Atmospheric Chemistry and Physics Discussions, 2012
ABSTRACT The extinction coefficient and growth factor of humidified aerosols, at 80% and 90% RH, ... more ABSTRACT The extinction coefficient and growth factor of humidified aerosols, at 80% and 90% RH, and at 532 nm and 355 nm wavelengths were measured for size-selected particles for ammonium sulfate, IHSS Pahokee peat (a lightly absorbing humic-like substance proxy), nigrosine (a black dye to model highly absorbing substances), and a mixture of AS and nigrosine. The ratio of the humidified extinction coefficients to the dry (fRHext(%RH, Dry)) was explored. The measured fRHext(%RH, Dry) was compared to theoretical calculations based on Mie theory, using the measured growth factors and assuming homogeneous mixing. The expected complex refractive indices (RIs) using the volume weighted mixing rule were compared to the RIs derived from the extinction measurements. Moreover, the differences between assuming a core-shell structure or a homogeneous mixing of the substances is examined. The laboratory results were used as a basis to model the change in the total extinction, the single scattering albedo (ω), and the asymmetry parameter (g) in the twilight zone of clouds at 355 nm and 532 nm. We found slightly linear to no dependency of fRH(%RH, Dry) with size for absorbing substances in contrast to the decreasing exponential behavior with size for purely scattering substances. However, no discernable difference could be made between the two wavelengths used. Less than 5% differences were found between the real parts of the complex refractive indices derived and those calculated using the volume weighted mixing rule, and the imaginary parts had up to a 20% difference. Moreover, for substances with growth factor less than 1.15 there was, in average, less than 5% difference between the extinction efficiencies calculated using a core-shell model and assuming homogeneous mixing for size parameters less than 2.5. For x>2.5 the differences were greater causing and overestimation of the extinction efficiency (Qext) values if homogenous mixing was assume instead of a core-shell structure. The total extinction as a function of distance from the nearest cloud was found to be independent from the imaginary component (k) of the dry RI of the absorbing aerosols modeled. On the other hand, the single scattering albedo, as expected, decreased with larger values of k, whereas the asymmetry parameter increased suggesting a reduction in the reflectivity of the twilight zone with more absorbing aerosols and a reduction of cloud edge 3-D radiative effects.
New particle formation by gas to particle conversion in the tropical upper troposphere and lower ... more New particle formation by gas to particle conversion in the tropical upper troposphere and lower stratosphere (UT/LS) may be a major source of aerosols for origin and maintenance of the global stratospheric Junge aerosol layer. Once created in the tropical upper troposphere these ultrafine particles can be carried aloft towards the tropopause by the -radiatively driven- slowly rising air, and
Bulletin of the American Meteorological Society, 2012
ABSTRACT A meeting of 31 international experts on in situ measurements from aircraft was held to ... more ABSTRACT A meeting of 31 international experts on in situ measurements from aircraft was held to identify unresolved questions concerning ice formation and evolution in ice clouds, assess the current state of instrumentation that can address these problems, introduce emerging technology that may overcome current measurement issues, and recommend future courses of action to improve our understanding of ice cloud microphysical processes and their impact on the environment.
ABSTRACT Sulfate and sulfur dioxide play an important role in environmental chemistry and climate... more ABSTRACT Sulfate and sulfur dioxide play an important role in environmental chemistry and climate, particularly through their effect on aerosols. Processing of aerosol through sulfate addition in clouds, which causes both hygroscopicity changes and mass increases, has been shown to modify the cloud condensation nucleus spectrum, leading to important climatological effects (Bower et al. 1997, Hegg et al. 2004). However, the uptake of sulfate and SO2 to aerosol in clouds is not well constrained, nor is it resolved for different particle types and sizes (Kasper-Giebl et al. 2000, Barrie et al. 2001). Measurements of stable sulfur isotopes can be used to investigate the chemistry of SO2 in the environment, providing insight into sources, sinks and oxidation pathways. Typical isotopic compositions for many sources have been measured, and the major current limitation is the lack of reliable fractionation factors - characteristic changes in isotopic composition caused by chemical reactions - with which to interpret the data. Laboratory values of fractionation factors for the major oxidation reactions have been measured in previous work, however there are no measurements or models to represent isotopic fractionation during heterogeneous oxidation on complex atmospheric surfaces. In this work the sulfur isotopic fractionation factors for SO2 oxidation have been measured on Sahara dust, obtained from the Cape Verde Islands, and sea salt aerosol, which was synthesised in the laboratory according to Millero (1974), modified to contain no sulfate. Sulfur dioxide with a known isotopic composition was oxidised on these surfaces under a variety of conditions including irradiation and ozonation, and the sulfur isotopic composition of the product sulfate was measured with the Cameca NanoSIMS 50. These laboratory results were then used to investigate the uptake of sulfur to particles in an orographic cloud during the HCCT campaign. The campaign took place at the Schmücke mountain in Germany and used connected flow conditions between upwind, in-cloud and downwind measurement sites to study the evolution of air masses due to cloud processing. NanoSIMS isotopic analysis was used to constrain the formation and uptake of sulfate in the cloud. Oxidation by a radical chain reaction catalysed by transition metals was seen to be the most important oxidation reaction occurring in two out of three measured cloud events.
ABSTRACT Upper tropospheric observations outside and inside of cirrus clouds of water vapour mixi... more ABSTRACT Upper tropospheric observations outside and inside of cirrus clouds of water vapour mixing ratios sometimes exceeding water saturation, yielding up to more than 200% relative humidities over ice (RHice) have been reported from aircraft and balloon measurements in recent years. From these observations a lively continuous discussion arose on whether there is a lack of understanding of ice cloud microphysics or if the water measurements are tainted with large uncertainties or flaws. Here, RHice in clear air and in ice clouds is investigated: strictly quality checked aircraft in-situ observations of RHice were performed during 28 flights in tropical, mid-latitude and Arctic field experiments in the temperature range 183-250 K. In our field measurements, no supersaturations above water saturation are found. Nevertheless, super- or subsaturations inside of cirrus are frequently observed at low temperatures (<205 K) in our field data set. To explain persistent RHice deviating from saturation, we analysed the number densities of ice crystals recorded during 20 flights. From the combined analysis - using conventional microphysics - of supersaturations and ice crystal numbers, we show that the high, persistent supersaturations observed inside of cirrus are caused by unexpected, frequent very low ice crystal numbers that could hardly be explained by homogeneous ice nucleation. Heterogeneous ice formation or the suppression of freezing might better explain the observed ice crystal numbers. Thus, our lack of understanding of the high supersaturations with implications to the microphysical and radiative properties of cirrus, the vertical redistribution of water and climate, is traced back to the understanding of the freezing process at low temperatures.
The Amazonian Aerosol Characterization Experiment (AMAZE-08), carried out in the northern Amazon ... more The Amazonian Aerosol Characterization Experiment (AMAZE-08), carried out in the northern Amazon during the wet season (February and March, 2008), provided an opportunity to investigate the chemical and microphysical properties of nearly pure biogenic organic aerosol. The non-refractory components (i.e., ammonium, chloride, nitrate, sulfate, and organic) of submicron particles were analyzed with the Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS)
Atmospheric Chemistry and Physics Discussions, 2012
ABSTRACT The extinction coefficient and growth factor of humidified aerosols, at 80% and 90% RH, ... more ABSTRACT The extinction coefficient and growth factor of humidified aerosols, at 80% and 90% RH, and at 532 nm and 355 nm wavelengths were measured for size-selected particles for ammonium sulfate, IHSS Pahokee peat (a lightly absorbing humic-like substance proxy), nigrosine (a black dye to model highly absorbing substances), and a mixture of AS and nigrosine. The ratio of the humidified extinction coefficients to the dry (fRHext(%RH, Dry)) was explored. The measured fRHext(%RH, Dry) was compared to theoretical calculations based on Mie theory, using the measured growth factors and assuming homogeneous mixing. The expected complex refractive indices (RIs) using the volume weighted mixing rule were compared to the RIs derived from the extinction measurements. Moreover, the differences between assuming a core-shell structure or a homogeneous mixing of the substances is examined. The laboratory results were used as a basis to model the change in the total extinction, the single scattering albedo (ω), and the asymmetry parameter (g) in the twilight zone of clouds at 355 nm and 532 nm. We found slightly linear to no dependency of fRH(%RH, Dry) with size for absorbing substances in contrast to the decreasing exponential behavior with size for purely scattering substances. However, no discernable difference could be made between the two wavelengths used. Less than 5% differences were found between the real parts of the complex refractive indices derived and those calculated using the volume weighted mixing rule, and the imaginary parts had up to a 20% difference. Moreover, for substances with growth factor less than 1.15 there was, in average, less than 5% difference between the extinction efficiencies calculated using a core-shell model and assuming homogeneous mixing for size parameters less than 2.5. For x>2.5 the differences were greater causing and overestimation of the extinction efficiency (Qext) values if homogenous mixing was assume instead of a core-shell structure. The total extinction as a function of distance from the nearest cloud was found to be independent from the imaginary component (k) of the dry RI of the absorbing aerosols modeled. On the other hand, the single scattering albedo, as expected, decreased with larger values of k, whereas the asymmetry parameter increased suggesting a reduction in the reflectivity of the twilight zone with more absorbing aerosols and a reduction of cloud edge 3-D radiative effects.
New particle formation by gas to particle conversion in the tropical upper troposphere and lower ... more New particle formation by gas to particle conversion in the tropical upper troposphere and lower stratosphere (UT/LS) may be a major source of aerosols for origin and maintenance of the global stratospheric Junge aerosol layer. Once created in the tropical upper troposphere these ultrafine particles can be carried aloft towards the tropopause by the -radiatively driven- slowly rising air, and
Bulletin of the American Meteorological Society, 2012
ABSTRACT A meeting of 31 international experts on in situ measurements from aircraft was held to ... more ABSTRACT A meeting of 31 international experts on in situ measurements from aircraft was held to identify unresolved questions concerning ice formation and evolution in ice clouds, assess the current state of instrumentation that can address these problems, introduce emerging technology that may overcome current measurement issues, and recommend future courses of action to improve our understanding of ice cloud microphysical processes and their impact on the environment.
ABSTRACT Sulfate and sulfur dioxide play an important role in environmental chemistry and climate... more ABSTRACT Sulfate and sulfur dioxide play an important role in environmental chemistry and climate, particularly through their effect on aerosols. Processing of aerosol through sulfate addition in clouds, which causes both hygroscopicity changes and mass increases, has been shown to modify the cloud condensation nucleus spectrum, leading to important climatological effects (Bower et al. 1997, Hegg et al. 2004). However, the uptake of sulfate and SO2 to aerosol in clouds is not well constrained, nor is it resolved for different particle types and sizes (Kasper-Giebl et al. 2000, Barrie et al. 2001). Measurements of stable sulfur isotopes can be used to investigate the chemistry of SO2 in the environment, providing insight into sources, sinks and oxidation pathways. Typical isotopic compositions for many sources have been measured, and the major current limitation is the lack of reliable fractionation factors - characteristic changes in isotopic composition caused by chemical reactions - with which to interpret the data. Laboratory values of fractionation factors for the major oxidation reactions have been measured in previous work, however there are no measurements or models to represent isotopic fractionation during heterogeneous oxidation on complex atmospheric surfaces. In this work the sulfur isotopic fractionation factors for SO2 oxidation have been measured on Sahara dust, obtained from the Cape Verde Islands, and sea salt aerosol, which was synthesised in the laboratory according to Millero (1974), modified to contain no sulfate. Sulfur dioxide with a known isotopic composition was oxidised on these surfaces under a variety of conditions including irradiation and ozonation, and the sulfur isotopic composition of the product sulfate was measured with the Cameca NanoSIMS 50. These laboratory results were then used to investigate the uptake of sulfur to particles in an orographic cloud during the HCCT campaign. The campaign took place at the Schmücke mountain in Germany and used connected flow conditions between upwind, in-cloud and downwind measurement sites to study the evolution of air masses due to cloud processing. NanoSIMS isotopic analysis was used to constrain the formation and uptake of sulfate in the cloud. Oxidation by a radical chain reaction catalysed by transition metals was seen to be the most important oxidation reaction occurring in two out of three measured cloud events.
ABSTRACT Upper tropospheric observations outside and inside of cirrus clouds of water vapour mixi... more ABSTRACT Upper tropospheric observations outside and inside of cirrus clouds of water vapour mixing ratios sometimes exceeding water saturation, yielding up to more than 200% relative humidities over ice (RHice) have been reported from aircraft and balloon measurements in recent years. From these observations a lively continuous discussion arose on whether there is a lack of understanding of ice cloud microphysics or if the water measurements are tainted with large uncertainties or flaws. Here, RHice in clear air and in ice clouds is investigated: strictly quality checked aircraft in-situ observations of RHice were performed during 28 flights in tropical, mid-latitude and Arctic field experiments in the temperature range 183-250 K. In our field measurements, no supersaturations above water saturation are found. Nevertheless, super- or subsaturations inside of cirrus are frequently observed at low temperatures (<205 K) in our field data set. To explain persistent RHice deviating from saturation, we analysed the number densities of ice crystals recorded during 20 flights. From the combined analysis - using conventional microphysics - of supersaturations and ice crystal numbers, we show that the high, persistent supersaturations observed inside of cirrus are caused by unexpected, frequent very low ice crystal numbers that could hardly be explained by homogeneous ice nucleation. Heterogeneous ice formation or the suppression of freezing might better explain the observed ice crystal numbers. Thus, our lack of understanding of the high supersaturations with implications to the microphysical and radiative properties of cirrus, the vertical redistribution of water and climate, is traced back to the understanding of the freezing process at low temperatures.
The Amazonian Aerosol Characterization Experiment (AMAZE-08), carried out in the northern Amazon ... more The Amazonian Aerosol Characterization Experiment (AMAZE-08), carried out in the northern Amazon during the wet season (February and March, 2008), provided an opportunity to investigate the chemical and microphysical properties of nearly pure biogenic organic aerosol. The non-refractory components (i.e., ammonium, chloride, nitrate, sulfate, and organic) of submicron particles were analyzed with the Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS)
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