Water and nitric acid in cirrus clouds:microphysical kinetical modeling and a closure to field observations

Upper tropospheric relative humidities over ice (RH$_{ice}$) of up to 200% have been reported frequently in recent times. This unexpectedly high supersaturation out- and inside of cold cirrus clouds may have significant impact on the Earth’s climate. In the first case, clear sky supersaturation could be justified when the critical supersaturation for ice cloud formation is higher than until now assumed. This would lead to a decrease in high cloud cover and thus impact on the radiation budget. In the second case, high supersaturation inside of cirrus clouds could suggest the existence of unknown microphysical and radiative properties with consequences for climate and the vertical redistribution of water and nitric acid. Peter et al. (2006) summarized possible reasons for the observed supersaturation in a ’supersaturation puzzle’, calling into question whether this puzzle can be solved by solely using the conventional ice cloud microphysics. Another important question raised in this study is whether the supersaturation may result from uncertainties or flaws in the water measurements. The aim of this PhD thesis is to puzzle out these questions. Therefore upper tropospheric field observations are simulated with an adequate conventional kinetic model in order to analyze the origin and persistence of high ice supersaturation, particularly inside cold cirrus clouds. The proposed $\underline{M}$odel for $\underline{A}$erosol and $\underline{I}$ce $\underline{D}$ynamics (MAID) handles widely aerosol and ice microphysics, including: gas-diffusive particle growth and evaporation, homogeneous and heterogeneous ice nucleation, water vapor deposition and nitric acid uptake on growing ice crystal. Special emphasis of MAID is the exact balancing of chemical species among different physical states. MAID is validated here, based on observations during the field campaign POLSTAR–1 1997 (Polar Stratospheric Aerosol Experiment). Further, a detailed analysis of cirrus cloud observations during CR-AVE 2006, the tropical Costa Rica – Aura Validation Experiment, is performed with MAID. The model is initialized with different aerosol properties, water mixing ratios, accommodation factors of water on ice and amplitudes of mesoscale temperature fluctuations. A notable feature here is to vary the freezing mechanism in the simulations. The model results indicate high sensitivity of the cloud microphysical evolution to the freezing pathway. The ice microphysics, as well as the partitioning of water and nitric acid inside the cloud derived from all sensitivity studies are compared at last with the the microphysical and chemical in-situ observations, to determine the most probable constellation of initial conditions and processes that led to the very cold, sub-visible tropical cirrus cloud observed during CR–AVE. The best agreement between model results and measurements is given when the cirrus cloud forms heterogeneously, with total accommodation of water on ice. By varying the freezing pathway, the accommodation factor of water on ice, or the amount of available water, clouds with completely different microphysical properties form. As a summary, this work demonstrates that it is possible to simulate significant supersaturation inside cold cirrus (T<200 K) with conventional microphysics when assuming heterogeneous ice nucleation as the freezing mechanism. Thus, heterogeneous freezing appears to be an important pathway for cold cirrus cloud formation. More generally, a freezing mechanism producing low number densities of ice crystals could explain the frequent high supersaturation inside cirrus clouds observed in this temperature range

Isotope ratio studies of atmospheric organic compounds: Principles, methods, applications and potential

In the atmosphere, both gas and particle phase organic trace compounds (OTC) have multiple effects on air quality and climate. Gaps exist in a fundamental understanding of the sources and sinks of organics and thus, knowledge needed to steer regulatory purposes is far from complete. Isotopes provide specific “fingerprints” in OTC. These fingerprints result from the isotopic composition at emission, as well as from chemical and physical processes in the atmosphere. Compound specific isotope ratio mass spectrometry (IRMS) in atmospheric OTC is therefore a promising tool to improve our understanding of sources and the atmospheric fate of OTC. Due to analytical challenges originating from the small sample amounts and a huge variety of physical and chemical properties of OTC present in the atmosphere, such measurements are not routinely performed. We present an overview of basic concepts as well as instrumental and measurement procedures used for compound specific IRMS in atmospheric OTC. Concepts for the interpretation of ambient observations are reviewed together with available literature data on source specific and ambient δ13C values of gas and particle phase OTC. Full deployment of the IRMS potential in future atmospheric studies will depend on the availability of laboratory kinetic data. Further method developments, such as increasing sensitivity and accuracy, as well as techniques for simultaneous isotope ratio measurement of multiple atoms are expected to further extend the potential use of isotope ratios for studies of atmospheric OTC

Experimental determination of the partitioning coefficient of β-pinene oxidation products in SOAs

The composition of secondary organic aerosols (SOAs) formed by β-pinene ozonolysis was experimentally investigated in the Juelich aerosol chamber. Partitioning of oxidation products between gas and particles was measured through concurrent concentration measurements in both phases. Partitioning coefficients (Kp) of 2.23 × 10−5 ± 3.20 × 10−6 m3 μg−1 for nopinone, 4.86 × 10−4 ± 1.80 × 10−4 m3 μg−1 for apoverbenone, 6.84 × 10−4 ± 1.52 × 10−4 m3 μg−1 for oxonopinone and 2.00 × 10−3 ± 1.13 × 10−3 m3 μg−1 for hydroxynopinone were derived, showing higher values for more oxygenated species. The observed Kp values were compared with values predicted using two different semi-empirical approaches. Both methods led to an underestimation of the partitioning coefficients with systematic differences between the methods. Assuming that the deviation between the experiment and the model is due to non-ideality of the mixed solution in particles, activity coefficients of 4.82 × 10−2 for nopinone, 2.17 × 10−3 for apoverbenone, 3.09 × 10−1 for oxonopinone and 7.74 × 10−1 for hydroxynopinone would result using the vapour pressure estimation technique that leads to higher Kp. We discuss that such large non-ideality for nopinone could arise due to particle phase processes lowering the effective nopinone vapour pressure such as diol- or dimer formation. The observed high partitioning coefficients compared to modelled results imply an underestimation of SOA mass by applying equilibrium conditions

Benchmarking source specific isotopic ratios of levoglucosan to better constrain the contribution of domestic heating to the air pollution

We report source specific isotope ratios of levoglucosan, the specific biomass burning tracer, in aerosol particle from the combustion of selected woods used for domestic heating in Europe, of coals containing cellulose (lignites) as well as of corn, a C4 plant. Here, we combine compound specific δ13C measurements of levoglucosan with total carbon δ13C of parent materials, to assess isotopic fractionations due to biosynthetic pathways or combustion processes. Levoglucosan formed during the combustion of cellulose from coals shows with δ13C of −21.1‰ and −18.6‰ a moderate enrichment in the heavier isotope compared to the C3 plant samples. Contrarily, observed levoglucosan isotope ratios of −25.0 to −21.5‰ for C3 plant samples are significantly lower than for the C4 plant sample (−12.4‰), as expected from the stronger 13C discrimination during the carbon fixation process by C3 compared to C4 plants. Overall, the C4 plant sample shows a 13C enrichment in all bulk measurements, on average by 12.2‰, 14.2‰ and 14.2‰ for total carbon (TC) in aerosol particle, whole plant/coal material and cellulose samples, respectively. Further, δ13C measurements of levoglucosan and TC of biomass burning aerosol particles, bulk plant/coal and cellulose in C3 plant samples agree well with the published observations. The combined levoglucosan/TC isotopic analyses can be used to differentiate among C3/coal/C4 origin of the smoke emissions from the cellulose-containing-fuel combustion. Noticeably, there is a consistent δ13C offset between C3 plant material and levoglucosan, which allows deriving emission levoglucosan isotope ratios when the combusted plant types are known