A nocturnal atmospheric loss of CH2I2 in the remote marine boundary layer.

Ocean emissions of inorganic and organic iodine compounds drive the biogeochemical cycle of iodine and produce reactive ozone-destroying iodine radicals that influence the oxidizing capacity of the atmosphere. Di-iodomethane (CH2I2) and chloro-iodomethane (CH2ICl) are the two most important organic iodine precursors in the marine boundary layer. Ship-borne measurements made during the TORERO (Tropical Ocean tRoposphere Exchange of Reactive halogens and Oxygenated VOC) field campaign in the east tropical Pacific Ocean in January/February 2012 revealed strong diurnal cycles of CH2I2 and CH2ICl in air and of CH2I2 in seawater. Both compounds are known to undergo rapid photolysis during the day, but models assume no night-time atmospheric losses. Surprisingly, the diurnal cycle of CH2I2 was lower in amplitude than that of CH2ICl, despite its faster photolysis rate. We speculate that night-time loss of CH2I2 occurs due to reaction with NO3 radicals. Indirect results from a laboratory study under ambient atmospheric boundary layer conditions indicate a k CH2I2+NO3 of ≤4 × 10-13 cm3 molecule-1 s-1; a previous kinetic study carried out at ≤100 Torr found k CH2I2+NO3 of 4 × 10-13 cm3 molecule-1 s-1. Using the 1-dimensional atmospheric THAMO model driven by sea-air fluxes calculated from the seawater and air measurements (averaging 1.8 +/- 0.8 nmol m-2 d-1 for CH2I2 and 3.7 +/- 0.8 nmol m-2 d-1 for CH2ICl), we show that the model overestimates night-time CH2I2 by >60 % but reaches good agreement with the measurements when the CH2I2 + NO3 reaction is included at 2-4 × 10-13 cm3 molecule-1 s-1. We conclude that the reaction has a significant effect on CH2I2 and helps reconcile observed and modeled concentrations. We recommend further direct measurements of this reaction under atmospheric conditions, including of product branching ratios.

Atmospheric ethanol in London and the potential impacts of future fuel formulations.

There is growing global consumption of non-fossil fuels such as ethanol made from renewable biomass. Previous studies have shown that one of the main air quality disadvantages of using ethanol blended fuels is a significant increase in the production of acetaldehyde, an unregulated and toxic pollutant. Most studies on the impacts of ethanol blended gasoline have been carried out in the US and Brazil, with much less focus on the UK and Europe. We report time resolved measurements of ethanol in London during the winter and summer of 2012. In both seasons the mean mixing ratio of ethanol was around 5 ppb, with maximum values over 30 ppb, making ethanol currently the most abundant VOC in London air. We identify a road transport related source, with 'rush-hour' peaks observed. Ethanol is strongly correlated with other road transport-related emissions, such as small aromatics and light alkanes, and has no relationship to summer biogenic emissions. To determine the impact of road transport-related ethanol emission on secondary species (i.e. acetaldehyde and ozone), we use both a chemically detailed box model (incorporating the Master Chemical Mechanism, MCM) and a global and nested regional scale chemical transport model (GEOS-Chem), on various processing time scales. Using the MCM model, only 16% of the modelled acetaldehyde was formed from ethanol oxidation. However, the model significantly underpredicts the total levels of acetaldehyde, indicating a missing primary emission source, that appears to be traffic-related. Further support for a primary emission source comes from the regional scale model simulations, where the observed concentrations of ethanol and acetaldehyde can only be reconciled with the inclusion of large primary emissions. Although only constrained by one set of observations, the regional modelling suggests a European ethanol source similar in magnitude to that of ethane (∼60 Gg per year) and greater than that of acetaldehyde (∼10 Gg per year). The increased concentrations of ethanol and acetaldehyde from primary emissions impacts both radical and NOx cycling over Europe, resulting in significant regional impacts on NOy speciation and O3 concentrations, with potential changes to human exposure to air pollutants.

Improving the quantification of secondary organic aerosol using a microflow reactor coupled to HPLC-MS and NMR to manufacture ad hoc calibration standards.

Secondary organic aerosol (SOA) is a key uncertainty in quantifying the impact of humans on Earth's climate. SOA is a complex mixture of oxidized organic species, and a fundamental hurdle in determining its composition is the lack of authentic standards for comparison and quantification. Organic synthesis can be used to produce pure standards, but is limited to compounds for which there is a degree of confidence in the proposed structure and can be expensive and time-consuming. In this study, a flow reactor was developed to form SOA in sufficient quantities to be collected and pure compounds subsequently isolated from the mixture using semipreparative high performance liquid chromatography. The purity and yield of each isolated compound were obtained using proton nuclear magnetic resonance ((1)H NMR), whereas molecular formulas were confirmed by high resolution Fourier transform ion cyclotron mass spectrometry (FTICR-MS). The effectiveness of the methodology has been evaluated here by using α-pinene as the precursor because it is the monoterpene with the most well characterized SOA chemistry. Eleven individual α-pinene SOA compounds were produced from α-pinene oxidation experiments and used for quantitative analysis of SOA formed during chamber experiments carried out close to ambient conditions. These compounds represented 25% of the total SOA mass, a significant improvement in mass balance compared to previous studies. This relatively simple approach may be extended to produce other SOA components not available commercially to improve quantification of aerosol sources.

The application of two total transfer valve modulators for comprehensive two-dimensional gas chromatography of volatile organic compounds.

Rotary and diaphragm 6-port 2-position valves have been evaluated as modulators for comprehensive GC. A total transfer methodology was used such that all analyte materials from the primary column were passed to the second separating column. The transfer methodology used repeated short periods of stop flow, but retained good primary column performance since primary carrier flow was partially maintained as pressure equilibrated along the length of the first column. Highly resolved separations of complex samples such as petrol and essential oils were achieved with equivalent performance (in terms of second column resolution) to thermal modulation with flame ionisation detection (FID). The valve modulators tested could outperform thermal modulators for very highly volatile organic species since the modulation process was insensitive to analyte vapour pressure. The diaphragm valve tested showed the best performance and was incorporated with a thermal desorption system to provide a high sensitivity separation of volatile organic compounds (VOCs) in air with LODs for individual VOCs of 2-4 pptV for a 1-L air sample.