Laboratory Studies of Homogeneous and Heterogeneous Atmospheric Processes (nur Englisch)

The hydroxyl radical, OH, is the primary oxidant in the Earth’s atmosphere, initiating the degradation of many common biogenic and anthropogenic trace gases. Most reactions of OH result in its removal from the radical pool, as long lived species which sequester HOx are formed. Some reactions, involving e.g. NOx, organic peroxy radical and peroxides can also regenerate OH (or HO₂) resulting in radical recycling. The degree of radical recycling for any chemical system will impact on the levels of OH and thus on oxidation rates. We employ Pulsed Laser Photolysis set-ups (see Methods, Lab1 and 2) to derive the efficiency of radical recycling in the destruction of various organic traces gases. For a recent publication see Dillon, T.J., and Crowley, J.N.: Atmos. Chem. Phys., 8, 4877-4889, 2008 and Lelieveld et al., Nature, 452, 737-740, 2008.

The upper troposphere (UT) is characterised by low temperatures and high photochemical activity Due to the low water vapour mixing ratios in the UT, the HOx chemistry of this region is somewhat different to that of the lower troposphere and, due to the presence of volatile organic species also distinct to that of the lower stratosphere. Field measurements have revealed the almost ubiquitous presence of high mixing ratios of partially oxidised organics, which may have a large impact on the HOx budget. Our laboratory work has thus focused on elucidating gas-phase oxidation mechanisms for species such as ketones, aldehydes and alcohols using Pulsed Laser Photolysis set-ups (see below) to derive rate coefficients, products, absorption cross sections and quantum yields. For a recent publication see Dillon et al., Atmos. Chem. Phys. Discuss., 10, 16747–16773, 2010 or  Karunanandan et al J. Phys. Chem. A, 111, 897-908, 2007 or Khamaganov et al. Phys. Chem. Chem. Phys., 11, 6173-6181, 2009.

The low temperatures of the UT ensure significant coverage by cirrus clouds and thus the provision of a surface with which trace gases may interact. The scavenging of trace gases to ice surfaces and the resulting modification of ice properties can have a significant impact on e.g. NOy in the UT, and potentially on ice properties. Our laboratory studies on trace-gas ice interactions aims to investigate the physical chemistry of such processes by measuring e.g. equilibrium adsorption isotherms (reversible uptake for e.g. HNO₃) and uptake coefficients (reactive uptake, e.g. N₂O₅). The trace gases under investigation are members of the NOy family, organics acids, peroxides and radicals, which are all examined using the Coated Wall Flow Tube (see Methods, Lab 5) attached to a highly sensitive chemical ionisation mass spectrometer. For a recent publication see Pouvesle et al., Phys. Chem. Chem. Phys, 12, 15544-15550, 2010  or von Hessberg et al., Phys. Chem. Chem. Phys., 10, 2345-2355, 2008.

Mineral aerosol is the largest component of the coarse fraction of atmospheric aerosol, and with emissions of > 1 Tg per year is the most important aerosol (by mass) in the troposphere. The potential for mineral aerosol – trace gas interactions to modify the photochemistry of the free troposphere has been documented in several modeling studies, yet the physico-chemical parameters needed to describe such processes are very poorly characterised. We have embarked on a series of experiments to examine the interactions of mineral aerosol with a number of NOy trace gases such as HNO₃ and N₂O₅. This research, presently employing an aerosol flow tube with chemiluminescence detection will be extended to allow detection of other trace gases (e.g. SO2, H2O2) by atmospheric pressure chemical ionisation mass spectrometry (see methods, Lab 4). For a recent publication see Tang et al., Atmos. Chem. Phys., 10, 2965-2974, 2010 or Wagner et al., Atmos. Environ., Part A, 43, 5001-5008, 2009.

The potential of a species to act as a greenhouse gas depends on its emission rate, the strength of its absorption features, and its atmospheric lifetime. Important loss processes may include: reaction with O₃, OH, O(¹D); photolysis; uptake to surfaces e.g. the ocean. We thus use a suite of experimental setups such as infrared spectroscopy, laser photolysis and wetted wall flow tubes (see Method, Lab 1-3 and 6) to investigate the photochemical processes that lead to removal of greenhouse gases (e.g. CH₄, N₂O, SO₂F₂ and NF₃) from the troposphere or stratosphere. For a recent publication see Dillon et al., Atmos. Chem. Phys., 8, 1547-1557, 2008 or Dillon et al., Atmospheric Environment, 44, 1186-1191, 2010.