant increase in the thermal output of the volcanic edifice detected in infrared satellite imagery leading up to and after our measurements. Though the measurement conditions encountered at Sabancaya were very favorable for our experiment, we show that visible-light DOAS systems could be used to measure water vapor emissions at numerous other high-elevation volcanoes. Such measurements would provide observatories with additional information particularly useful for forecasting eruptions at volcanoes harboring significant hydrothermal systems.

The Sectional Stratospheric Sulfate Aerosol module (S3A-v1) within the LMDZ general circulation model: description and evaluation against stratospheric aerosol observations

Abstract: Stratospheric aerosols play an important role in the climate system by affecting the Earth's radiative budget as well as atmospheric chemistry, and the capabilities to simulate them interactively within global models are continuously improving. It is important to represent accurately both aerosol microphysical and atmospheric dynamical processes because together they affect the size distribution and the residence time of the aerosol particles in the stratosphere. The newly developed LMDZ-S3A model presented in this article uses a sectional approach for sulfate particles in the stratosphere and includes the relevant microphysical processes. It allows full interaction between aerosol radiative effects (e.g. radiative heating) and atmospheric dynamics, including e.g. an internally generated quasi-biennial oscillation (QBO) in the stratosphere. Sulfur chemistry is semi-prescribed via climatological lifetimes. LMDZ-S3A reasonably reproduces aerosol observations in periods of low (background) and high (volcanic) stratospheric sulfate loading, but tends to overestimate the number of small particles and to underestimate the number of large particles. Thus, it may serve as a tool to study the climate impacts of volcanic eruptions, as well as the deliberate anthropogenic injection of aerosols into the stratosphere, which has been proposed as a method of geoengineering to abate global warming.

Horizontal and vertical structure of reactive bromine events probedby bromine monoxide MAX-DOAS spectroscopy

Abstract: Heterogeneous photochemistry converts bromide (Br − ) to reactive bromine species (Br atoms and bromine monoxide, BrO) that dominate Arctic springtime chemistry. This phenomenon has many impacts such as boundary-layer ozone depletion, mercury oxidation and deposition, and modification of the fate of hydrocarbon species. To study environmental controls on reactive bromine events, the BRomine, Ozone, and Mercury EXperiment (BROMEX) was carried out from early March to mid April 2012 near Barrow (Utqiaġvik), Alaska. We measured horizontal and vertical gradients in BrO with Multiple-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) instrumentation at three sites, two mobile and one fixed. During the campaign, a large crack in the sea ice (an open lead) formed pushing one instrument package ~ 250 km downwind from Barrow (Utqiaġvik). Convection associated with the open lead converted the BrO vertical structure from a surface-based event to a lofted event downwind of the lead influence. The column abundance of BrO downwind of the re-freezing lead was comparable to upwind amounts indicating direct reactions on frost flowers or open seawater was not a major reactive bromine source. When these three sites were separated by ~ 30 km length scales of unbroken sea ice, the BrO amount and vertical distributions were highly correlated for most of the time, indicating the horizontal length scales of BrO events were typically larger than ~ 30 km in the absence of sea-ice features. Although correlation dominated most of the time, rapid changes in BrO with edges significantly sharper than this ~ 30 km length scale episodically transported between the sites, indicating BrO events were large but with sharp edge contrasts. BrO was often found in shallow layers that recycled reactive bromine via heterogeneous reactions on snowpack. Episodically, these surface-based events propagated aloft, which required enhanced aerosol extinction aloft; however, the presence of aerosol particles aloft was not sufficient to produce BrO aloft. Highly depleted ozone ( −1 ) repartitioned reactive bromine away from BrO and drove BrO events aloft in cases. This work demonstrates the interplay between atmospheric mixing and heterogeneous chemistry that affects the vertical structure and horizontal extent of reactive bromine events.

The tilt effect in DOAS observations

Abstract: . Experience of differential atmospheric absorption spectroscopy (DOAS) shows that a spectral shift between measurement spectra and reference spectra is frequently required in order to achieve optimal fit results, while the straightforward calculation of the optical density proves inferior. The shift is often attributed to temporal instabilities of the instrument but implicitly solved the problem of the tilt effect discussed/explained in this paper. Spectral positions of Fraunhofer and molecular absorption lines are systematically shifted for different measurement geometries due to an overall slope – or tilt – of the intensity spectrum. The phenomenon has become known as the tilt effect for limb satellite observations, where it is corrected for in a first-order approximation, whereas the remaining community is less aware of its cause and consequences. It is caused by the measurement process, because atmospheric absorption and convolution in the spectrometer do not commute. Highly resolved spectral structures in the spectrum will first be modified by absorption and scattering processes in the atmosphere before they are recorded with a spectrometer, which convolves them with a specific instrument function. In the DOAS spectral evaluation process, however, the polynomial (or other function used for this purpose) accounting for broadband absorption is applied after the convolution is performed. In this paper, we derive that changing the order of the two modifications of the spectra leads to different results. Assuming typical geometries for the observations of scattered sunlight and a spectral resolution of 0.6 nm, this effect can be interpreted as a spectral shift of up to 1.5 pm, which is confirmed in the actual analysis of the ground-based measurements of scattered sunlight as well as in numerical radiative transfer simulations. If no spectral shift is allowed by the fitting routine, residual structures of up to 2.5 × 10−3 peak-to-peak are observed. Thus, this effect needs to be considered for DOAS applications aiming at an rms of the residual of 10−3 and below.

Detection of O 4 absorption around 328 and 419 nm in measured atmospheric absorption spectra

Abstract: . Retrieving the column of an absorbing trace gas from spectral data requires that all absorbers in the corresponding wavelength range are sufficiently well known. This is especially important for the retrieval of weak absorbers, whose absorptions are often in the 10−4 range. Previous publications on the absorptions of the oxygen dimer O2–O2 (or short: O4) list absorption peaks at 328 and 419 nm, for which no spectrally resolved literature cross sections are available. As these absorptions potentially influence the spectral retrieval of various trace gases, such as HCHO, BrO, OClO and IO, their shape and magnitude need to be quantified. We assume that the shape of the absorption peaks at 328 and 419 nm can be approximated by their respective neighbouring absorption peaks. Using this approach we obtain estimates for the wavelength of the absorption and its magnitude. Using long-path differential optical absorption spectroscopy (LP-DOAS) observations and multi-axis DOAS (MAX-DOAS) observations, we estimate the peak absorption cross sections of O4 to be (1.96 ± 0.20) × 10−47 cm5 molec−2 and determine the wavelength of its maximum at 328.59 ± 0.15 nm. For the absorption at 419.13 ± 0.42 nm a peak O4 cross-section value is determined to be (5.0 ± 3.5) × 10−48 cm5 molec−2.

Intra-pixel variability in satellite tropospheric NO 2 column densities derived from simultaneous spaceborne and airborne observations

Abstract: Aircraft measurements of NO 2 using an imaging differential optical absorption spectrometer (iDOAS) instrument over the South African Highveld region in August 2007 are presented and compared to satellite measurements from OMI and SCIAMACHY. In-situ aerosol and trace-gas vertical profile measurements, along with aerosol optical thickness and single-scattering albedo measurements from the Aerosol Robotic Network (AERONET), are used to devise scenarios for a radiative-transfer modelling sensitivity study. Uncertainty in the air-mass factor due to variations in the aerosol and NO 2 profile shape is constrained, and used to calculate vertical column densities (VCD), which are compared to co-located satellite measurements. The lower spatial resolution of the satellites cannot resolve the detailed plume structures revealed in the aircraft measurements. The airborne DOAS in general measured steeper horizontal gradients and higher peak NO 2 vertical column density. Aircraft measurements close to major sources, spatially-averaged to the satellite resolution, indicate NO 2 column densities more than twice those measured by the satellite. The agreement between the high-resolution aircraft instrument and the satellite instrument improves with distance from the source. Despite their low spatial resolution, satellite images reveal point sources and plumes that retain their structure for several hundred kilometers downwind.

Detection of O 4 absorption around 328 nm and 419 nm in measuredatmospheric absorption spectra

Abstract: Retrieving the column of an absorbing trace gas from spectral data requires that all absorbers in the corresponding wavelength range are sufficiently well known. This is especially important for the retrieval of weak absorbers, whose absorptions are often in the 10 -4 range. Previous publications on the absorptions of the oxygen dimer O 2 -O 2 (or short: O 4 ) list absorption peaks at 328 nm and 419 nm, for which no spectrally resolved literature cross-sections are available. As these absorptions potentially influence the spectral retrieval of various trace gases, such as HCHO, BrO, OClO and IO, their shape and magnitude needs to be quantified. We assume that the shape of the absorption peaks at 328 nm and 419 nm can be approximated by their respective neighboring absorption peaks. Using this approach we obtain estimates for the wavelength of the absorption and its magnitude. Using Longpath Differential Optical Absorption Spectroscopy (LP-DOAS) observations and Multi-Axis (MAX)-DOAS observations, we estimate the peak absorption cross-sections of O 4 to be (1.7 ± 0.2) x 10 -47 cm 5 molec -2 and determine the wavelength of its maximum at 328.51 ± 0.15 nm. For the absorption at 419.0 ± 0.4 nm a peak O 4 cross-section value is determined as (3.7 ± 2.7) x 10 -48 cm 5 molec -2 .

Verfahren und Vorrichtung zur Kalibration eines optischen Resonators, Verwendung der Vorrichtung, Verwendung eines optischen Modulators und Computerprogrammprodukt

Abstract: Die vorliegende Erfindung betrifft ein Verfahren und eine Vorrichtung zur Kalibration eines optischen Resonators. Dabei umfasst das Verfahren die folgenden Schritte: – Erzeugen von Lichtpulsen einer bekannten Pulsfrequenz mittels einer Lichtpulserzeugungseinheit; – Einkoppeln der Lichtpulse in den Resonator (20); – Modulieren des aus dem Resonator (20) austretenden Lichts mittels eines optischen Modulators (30), wobei eine Modulationsfrequenz des optischen Modulators (30) im Wesentlichen gleich der Pulsfrequenz der erzeugten Lichtpulse ist oder wobei eine Modulationsfrequenz des optischen Modulators (30) im Wesentlichen ein ganzzahliges Vielfaches der Pulsfrequenz der erzeugten Lichtpulse betragt oder wobei die Pulsfrequenz der erzeugten Lichtpulse im Wesentlichen ein ganzzahliges Vielfaches der Modulationsfrequenz des optischen Modulators (30) betragt; – Detektieren des modulierten Lichts mittels eines Detektors (40); – Kalibrieren des optischen Resonators (20) auf Basis des detektierten modulierten Lichts. Ferner betrifft die vorliegende Erfindung eine Verwendung der erfindungsgemasen Vorrichtung, eine Verwendung eines optischen Modulators sowie ein Computerprogrammprodukt.

Method and device for calibrating an optical resonator, use of the device, use of an optical modulator, and computer program product

Abstract: The invention relates to a method and to a device for calibrating an optical resonator. The method comprises the following steps: generating light pulses of a known pulse frequency by means of a light-pulse generation unit; coupling the light pulses into the resonator (20); detecting light exiting the resonator (20) by means of a detection apparatus in order to generate a detection signal, wherein the detection apparatus is designed to generate the detection signal by means of a modulator as a signal modulated with a modulation frequency, wherein the modulation frequency is substantially equal to the pulse frequency of the generated light pulses or wherein the modulation frequency is substantially an integer multiple of the pulse frequency of the generated light pulses or wherein the pulse frequency of the generated light pulses is substantially an integer multiple of the modulation frequency; and calibrating the optical resonator (20) on the basis of the detection signal. The invention further relates to a use of the device according to the invention, to a use of a switchable detector or of an optical modulator, and to a computer program product.