Key Role of NO3 Radicals in the Production of Isoprene Nitrates and Nitrooxyorganosulfates in Beijing

The formation of isoprene nitrates (IsN) can lead to significant secondary organic aerosol (SOA) production and they can act as reservoirs of atmospheric nitrogen oxides. In this work, we estimate the rate of production of IsN from the reactions of isoprene with OH and NO₃ radicals during the summertime in Beijing. While OH dominates the loss of isoprene during the day, NO₃ plays an increasingly important role in the production of IsN from the early afternoon onwards. Unusually low NO concentrations during the afternoon resulted in NO₃ mixing ratios of ca. 2 pptv at approximately 15:00, which we estimate to account for around a third of the total IsN production in the gas phase. Heterogeneous uptake of IsN produces nitrooxyorganosulfates (NOS). Two mono-nitrated NOS were correlated with particulate sulfate concentrations and appear to be formed from sequential NO₃ and OH oxidation. Di- and tri-nitrated isoprene-related NOS, formed from multiple NO₃ oxidation steps, peaked during the night. This work highlights that NO₃ chemistry can play a key role in driving biogenic-anthropogenic interactive chemistry in Beijing with respect to the formation of IsN during both the day and night.

Significant OH production under surface cleaning and air cleaning conditions: Impact on indoor air quality

We report measurements of hydroxyl (OH) and hydroperoxy (HO₂ ) radicals made by laser-induced fluorescence spectroscopy in a computer classroom (i) in the absence of indoor activities (ii) during desk cleaning with a limonene-containing cleaner (iii) during operation of a commercially available "air cleaning" device. In the unmanipulated environment, the one-minute averaged OH concentration remained close to or below the limit of detection (6.5×10⁵  molecule cm^-3 ), whilst that of HO₂ was 1.3×10⁷  molecule cm^-3 . These concentrations increased to ~4×10⁶ and 4×10⁸  molecule cm^-3 , respectively during desk cleaning. During operation of the air cleaning device, OH and HO₂ concentrations reached ~2×10⁷ and ~6×10⁸  molecule cm^-3 respectively. The potential of these OH concentrations to initiate chemical processing is explored using a detailed chemical model for indoor air (the INDCM). The model can reproduce the measured OH and HO₂ concentrations to within 50% and often within a few % and demonstrates that the resulting secondary chemistry varies with the cleaning activity. Whilst terpene reaction products dominate the product composition following surface cleaning, those from aromatics and other VOCs are much more important during the use of the air cleaning device.

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 NO_x cycling over Europe, resulting in significant regional impacts on NO_y speciation and O₃ concentrations, with potential changes to human exposure to air pollutants.