Spatially resolved flux measurements of NOx from London suggest significantly higher emissions than predicted by inventories

To date, direct validation of city-wide emissions inventories for air pollutants has been difficult or impossible. However, recent technological innovations now allow direct measurement of pollutant fluxes from cities, for comparison with emissions inventories, which are themselves commonly used for prediction of current and future air quality and to help guide abatement strategies. Fluxes of NOx were measured using the eddy-covariance technique from an aircraft flying at low altitude over London. The highest fluxes were observed over central London, with lower fluxes measured in suburban areas. A footprint model was used to estimate the spatial area from which the measured emissions occurred. This allowed comparison of the flux measurements to the UK's National Atmospheric Emissions Inventory (NAEI) for NOx, with scaling factors used to account for the actual time of day, day of week and month of year of the measurement. The comparison suggests significant underestimation of NOx emissions in London by the NAEI, mainly due to its under-representation of real world road traffic emissions. A comparison was also carried out with an enhanced version of the inventory using real world driving emission factors and road measurement data taken from the London Atmospheric Emissions Inventory (LAEI). The measurement to inventory agreement was substantially improved using the enhanced version, showing the importance of fully accounting for road traffic, which is the dominant NOx emission source in London. In central London there was still an underestimation by the inventory of 30-40% compared with flux measurements, suggesting significant improvements are still required in the NOx emissions inventory.

Evaluation of Novel Routes for NOx Formation in Remote Regions

Photochemical cycling of nitrogen oxides (NO_x) produces tropospheric ozone (O₃), and NO_x is traditionally considered to be directly emitted. The inability of current global models to accurately calculate NO_x levels, and concurrently, difficulties in performing direct NO_x measurements in low-NO_x regimes (several pptv or several tens of pptv) globally introduce a large uncertainty in the modeling of O₃ formation. Here, we use the near-explicit Master Chemical Mechanism (MCM v3.2) within a 0D box-model framework, to describe the chemistry of NO_x and O₃ in the remote marine boundary layer at Cape Verde. We explore the impact of a recently discovered NO_x recycling route, namely photolysis of particulate nitrate, on the modeling of NO_x abundance and O₃ formation. The model is constrained to observations of long-lived species, meteorological parameters, and photolysis frequencies. Only a model with this novel NO_x recycling route reproduces levels of gaseous nitrous acid, NO, and NO₂ within the model and measurement uncertainty. O₃ formation from NO oxidation is several times more efficient than previously considered. This study highlights the need for the inclusion of particulate nitrate photolysis in future models for O₃ and for the photolysis rate of particulate nitrate to be quantified under variable atmospheric conditions.

Insights into the photochemical transformation of iodine in aqueous systems: humic acid photosensitized reduction of iodate

Marine aerosol is highly enriched in iodine, mostly in the form of iodate (IO(3)(-)) ions, compared to its relative abundance in seawater. This paper describes a laboratory study of the photochemical reduction of IO(3)(-) in the presence of humic acid. Spectroscopic analysis showed that ~20% of IO(3)(-) was converted to "free" iodide (I(-)) ions and this fraction remained constant as a function of time. Direct detection of an organically fixed fraction (i.e., ∼ 80%) was not possible, but a number of test reactions with surrogate organic compounds containing functional groups identified in humic acid structures indicate that efficient substitution of iodine occurs at aromatic 1,2 diol sites. These iodinated humic acids are stable with respect to photolysis at near-UV/visible wavelengths and are likely to account for a significant proportion of the soluble iodine-containing organic material occurring within aerosols. In the lower atmosphere, oxidation of I(-) to I(2) in marine aerosol occurs mostly through the uptake of O(3), with H(2)O(2) playing a very minor role. A model of iodine chemistry in the open ocean tropical boundary layer, which incorporates these experimental results, is able to account for the observed enrichment of iodine in marine aerosol.