Determination of two dimensional trace gas distributions using tomographic LP-DOAS measurements in the city of Heidelberg, Germany

Tomographic Long path Differential Optical Absorption Spectroscopy (LP-DOAS) allows two and three dimensional determination of trace gas distributions by measuring the average concentration along 10 to 20 intersecting light paths and applying tomographic inversion techniques. In this thesis such a setup was developed and applied for the first time to determine the horizontal distribution of several trace gases in the open atmosphere. The measurements took place in the city of Heidelberg, Germany from 2005 to 2007 and focused on the trace gases NO2, SO2, O3, HCHO and HONO, which play a major role in the polluted atmosphere. The setup consisted of three Multi Beam LP-DOAS instruments and 20 retro reflector arrays all installed on different buildings in the city. The 20 realised intersecting light paths covered an area of 4 × 4 km² with different emission sources. The retrieved horizontal trace gas distributions had a temporal resolution of up to 15 minutes with best results for NO2, SO2 and O3. The highest trace gas concentrations (except for O3) and spatial variations arose during low wind situations. Emission sources varying in space and time could be distinguished and identified mainly as emissions from traffic (NO2 with O3 depletion) and power plants / industry (SO2). Several insights into chemical processes in the atmosphere could be gained by studying the interrelationship of the measured trace gases. HONO, for example, displayed much lower spatial variability than NO2 and was thus not directly emitted by the same source but rather formed in heterogeneous reactions. Transport processes of plumes were also investigated

New methods for the calibration of optical resonators : integrated calibration by means of optical modulation (ICOM) and narrow-band cavity ring-down (NB-CRD)

Optical resonators are used in spectroscopic measurements of atmospheric trace gases to establish long optical path lengths L with enhanced absorption in compact in-struments. In cavity-enhanced broad-band methods, the ex-act knowledge of both the magnitude of L and its spectral dependency on the wavelength lambda is fundamental for the correct retrieval of trace gas concentrations. L(lambda) is connected to the spectral mirror reflectivity R (lambda), which is often referred to instead. L(lambda) is also influenced by other quantities like broad-band absorbers or alignment of the optical resonator. The established calibration techniques to determine L(lambda), e.g. introducing gases with known optical properties or measuring the ring-down time, all have limitations: limited spectral resolution, insufficient absolute accuracy and precision, inconvenience for field deployment, or high cost of implementation. Here, we present two new methods that aim to overcome these limitations: (1) the narrow-band cavity ring-down (NB-CRD) method uses cavity ring-down spectroscopy and a tunable filter to retrieve spectrally resolved path lengths L(lambda); (2) integrated calibration by means of op-tical modulation (ICOM) allows the determination of the op-tical path length at the spectrometer resolution with high ac-curacy in a relatively simple setup. In a prototype setup we demonstrate the high accuracy and precision of the new approaches. The methods facilitate and improve the determination of L(lambda), thereby simplifying the use of cavity-enhanced absorption spectroscopy.Peer reviewe

Non-dispersive UV Absorption Spectroscopy: A Promising New Approach for in-situ Detection of Sulfur Dioxide

A new type of instrument for in-situ detection of volcanic sulfur dioxide is presented on the basis of non-dispersive UV absorption spectroscopy. It is a promising alternative to presently used compact and low-cost SO2 monitoring techniques, over which it has a series of advantages, including an inherent calibration, fast response times (< 2 s to reach 90 % of the applied concentration), a measurement range spanning about 5 orders of magnitude and small, well-known cross sensitivities to other gases. Compactness, cost-efficiency and detection limit (< 1 ppm, few ppb under favorable conditions) are comparable to other presently used in-situ instruments. Our instrument prototype has been extensively tested in comparison studies with established methods. In autumn 2015, diverse volcanic applications were investigated such as fumarole sampling, proximal plume measurements and airborne measurements several kilometers downwind from the vent on Mt. Etna and White Island. General capabilities and limitations of the measurement principle are discussed, considering different instrument configurations and future applications

Analysis of spatial and temporal patterns of on-road NO2 concentrations in Hong Kong

In this paper we present an investigation of the spatial and temporal variability of street-level concentrations of NO2 in Hong Kong as an example of a densely populated megacity with heavy traffic. For the study we use a combination of open-path remote sensing and in situ measurement techniques that allows us to separate temporal changes and spatial patterns and analyse them separately. Two measurement campaigns have been conducted, one in December 2010 and one in March 2017. Each campaign lasted for a week which allowed us to examine diurnal cycles, weekly patterns as well as spatially resolved long-term changes. We combined a long-path differential optical absorption spectroscopy (DOAS) instrument with a cavity-enhanced DOAS and applied several normalizations to the data sets in order to make the different measurement routes comparable. For the analysis of long-term changes we used the entire unfiltered data set and for the comparison of spatial patterns we filtered out the accumulation of NO2 when stopping at traffic lights for focusing on the changes of NO2 spatial distribution instead of comparing traffic flow patterns. For the generation of composite maps the diurnal cycle has been normalized by scaling the mobile data with coinciding citywide path-averaged measurement results. An overall descending trend from 2010 to 2017 could be observed, consistent with the observations of the Ozone Monitoring Instrument (OMI) and the Environment Protection Department (EPD) air quality monitoring network data. However, long-term difference maps show pronounced spatial structures with some areas, e.g. around subway stations, revealing an increasing trend. We could also show that the weekend effect, which for the most part of Hong Kong shows reduced NO2 concentrations on Sundays and to a lesser degree on Saturdays, is reversed around shopping malls. Our study shows that spatial differences have to be considered when discussing citywide trends and can be used to put local point measurements into perspective. The resulting data set provides a better insight into on-road NO2 characteristics in Hong Kong, which helps to identify heavily polluted areas and represents a useful database for urban planning and the design of pollution control measures

Towards better air quality using the plume chasing method : validation studies of real driving NOx emission measurements of vehicles

The reliability of Plume Chasing as a Remote Emission Measurement Technique in detecting NOx emissions is investigated. It allows, for example, to identify high-NOx-emitting vehicles with high precision. During a 5-day study of the CARES project, controlled Plume Chasing measurements of different types of vehicles were performed on a test track. The test track experiments included 21 different sessions with different driving properties and different test vehicles representative for a common vehicle fleet. During the experiments, the emission control systems were activated and deactivated in a blind comparison experiment. The Plume Chasing method showed excellent correlation with the averaged reference SEMS NOx data. The main cause for deviations was found to be situations when emissions are significantly influenced by plumes from high emitting vehicles driving ahead

The Role of Open Lead Interactions in Atmospheric Ozone Variability Between Arctic Coastal and Inland Sites

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 (O3 \u3c 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

Glyoxal observations in the global marine boundary layer

Glyoxal is an important intermediate species formed by the oxidation of common biogenic and anthropogenic volatile organic compounds such as isoprene, toluene and acetylene. Although glyoxal has been shown to play an important role in urban and forested environments, its role in the open ocean environment is still not well understood, with only a few observations showing evidence for its presence in the open ocean marine boundary layer (MBL). In this study, we report observations of glyoxal from ten field campaigns in different parts of the world's oceans. These observations together represent the largest database of glyoxal in the MBL. The measurements are made with similar instruments that have been used in the past, although the open ocean values reported here, average of about 25 pptv with an upper limit of 40 pptv, are much lower than previously reported observations that were consistently higher than 40 pptv and had an upper limit of 140 pptv, highlighting the uncertainties in the Differential Optical Absorption Spectroscopy (DOAS) method for the retrieval of glyoxal. Despite retrieval uncertainties, the results reported in this work support previous suggestions that the currently known sources of glyoxal are insufficient to explain the average MBL concentrations. This suggests that there is an additional missing source, more than a magnitude larger than currently known sources, which is necessary to account for the observed atmospheric levels of glyoxal. Therefore it could play a more important role in the MBL than previously considered

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

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

Emission ratio determination from road vehicles using a range of remote emission sensing techniques

The development of remote emission sensing techniques such as plume chasing and point sampling has progressed significantly and is providing new insight into vehicle emissions behaviour. However, the analysis of remote emission sensing data can be highly challenging and there is currently no standardised method available. In this study we present a single data processing approach to quantify vehicle exhaust emissions measured using a range of remote emission sensing techniques. The method uses rolling regression calculated over short time intervals to derive the characteristics of diluting plumes. We apply the method to high time-resolution plume chasing and point sampling data to quantify gaseous exhaust emission ratios from individual vehicles. Data from a series of vehicle emission characterisation experiments conducted under controlled conditions is used to demonstrate the potential of this approach. First, the method is validated through comparison with on-board emission measurements. Second, the ability of this approach to detect changes in NOx / CO2 ratios associated with aftertreatment system tampering and different engine operating conditions is shown. Third, the flexibility of the approach is demonstrated by varying the pollutants used as regression variables and quantifying the NO2 / NOx ratios for different vehicle types. A higher proportion of total NOx is emitted as NO2 when the selective catalytic reduction system of the measured heavy duty truck is tampered. In addition, the applicability of this approach to urban environments is illustrated using mobile measurements conducted in Milan, Italy in 2021. Emissions from local combustion sources are distinguished from a complex urban background and the spatiotemporal variability in emissions is shown. The mean NOx / CO2 ratio of 1.61 ppb/ppm is considered representative of the local vehicle fleet. It is envisaged that this approach can be used to quantify emissions from a range of mobile and stationary fuel combustion sources, including non-road vehicles, ships, trains, boilers and incinerators