Showing papers by "Ulrich Platt published in 2016"

Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol

Abstract: . Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO3) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO3-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO3-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO3 radical, the difficulty of characterizing the spatial distributions of BVOC and NO3 within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry–climate models. This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO3-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO3-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.

Detection of water vapour absorption around 363 nm in measured atmospheric absorption spectra and its effect on DOAS evaluations

Abstract: . Water vapour is known to absorb radiation from the microwave region to the blue part of the visible spectrum with decreasing efficiency. Ab initio approaches to model individual absorption lines of the gaseous water molecule predict absorption lines up to its dissociation limit at 243 nm. We present the first evidence of water vapour absorption near 363 nm from field measurements using data from multi-axis differential optical absorption spectroscopy (MAX-DOAS) and long-path (LP)-DOAS measurements. The identification of the absorptions was based on the recent POKAZATEL line list by Polyansky et al. (2017). For MAX-DOAS measurements, we observed absorption by water vapour in an absorption band around 363 nm with optical depths of up to 2 × 10−3. The retrieved column densities from 2 months of measurement data and more than 2000 individual observations at different latitudes correlate well with simultaneously measured well-established water vapour absorptions in the blue spectral range from 452 to 499 nm (R2 = 0.89), but the line intensities at around 363 nm are underestimated by a factor of 2.6 ± 0.5 by the ab initio model. At a spectral resolution of 0.5 nm, we derive a maximum cross section value of 2.7 × 10−27 cm2 molec−1 at 362.3 nm. The results were independent of the used literature absorption cross section of the O4 absorption, which overlays this water vapour absorption band. Also water vapour absorption around 376 nm was identified. Below 360 nm no water vapour absorption above 1.4 × 10−26 cm2 molec−1 was observed. The newly found absorption can have a significant impact on the spectral retrievals of absorbing trace-gas species in the spectral range around 363 nm. Its effect on the spectral analysis of O4, HONO and OClO is discussed.

Plume propagation direction determination with SO 2 cameras

Abstract: SO2 cameras are becoming an established tool for measuring sulfur dioxide (SO2) fluxes in volcanic plumes with good precision and high temporal resolution The primary result of SO2 camera measurements are time series of two-dimensional SO2 column density distributions (ie SO2 column density images) However, it is frequently overlooked that, in order to determine the correct SO2 fluxes, not only the SO2 column density, but also the distance between the camera and the volcanic plume, has to be precisely known This is because cameras only measure angular extents of objects while flux measurements require knowledge of the spatial plume extent The distance to the plume may vary within the image array (ie the field of view of the SO2 camera) since the plume propagation direction (ie the wind direction) might not be parallel to the image plane of the SO2 camera If the wind direction and thus the camera–plume distance are not well known, this error propagates into the determined SO2 fluxes and can cause errors exceeding 50 % This is a source of error which is independent of the frequently quoted (approximate) compensation of apparently higher SO2 column densities and apparently lower plume propagation velocities at non-perpendicular plume observation angles Here, we propose a new method to estimate the propagation direction of the volcanic plume directly from SO2 camera image time series by analysing apparent flux gradients along the image plane From the plume propagation direction and the known location of the SO2 source (ie volcanic vent) and camera position, the camera–plume distance can be determined Besides being able to determine the plume propagation direction and thus the wind direction in the plume region directly from SO2 camera images, we additionally found that it is possible to detect changes of the propagation direction at a time resolution of the order of minutes In addition to theoretical studies we applied our method to SO2 flux measurements at Mt Etna and demonstrate that we obtain considerably more precise (up to a factor of 2 error reduction) SO2 fluxes We conclude that studies on SO2 flux variability become more reliable by excluding the possible influences of propagation direction variations

Retrieval of absolute SO2 column amounts from scattered-light spectra - Implications for the evaluation of data from automated DOAS Networks

Abstract: Abstract. Scanning spectrometer networks using scattered solar radiation in the ultraviolet spectral region have become an increasingly important tool for monitoring volcanic sulfur dioxide (SO2) emissions. Often measured spectra are evaluated using the differential optical absorption spectroscopy (DOAS) technique. In order to obtain absolute column densities (CDs), the DOAS evaluation requires a Fraunhofer reference spectrum (FRS) that is free of absorption structures of the trace gas of interest. For measurements at volcanoes such a FRS can be readily obtained if the scan (i.e. series of measurements at different elevation angles) includes viewing directions where the plume is not seen. In this case, it is possible to use these viewing directions (e.g. zenith) as FRS. Possible contaminations of the FRS by the plume can then be corrected by calculating and subtracting an SO2 offset (e.g. the lowest SO2 CD) from all viewing directions of the respective scan. This procedure is followed in the standard evaluations of data from the Network for Observation of Volcanic and Atmospheric Change (NOVAC). While this procedure is very efficient in removing Fraunhofer structures and instrumental effects it has the disadvantage that one can never be sure that there is no SO2 from the plume in the FRS. Therefore, using a modelled FRS (based on a high-resolution solar atlas) has a great advantage. We followed this approach and investigated an SO2 retrieval algorithm using a modelled FRS. In this paper, we present results from two volcanoes that are monitored by NOVAC stations and which frequently emit large volcanic plumes: Nevado del Ruiz (Colombia) recorded between January 2010 and June 2012 and from Tungurahua (Ecuador) recorded between January 2009 and December 2011. Instrumental effects were identified with help of a principal component analysis (PCA) of the residual structures of the DOAS evaluation. The SO2 retrieval performed extraordinarily well with an SO2 DOAS retrieval error of 1 − 2 × 1016 [molecules cm−2]. Compared to a standard evaluation, we found systematic differences of the differential slant column density (dSCD) of only up to ≈ 15 % when looking at the variation of the SO2 within one scan. The major advantage of our new retrieval is that it yields absolute SO2 CDs and that it does not require complicated instrumental calibration in the field (e.g. by employing calibration cells or broadband light sources), since the method exploits the information available in the measurements. We compared our method to an evaluation that is similar to the NOVAC approach, where a spectrum that is recorded directly before the scan is used as an FRS and an SO2 CD offset is subtracted from all retrieved dSCD in the scan to correct for possible SO2 contamination of the FRS. The investigation showed that 21.4 % of the scans (containing significant amounts of SO2) at Nevado del Ruiz and 7 % of the scans at Tungurahua showed much larger SO2 CDs when evaluated using modelled FRS (more than a factor of 2). For standard evaluations the overall distribution of the SO2 CDs in a scan can in some cases indicate whether the plume affects all viewing directions and thus these scans need to be discarded for NOVAC emission rate evaluation. However, there are other cases where this is not possible and thus the reported SO2 emission rates would be underestimated. The new method can be used to identify these cases and thus it can considerably improve SO2 emission budgets.

Seasonal variation of tropospheric bromine monoxide over the Rann of Kutch salt marsh seen from space

Abstract: . The Rann of Kutch (India and Pakistan) is one of the largest salt deserts in the world. Being a so-called "seasonal salt marsh", it is regularly flooded during the Indian summer monsoon. We present 10 years of bromine monoxide (BrO) satellite observations by the Ozone Monitoring Instrument (OMI) over the Great and Little Rann of Kutch. OMI spectra were analysed using Differential Optical Absorption Spectroscopy (DOAS) and revealed recurring high BrO vertical column densities (VCDs) of up to 1.4 × 1014 molec cm−2 during April/May, but no significantly enhanced column densities during the monsoon season (June–September). In the following winter months, the BrO VCDs are again slightly enhanced while the salty surface dries up. We investigate a possible correlation of enhanced reactive bromine concentrations with different meteorological parameters and find a strong relationship between incident UV radiation and the total BrO abundance. In contrast, the second Global Ozone Monitoring Instrument (GOME-2) shows about 4 times lower BrO VCDs over the Rann of Kutch than found by OMI and no clear seasonal cycle is observed. One reason for this finding might be the earlier local overpass time of GOME-2 compared to OMI (around 09:30 vs. 13:30 LT), as the ambient conditions significantly differ for both satellite instruments at the time of the measurements. Further possible reasons are discussed and mainly attributed to instrumental issues. OMI additionally confirms the presence of enhanced BrO concentrations over the Dead Sea valley (Israel/Jordan), as suggested by former ground-based observations. The measurements indicate that the Rann of Kutch salt marsh is probably one of the strongest natural point sources of reactive bromine compounds outside the polar regions and is therefore supposed to have a significant impact on local and regional ozone chemistry.

Retrieval of absolute SO 2 column amounts from scattered-light spectra: implications for the evaluation of data from automated DOAS networks

Abstract: . Scanning spectrometer networks using scattered solar radiation in the ultraviolet spectral region have become an increasingly important tool for monitoring volcanic sulfur dioxide (SO2) emissions. Often measured spectra are evaluated using the differential optical absorption spectroscopy (DOAS) technique. In order to obtain absolute column densities (CDs), the DOAS evaluation requires a Fraunhofer reference spectrum (FRS) that is free of absorption structures of the trace gas of interest. For measurements at volcanoes such a FRS can be readily obtained if the scan (i.e. series of measurements at different elevation angles) includes viewing directions where the plume is not seen. In this case, it is possible to use these viewing directions (e.g. zenith) as FRS. Possible contaminations of the FRS by the plume can then be corrected by calculating and subtracting an SO2 offset (e.g. the lowest SO2 CD) from all viewing directions of the respective scan. This procedure is followed in the standard evaluations of data from the Network for Observation of Volcanic and Atmospheric Change (NOVAC). While this procedure is very efficient in removing Fraunhofer structures and instrumental effects it has the disadvantage that one can never be sure that there is no SO2 from the plume in the FRS. Therefore, using a modelled FRS (based on a high-resolution solar atlas) has a great advantage. We followed this approach and investigated an SO2 retrieval algorithm using a modelled FRS. In this paper, we present results from two volcanoes that are monitored by NOVAC stations and which frequently emit large volcanic plumes: Nevado del Ruiz (Colombia) recorded between January 2010 and June 2012 and from Tungurahua (Ecuador) recorded between January 2009 and December 2011. Instrumental effects were identified with help of a principal component analysis (PCA) of the residual structures of the DOAS evaluation. The SO2 retrieval performed extraordinarily well with an SO2 DOAS retrieval error of 1 − 2 × 1016 [molecules cm−2]. Compared to a standard evaluation, we found systematic differences of the differential slant column density (dSCD) of only up to ≈ 15 % when looking at the variation of the SO2 within one scan. The major advantage of our new retrieval is that it yields absolute SO2 CDs and that it does not require complicated instrumental calibration in the field (e.g. by employing calibration cells or broadband light sources), since the method exploits the information available in the measurements. We compared our method to an evaluation that is similar to the NOVAC approach, where a spectrum that is recorded directly before the scan is used as an FRS and an SO2 CD offset is subtracted from all retrieved dSCD in the scan to correct for possible SO2 contamination of the FRS. The investigation showed that 21.4 % of the scans (containing significant amounts of SO2) at Nevado del Ruiz and 7 % of the scans at Tungurahua showed much larger SO2 CDs when evaluated using modelled FRS (more than a factor of 2). For standard evaluations the overall distribution of the SO2 CDs in a scan can in some cases indicate whether the plume affects all viewing directions and thus these scans need to be discarded for NOVAC emission rate evaluation. However, there are other cases where this is not possible and thus the reported SO2 emission rates would be underestimated. The new method can be used to identify these cases and thus it can considerably improve SO2 emission budgets.

The role of open lead interactions in atmospheric ozone variability between Arctic coastal and inland sites

Abstract: Boundary layer atmospheric ozone depletion events (ODEs) are commonly observed across polar sea ice regions following polar sunrise. During March-April 2005 in Alaska, the coastal site of Barrow and inland site of Atqasuk experienced ODEs (O 3 10 nmol mol -1 ) concurrently for 31% of the observations, consistent with large spatial scale ozone depletion. However, 7% of the time ODEs were exclusively observed inland at Atqasuk. This phenomenon also occurred during one of nine flights during the BRomine, Ozone, and Mercury EXperiment (BROMEX), when atmospheric vertical profiles at both sites showed near-surface ozone depletion only at Atqasuk on 28 March 2012. Concurrent in-flight BrO measurements made using nadir scanning differential optical absorption spectroscopy (DOAS) showed the differences in ozone vertical profiles at these two sites could not be attributed to differences in locally occurring halogen chemistry. During both studies, backward air mass trajectories showed that the Barrow air masses observed had interacted with open sea ice leads, causing increased vertical mixing and recovery of ozone at Barrow and not Atqasuk, where the air masses only interacted with tundra and consolidated sea ice. These observations suggest that, while it is typical for coastal and inland sites to have similar ozone conditions, open leads may cause heterogeneity in the chemical composition of the springtime Arctic boundary layer over coastal and inland areas adjacent to sea ice regions.

Detection of water vapour absorption around 363nm in measured atmospheric absorption spectra and its effect on DOAS evaluations

Abstract: Abstract. Water vapour is known to absorb radiation from the microwave region to the blue part of the visible spectrum with decreasing efficiency. Ab initio approaches to model individual absorption lines of the gaseous water molecule predict absorption lines up to its dissociation limit at 243 nm. We present the first evidence of water vapour absorption near 363 nm from field measurements using data from multi-axis differential optical absorption spectroscopy (MAX-DOAS) and long-path (LP)-DOAS measurements. The identification of the absorptions was based on the recent POKAZATEL line list by Polyansky et al. (2017). For MAX-DOAS measurements, we observed absorption by water vapour in an absorption band around 363 nm with optical depths of up to 2 × 10−3. The retrieved column densities from 2 months of measurement data and more than 2000 individual observations at different latitudes correlate well with simultaneously measured well-established water vapour absorptions in the blue spectral range from 452 to 499 nm (R2 = 0.89), but the line intensities at around 363 nm are underestimated by a factor of 2.6 ± 0.5 by the ab initio model. At a spectral resolution of 0.5 nm, we derive a maximum cross section value of 2.7 × 10−27 cm2 molec−1 at 362.3 nm. The results were independent of the used literature absorption cross section of the O4 absorption, which overlays this water vapour absorption band. Also water vapour absorption around 376 nm was identified. Below 360 nm no water vapour absorption above 1.4 × 10−26 cm2 molec−1 was observed. The newly found absorption can have a significant impact on the spectral retrievals of absorbing trace-gas species in the spectral range around 363 nm. Its effect on the spectral analysis of O4, HONO and OClO is discussed.

Numerical Analysis of the Role of Snowpack in the Ozone Depletion Events during the Arctic Spring

Abstract: . The tropospheric ozone depletion events (ODEs) and the related enhancement of reactive bromine in the boundary layer were observed in the springtime of Arctic almost 40 years ago. It is found that various substrates in polar regions such as the snowpack are able to release bromine, which is responsible for the consumption of ozone in the boundary layer. In the present simulation, a snowpack module which represents the mass transfer between the ambient air and the snowpack is implemented in a box model, aiming to clarify the influences of the snowpack on ODEs and the associated bromine explosion in the ambient air as well as in the interstitial air of the snowpack. In the snowpack module, the processes including the deposition of bromine containing compounds onto the snowpack, the mass exchange between the snow interstitial air and snow particles, and the release of Br2 from the snowpack to the ambient air are parameterized by estimating the transfer resistances which an air parcel experiences when being transported through the boundary layer into the snowpack. The present model successfully captures the complete removal of ozone both in the boundary layer and in the snow interstitial air. The temporal and spatial distributions of bromine species such as Br2 are shown and compared with observations. By changing the properties of the snowpack, it is found that the size of snow grains, volume fraction of the liquid-like layer (LLL), and the rate of the mass exchange between the snow interstitial air and the snow particles are the critical parameters which determine the occurrence of ODEs. The simulation results show that a smaller size of the snow grains considerably accelerates the ozone depletion process. Moreover, the decrease of LLL volume fraction in snow grains is found to slow down the scavenging process of HOBr by the snow particles, which prohibits the occurrence of ODEs in the snowpack. In addition, according to the simulations with the modification of the snowpack thickness, the depletion of ozone in the ambient air is shown to be influenced more heavily by the bromine explosion occurring in the surface snow layers instead of the deep snow layers. The importance of each step in the mass transfer processes occurring between the boundary layer and the snowpack is identified by conducting a local concentration sensitivity analysis. It is shown that the snow chemistry occurring in the surface snow layers has a relatively larger impact on the depletion of ozone in the ambient air compared to that within the deep snow layers. Besides, during the period of the ozone depletion, the mixing ratio of ozone in the boundary layer is mostly influenced by the deposition of HOBr onto the surface snow layers and the release of Br2 from the snow layers close to the ground surface. In contrast to that, in the interstitial air of the surface snow layer, the uptake of HOBr by snow particles is indicated as the most dominant step for the ODE.

Determination of NO x emissions from Frankfurt Airport by opticalspectroscopy (DOAS) – A feasibility study

Abstract: . Standard methods like in-situ measurements can hardly register NOx (= NO + NO2) emissions from aircrafts during take-off, when engines run at high load and thus an important amount of fuel is consumed and most of the harmful emissions are produced . The goal of this work is to show that it is possible to measure aircraft emissions generated during take-off (and initial part of the climb) by a remote spectroscopic method like automobile – based – Differential Optical Absorption Spectroscopy (Mobile-DOAS), which uses scattered solar radiation in the blue spectral range (around 445 nm). In order to test its feasibility, total column measurements of NO2 encircling Frankfurt Airport were carried out on 23 February 2012 using Mobile-DOAS. Also, NOx fluxes were derived from the NO2 observations. Unlike standard mobile-DOAS measures using a spectrometer looking at zenith, the measurements were performed looking at 22° elevation angle leading to a roughly two to three times higher sensitivity compared to zenith observations. The origin of the observed NO2 is discussed and the total NOx fluxes are calculated. As result of three round-trips encircling the Frankfurt Airport, the mean NOx flux was found to correlate with the number of aircrafts taking-off. Our results demonstrate that mobile-DOAS method is suitable for quantifying emissions from airports and to study their impact in the planetary boundary layer, which is most relevant concerning the impact on the environment and the human health.