Transport of very short-lived substances from the Indian Ocean to the stratosphere through the Asian monsoon

Anthropogenic halogenated substances cause the ozone hole above Antarctica through catalytic ozone destruction and depletion of the stratospheric ozone layer, which shields the Earth from harmful ultraviolet radiation. Their emissions were regulated through the Montreal Protocol in 1989. Since the beginning of the 21st century, the amount of chlorine and bromine in the stratosphere from long-lived ozone depleting substances (ODS) has been decreasing and stratospheric ozone has started to increase slowly. Under these circumstances the importance of natural halogenated substances for atmospheric composition and chemistry will increase in the future. Trace-gases with atmospheric lifetimes of less than half a year belong to the so-called very short-lived substances(VSLS). The most important bromine containing VSLS bromoform (CHBr3, 17 days lifetime) and dibromomethane (CH2Br2, 150 days) from marine sources currently contribute about 25% to the observed stratospheric bromine loading. In addition, the short-lived VSLS methyl iodide (CH3I, 3.5 days) contributes to stratospheric iodine levels. Sulfur containing compounds, such as dimethylsulfide (DMS, 1 day), also influence stratospheric ozone. Sulfur supplies the stratospheric aerosol layer, which amplifies heterogeneous chemical ozone depleting reactions under high chlorine levels. DMS is a potential source of sulfur to the stratosphere. VSLS are naturally produced in the oceans by phytoplankton, macro algae, and photochemistry. They are primarily transported to the stratosphere with deep convection in the tropics and mainly enter the stratosphere over the Pacific warm pool in boreal winter and the Asian monsoon region in boreal summer. Major uncertainties still exist with respect to the oceanic emissions of halogenated VSLS from the Indian Ocean and their stratospheric entrainment through the Asian monsoon circulation. This thesis investigates the emissions of VSLS from the Indian Ocean and their transport to the stratosphere with novel combinations of data and modeling

Transport von sehr kurzlebigen Substanzen vom Indischen Ozean in die Stratosphäre durch den asiatischen Monsun

Anthropogenic halogenated substances cause the ozone hole above Antarctica through catalytic ozone destruction and depletion of the stratospheric ozone layer, which shields the Earth from harmful ultraviolet radiation. Their emissions were regulated through the Montreal Protocol in 1989. Since the beginning of the 21st century, the amount of chlorine and bromine in the stratosphere from long-lived ozone depleting substances (ODS) has been decreasing and stratospheric ozone has started to increase slowly. Under these circumstances the importance of natural halogenated substances for atmospheric composition and chemistry will increase in the future. Trace-gases with atmospheric lifetimes of less than half a year belong to the so-called very short-lived substances(VSLS). The most important bromine containing VSLS bromoform (CHBr3, 17 days lifetime) and dibromomethane (CH2Br2, 150 days) from marine sources currently contribute about 25% to the observed stratospheric bromine loading. In addition, the short-lived VSLS methyl iodide (CH3I, 3.5 days) contributes to stratospheric iodine levels. Sulfur containing compounds, such as dimethylsulfide (DMS, 1 day), also influence stratospheric ozone. Sulfur supplies the stratospheric aerosol layer, which amplifies heterogeneous chemical ozone depleting reactions under high chlorine levels. DMS is a potential source of sulfur to the stratosphere. VSLS are naturally produced in the oceans by phytoplankton, macro algae, and photochemistry. They are primarily transported to the stratosphere with deep convection in the tropics and mainly enter the stratosphere over the Pacific warm pool in boreal winter and the Asian monsoon region in boreal summer. Major uncertainties still exist with respect to the oceanic emissions of halogenated VSLS from the Indian Ocean and their stratospheric entrainment through the Asian monsoon circulation. This thesis investigates the emissions of VSLS from the Indian Ocean and their transport to the stratosphere with novel combinations of data and modeling.Anthropogene halogenierte Substanzen verursachen das Ozonloch über der Antarktis durch katalytische Ozonzerstörung und einen Schwund der stratosphärischen Ozonschicht,welche die Erde vor schadhafter ultravioletter Strahlung schützt. Seit 1989 reguliert das Montrealer Protokoll die Emissionen von langlebigen halogenierten Fluorchlorkohlenwasserstoffen. Seit dem Beginn des 21. Jahrhundert sinkt die atmosphärische Konzentration von Chlor und Brom aus den langlebigen anthropogenen Substanzen und das stratosphärische Ozon nimmt langsam wieder zu. Unter diesen Voraussetzungen wird die Bedeutung natürlicher halogenhaltiger Substanzen, vor allem sehr kurzlebiger Substanzen (engl. very short-lives substances, VSLS) mit atmosphärischen Lebenszeiten kürzer als ein halbes Jahr, für die Zusammensetzung und Chemie der Atmosphäre in der Zukunft zunehmen. Momentan beträgt der Beitrag von VSLS zum stratosphärischen Brom etwa 25%. Die beiden wichtigsten bromierten VSLS sind Bromoform (CHBr3, 17 Tage Lebenszeit) und Dibrommethan (CH2Br2, 150 Tage). Weiterhin wird ein stratosphärischer Eintrag von Methyliodid (CH3I, 3,5 Tage) und schwefelhaltigem Dimethylsulfid (DMS, 1 Tag) vermutet. Schwefel verstärkt die heterogene chemische Ozonzerstörung bei hohem Chlorgehalt in der Stratosphäre. VSLS werden im Ozean auf natürlichem Wege von Phytoplankton, Makroalgen und durch chemische Reaktionen produziert. Sie werden in tropischen Gebieten mit hochreichender Konvektion in die Stratosphäre eingetragen, hauptsächlich über dem tropischen Westpazifik im borealen Winter und der asiatischen Monsunzirkulation im borealen Sommer. Die Unsicherheiten bezüglich der VSLS-Emissionen aus dem Indischen Ozean und des Transportes durch den asiatischen Monsun in die Stratosphäre sind groß. Diese Arbeit untersucht erstmalig VSLS Emissionen aus dem Indischen Ozean und ihren Transport in die Stratosphäre mit einer neuartigen Kombination aus Daten und Modellierung

Meteorological constraints on oceanic halocarbons above the Peruvian Upwelling

During a cruise of R/V METEOR in December 2012 the oceanic sources and emissions of various halogenated trace gases and their mixing ratios in the marine atmospheric boundary layer (MABL) were investigated above the Peruvian upwelling. This study presents novel observations of the three very short lived substances (VSLSs) – bromoform, dibromomethane and methyl iodide – together with high-resolution meteorological measurements, Lagrangian transport and source–loss calculations. Oceanic emissions of bromoform and dibromomethane were relatively low compared to other upwelling regions, while those for methyl iodide were very high. Radiosonde launches during the cruise revealed a low, stable MABL and a distinct trade inversion above acting as strong barriers for convection and vertical transport of trace gases in this region. Observed atmospheric VSLS abundances, sea surface temperature, relative humidity and MABL height correlated well during the cruise. We used a simple source–loss estimate to quantify the contribution of oceanic emissions along the cruise track to the observed atmospheric concentrations. This analysis showed that averaged, instantaneous emissions could not support the observed atmospheric mixing ratios of VSLSs and that the marine background abundances below the trade inversion were significantly influenced by advection of regional sources. Adding to this background, the observed maximum emissions of halocarbons in the coastal upwelling could explain the high atmospheric VSLS concentrations in combination with their accumulation under the distinct MABL and trade inversions. Stronger emissions along the nearshore coastline likely added to the elevated abundances under the steady atmospheric conditions. This study underscores the importance of oceanic upwelling and trade wind systems on the atmospheric distribution of marine VSLS emissions

Importance of seasonally resolved oceanic emissions for bromoform delivery from the tropical Indian Ocean and west Pacific to the stratosphere

Oceanic very short-lived substances (VSLSs), such as bromoform (CHBr3), contribute to stratospheric halogen loading and, thus, to ozone depletion. However, the amount, timing, and region of bromine delivery to the stratosphere through one of the main entrance gates, the Indian summer monsoon circulation, are still uncertain. In this study, we created two bromoform emission inventories with monthly resolution for the tropical Indian Ocean and west Pacific based on new in situ bromoform measurements and novel ocean biogeochemistry modeling. The mass transport and atmospheric mixing ratios of bromoform were modeled for the year 2014 with the particle dispersion model FLEXPART driven by ERA-Interim reanalysis. We compare results between two emission scenarios: (1) monthly averaged and (2) annually averaged emissions. Both simulations reproduce the atmospheric distribution of bromoform from ship- and aircraft-based observations in the boundary layer and upper troposphere above the Indian Ocean reasonably well. Using monthly resolved emissions, the main oceanic source regions for the stratosphere include the Arabian Sea and Bay of Bengal in boreal summer and the tropical west Pacific Ocean in boreal winter. The main stratospheric injection in boreal summer occurs over the southern tip of India associated with the high local oceanic sources and strong convection of the summer monsoon. In boreal winter more bromoform is entrained over the west Pacific than over the Indian Ocean. The annually averaged stratospheric injection of bromoform is in the same range whether using monthly averaged or annually averaged emissions in our Lagrangian calculations. However, monthly averaged emissions result in the highest mixing ratios within the Asian monsoon anticyclone in boreal summer and above the central Indian Ocean in boreal winter, while annually averaged emissions display a maximum above the west Indian Ocean in boreal spring. In the Asian summer monsoon anticyclone bromoform atmospheric mixing ratios vary by up to 50% between using monthly averaged and annually averaged oceanic emissions. Our results underline that the seasonal and regional stratospheric bromine injection from the tropical Indian Ocean and west Pacific critically depend on the seasonality and spatial distribution of the VSLS emissions

Quantifying nitrous oxide emissions in the U.S. Midwest: a top‐down study using high resolution airborne in‐situ observations

The densely farmed U.S. Midwest is a prominent source of nitrous oxide (N2O) but top‐down and bottom‐up N2O emission estimates differ significantly. We quantify Midwest N2O emissions by combining observations from the Atmospheric Carbon and Transport‐America campaign with model simulations to scale the Emissions Database for Global Atmospheric Research (EDGAR). In October 2017 we scaled agricultural EDGAR v4.3.2 and v5.0 emissions by factors of 6.3 and 3.5, respectively, resulting in 0.42 nmol m−2 s−1 Midwest N2O emissions. In June/July 2019, a period when extreme flooding was occurring in the Midwest, agricultural scaling factors were 11.4 (v4.3.2) and 9.9 (v5.0), resulting in 1.06 nmol m−2 s−1 Midwest emissions. Uncertainties are on the order of 50 %. Agricultural emissions estimated with the process‐based model DayCent (Daily version of the CENTURY ecosystem model) were larger than in EDGAR but still substantially smaller than our estimates. The complexity of N2O emissions demands further studies to fully characterize Midwest emissions

The influence of air-sea fluxes on atmospheric aerosols during the summer monsoon over the tropical Indian Ocean

During the summer monsoon, the western tropical Indian Ocean is predicted to be a hot spot for dimethylsulfide emissions, the major marine sulfur source to the atmosphere, and an important aerosol precursor. Other aerosol relevant fluxes, such as isoprene and sea spray, should also be enhanced, due to the steady strong winds during the monsoon. Marine air masses dominate the area during the summer monsoon, excluding the influence of continentally derived pollutants. During the SO234-2/235 cruise in the western tropical Indian Ocean from July to August 2014, directly measured eddy covariance DMS fluxes confirm that the area is a large source of sulfur to the atmosphere (cruise average 9.1 μmol m−2 d−1). The directly measured fluxes, as well as computed isoprene and sea spray fluxes, were combined with FLEXPART backward and forward trajectories to track the emissions in space and time. The fluxes show a significant positive correlation with aerosol data from the Terra and Suomi-NPP satellites, indicating a local influence of marine emissions on atmospheric aerosol numbers

Meteorological and air quality measurements in a city region with complex terrain: influence of meteorological phenomena on urban climate

On 8 and 9 July 2018 extensive observations were conducted under fair-weather conditions in the German city of Stuttgart and its surroundings. This intensive observation period, part of the four weeks Urban Climate Under Change (UC)2 campaign, intended to provide a comprehensive data set to understand the complex interactions of thermally induced wind systems, vertical turbulent mixing and air pollutant concentration distribution in the atmospheric boundary layer of the city. Stuttgart has a very special and complex topography with a city center located in a basin surrounded by hills with heights of 250 to 300 m influencing the wind and flow system, reducing the wind speed, causing inhibited dispersion of air pollutants. Cold air flows from the surrounding plains can penetrate into the urban areas and influence the urban climate including the air quality. For investigating these effects with a focus on urban climate, combinations of different measurement platforms and techniques were used, such as in situ stationary and mobile measurements with cars, vertical profiling by means of tethered balloons, radiosondes, a drone, and aircraft observations, remote sensing devices and satellite-based instruments. Numerous atmospheric processes in an urban area regarding boundary layer evolution, inversion, local wind systems, urban heat island, etc. were observed. Some important findings are: Temperature observations provide local information about the warmest areas in the city and about the city and its surroundings. The urban heat island effect was evident from the results of stationary and mobile air temperature measurements as higher air temperature was measured in Stuttgart basin compared to its surroundings. Considerable spatio-temporal differences concerning the wind (speed and direction), turbulence and the convective boundary depth are evident. Lower wind speeds were observed during the nighttime and the main wind direction in the Stuttgart valley was measured to be southwest, which carried cold air from the hillsides into the city and pollutants to the windward side of the city into the Neckar valley. The low wind speed favored the accumulation of pollutants in a shallow nocturnal boundary layer close to the surface. During the day, the overall pollutant concentration was reduced by vertical convective mixing. The vertical profile measurements have shown that the applied techniques provided a good overview to understand the vertical characteristics of meteorological parameters and pollutants as well as the stability of the atmosphere and extent of the urban boundary layer. It also showed that the extent of atmospheric mixing determines the dispersion, dilution and mixing of emitted pollutants. Finally, the additional comprehensive air-chemical observations (surface and satellite based) allow understanding the diurnal cycle of air pollutants in the atmospheric boundary layer of the city of Stuttgart. Satellite-based observations from Sentinel‑5P/TROPOMI have shown their potential for mapping urban pollution islands and urban pollution plumes even in cities with a complex terrain like Stuttgart. These observations assisted to obtain a comprehensive data set intended for the validation of a novel urban climate model, PALM‑4U