Production of HO2 and OH radicals from near-UV irradiated airborne TiO2 nanoparticles.

The production of gas-phase hydroperoxyl radicals, HO2, is observed directly from sub-micron airborne TiO2 nanoparticles irradiated by 300-400 nm radiation. The rate of HO2 production as a function of O2 pressure follows Langmuir isotherm behaviour suggesting O2 is involved in the production of HO2 following its adsorption onto the surface of the TiO2 aerosol. Reduction of adsorbed O2 by photogenerated electrons is likely to be the initial step followed by reaction with a proton produced via oxidation of adsorbed water with a photogenerated hole. The rate of HO2 production decreased significantly over the range of relative humidities between 8.7 and 36.9%, suggesting competitive adsorption of water vapour inhibits HO2 production. From the data, the adsorption equilibrium constants were calculated to be: KO2 = 0.27 ± 0.02 Pa-1 and KH2O = 2.16 ± 0.12 Pa-1 for RH = 8.7%, decreasing to KO2 = 0.18 ± 0.01 Pa-1 and KH2O = 1.33 ± 0.04 Pa-1 at RH = 22.1%. The increased coverage of H2O onto the TiO2 aerosol surface may inhibit HO2 production by decreasing the effective surface area of the TiO2 particle and lowering the binding energy of O2 on the aerosol surface, hence shortening its desorption lifetime. The maximum yield (i.e. when [O2] is projected to atmospherically relevant levels) for production of gas-phase HO2, normalised for surface area and light intensity, was found to be at a RH of 8.7% for the 80% anatase and 20% rutile formulation of TiO2 used here. This yield decreased to as the RH was increased to 22.1%. Using this value, the rate of production of HO2 from TiO2 surfaces under atmospheric conditions was estimated to be in the range 5 × 104-1 × 106 molecule cm-3 s-1 using observed surface areas of mineral dust at Cape Verde, and assuming a TiO2 fraction of 4.5%. For the largest loadings of dust in the troposphere, the rate of this novel heterogeneous production mechanism begins to approach that of HO2 production from the gas-phase reaction of OH with CO in unpolluted regions. The production of gas-phase OH radicals could only be observed conclusively at high aerosol surface areas, and was attributed to the decomposition of H2O2 at the surface by photogenerated electrons.

Measurements of the HO2 uptake coefficients onto single component organic aerosols.

Measurements of HO2 uptake coefficients (γ) were made onto a variety of organic aerosols derived from glutaric acid, glyoxal, malonic acid, stearic acid, oleic acid, squalene, monoethanol amine sulfate, monomethyl amine sulfate, and two sources of humic acid, for an initial HO2 concentration of 1 × 10(9) molecules cm(-3), room temperature and at atmospheric pressure. Values in the range of γ < 0.004 to γ = 0.008 ± 0.004 were measured for all of the aerosols apart from the aerosols from the two sources of humic acid. For humic acid aerosols, uptake coefficients in the range of γ = 0.007 ± 0.002 to γ = 0.09 ± 0.03 were measured. Elevated concentrations of copper (16 ± 1 and 380 ± 20 ppb) and iron (600 ± 30 and 51 000 ± 3000 ppb) ions were measured in the humic acid atomizer solutions compared to the other organics that can explain the higher uptake values measured. A strong dependence upon relative humidity was also observed for uptake onto humic acid, with larger uptake coefficients seen at higher humidities. Possible hypotheses for the humidity dependence include the changing liquid water content of the aerosol, a change in the mass accommodation coefficient or in the Henry's law constant.

Measurements of uptake coefficients for heterogeneous loss of HO2 onto submicron inorganic salt aerosols.

Laboratory studies were conducted to investigate the kinetics of HO2 radical uptake onto submicron inorganic salt aerosols. HO2 reactive uptake coefficients were measured at room temperature using an aerosol flow tube and the Fluorescence Assay by Gas Expansion (FAGE) technique that allowed for measurements to be conducted under atmospherically relevant HO2 concentrations ([HO2] = 10(8) to 10(9) molecule cm(-3)). The uptake coefficient for HO2 uptake onto dry inorganic salt aerosols was consistently below the detection limit (γ(HO2) < 0.004). The mass accommodation coefficient of HO2 radicals onto Cu(II)-doped (NH4)2SO4 aerosols was measured to be α(HO2) = 0.4 ± 0.3 representing the kinetic upper limit to γ. For aqueous (NH4)2SO4, NaCl and NH4NO3 aerosols not containing traces of transition metal ions, a range of γ(HO2) = 0.003-0.02 was measured. These values were much lower than γ values previously measured on aqueous (NH4)2SO4 and NaCl aerosols and also those typically used in atmospheric models (γ(HO2) = 0.1-1.0). Evidence is presented showing that the HO2 uptake coefficients onto aqueous salt aerosol particles are dependent both on the exposure time to the aerosol and on the HO2 concentration used.

The atmospheric chemistry of trace gases and particulate matter emitted by different land uses in Borneo.

We report measurements of atmospheric composition over a tropical rainforest and over a nearby oil palm plantation in Sabah, Borneo. The primary vegetation in each of the two landscapes emits very different amounts and kinds of volatile organic compounds (VOCs), resulting in distinctive VOC fingerprints in the atmospheric boundary layer for both landscapes. VOCs over the Borneo rainforest are dominated by isoprene and its oxidation products, with a significant additional contribution from monoterpenes. Rather than consuming the main atmospheric oxidant, OH, these high concentrations of VOCs appear to maintain OH, as has been observed previously over Amazonia. The boundary-layer characteristics and mixing ratios of VOCs observed over the Borneo rainforest are different to those measured previously over Amazonia. Compared with the Bornean rainforest, air over the oil palm plantation contains much more isoprene, monoterpenes are relatively less important, and the flower scent, estragole, is prominent. Concentrations of nitrogen oxides are greater above the agro-industrial oil palm landscape than over the rainforest, and this leads to changes in some secondary pollutant mixing ratios (but not, currently, differences in ozone). Secondary organic aerosol over both landscapes shows a significant contribution from isoprene. Primary biological aerosol dominates the super-micrometre aerosol over the rainforest and is likely to be sensitive to land-use change, since the fungal source of the bioaerosol is closely linked to above-ground biodiversity.

Two high-speed, portable GC systems designed for the measurement of non-methane hydrocarbons and PAN: results from the Jungfraujoch High Altitude Observatory.

Near real-time measurements of light non-methane hydrocarbons (NMHCs) and peroxyacetyl nitrate (PAN) have been performed in the free troposphere using two fast gas chromatography (GC) instruments designed for use on aircraft. A GC-helium ionisation detector (HID) system measured 15 C(2)-C(5) hydrocarbons with 5 min time resolution and a dual channel GC-Electron Capture Detector (ECD) measured PAN with 90 s resolution. Both instruments had low parts per trillion by volume (pptV) detection limits and ran continuously at the remote Jungfraujoch (JFJ) research station in the Swiss Alps (46.55[degree]N, 7.98[degree]E), 3580 m above mean sea level (AMSL), during February/March 2003. Carbon monoxide, ozone, nitrogen oxide and nitrogen dioxide and all odd nitrogen species (NO(y)) were also measured continuously. Hydrocarbons and CO were strongly correlated in all air-masses whilst PAN exhibited both positive and negative correlations with respect to O(3), dependent on age and origin of the air-mass sampled. PAN was found to contribute [similar]20% to the NO(y) sampled on average. The experiment, as well as providing interesting datasets from this remote location, also demonstrated that when optimised, GC techniques have the potential to measure at a time resolution significantly greater than is traditionally considered, with high sensitivity and low uncertainty.