Global inorganic nitrate production mechanisms: Comparison of a global model with nitrate isotope observations

© Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. The formation of inorganic nitrate is the main sink for nitrogen oxides (NOx DNOCNO2). Due to the importance of NOx for the formation of tropospheric oxidants such as the hydroxyl radical (OH) and ozone, understanding the mechanisms and rates of nitrate formation is paramount for our ability to predict the atmospheric lifetimes of most reduced trace gases in the atmosphere. The oxygen isotopic composition of nitrate (117O(nitrate)) is determined by the relative importance of NOx sinks and thus can provide an observational constraint for NOx chemistry. Until recently, the ability to utilize 117O(nitrate) observations for this purpose was hindered by our lack of knowledge about the oxygen isotopic composition of ozone (117O.O3/). Recent and spatially widespread observations of 117O.O3/ motivate an updated comparison of modeled and observed 117O(nitrate) and a reassessment of modeled nitrate formation pathways. Model updates based on recent laboratory studies of heterogeneous reactions render dinitrogen pentoxide (N2O5) hydrolysis as important as NO2 COH (both 41 %) for global inorganic nitrate production near the surface (below 1 km altitude). All other nitrate production mechanisms individually represent less than 6% of global nitrate production near the surface but can be dominant locally. Updated reaction rates for aerosol uptake of NO2 result in significant reduction of nitrate and nitrous acid (HONO) formed through this pathway in the model and render NO2 hydrolysis a negligible pathway for nitrate formation globally. Although photolysis of aerosol nitrate may have implications for NOx , HONO, and oxidant abundances, it does not significantly impact the relative importance of nitrate formation pathways. Modeled 117O(nitrate) (28:64:5 ) compares well with the average of a global compilation of observations (27:65:0 ) when assuming 117O.O3/D26 , giving confidence in the model\u27s representation of the relative importance of ozone versus HOx (DOHCHO2 CRO2) in NOx cycling and nitrate formation on the global scale

Biofuels and their potential to aid the UK towards achieving emissions reduction policy targets

The potential of biofuels contributing to the UK emission reduction targets in the formulated UK Low Carbon Transition Plan (LCTP) and the UK’s obligation in the wider EU emissions reduction targets are assessed using four scenarios. The scenarios were evaluated using hybrid lifecycle assessment developed in a multi-regional input–output (MRIO) framework. In the hybrid MRIO LCA framework, technology-specific processes in the biofuels and fossil fuels LCA systems are integrated into a generalised 2-region (UK and Rest of the World) environmental-economic input–output framework in order to account for economy-wide indirect GHG emissions in the biofuels and fossil fuels LCA systems in addition to other indirect impacts such as indirect land use change. The lifecycle greenhouse gas emissions of biodiesel (soybean, palm, rape, waste cooking oil) and bio-ethanol (sugarcane, sugarbeet, corn) were assessed and compared to fossil fuel (diesel and petrol) baseline. From one of the scenarios, biodiesel production from waste cooking oil and bioethanol from sugarbeet offer the biggest potential for emissions savings relative to fossil fuel equivalent and offering a maximum emission savings of 4.1% observed with a biofuel market share of 10% reached in 2020. It was also established that under current biofuel feedstock mix, to achieve the 6% emissions saving primarily from biofuels as proposed in the LCTP, 23.8% of the transport fuels market would be required to be held by biofuels by 2020

Nitrogen oxides in the free troposphere : Implications for tropospheric oxidants and the interpretation of satellite NO2 measurements

Satellite-based retrievals of tropospheric NO2 columns are widely used to infer NOx (gNOg+gNO2) emissions. These retrievals rely on model information for the vertical distribution of NO2. The free tropospheric background above 2gkm is particularly important because the sensitivity of the retrievals increases with altitude. Free tropospheric NOx also has a strong effect on tropospheric OH and ozone concentrations. Here we use observations from three aircraft campaigns (SEAC4RS, DC3, and ATom) and four atmospheric chemistry models (GEOS-Chem, GMI, TM5, and CAMS) to evaluate the model capabilities for simulating NOx in the free troposphere and attribute it to sources. NO2 measurements during the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS) and Deep Convective Clouds and Chemistry (DC3) campaigns over the southeastern U.S. in summer show increasing concentrations in the upper troposphere above 10gkm, which are not replicated by the GEOS-Chem, although the model is consistent with the NO measurements. Using concurrent NO, NO2, and ozone observations from a DC3 flight in a thunderstorm outflow, we show that the NO2 measurements in the upper troposphere are biased high, plausibly due to interference from thermally labile NO2 reservoirs such as peroxynitric acid (HNO4) and methyl peroxy nitrate (MPN). We find that NO2 concentrations calculated from the NO measurements and NO-NO2 photochemical steady state (PSS) are more reliable to evaluate the vertical profiles of NO2 in models. GEOS-Chem reproduces the shape of the PSS-inferred NO2 profiles throughout the troposphere for SEAC4RS and DC3 but overestimates NO2 concentrations by about a factor of 2. The model underestimates MPN and alkyl nitrate concentrations, suggesting missing organic NOx chemistry. On the other hand, the standard GEOS-Chem model underestimates NO observations from the Atmospheric Tomography Mission (ATom) campaigns over the Pacific and Atlantic oceans, indicating a missing NOx source over the oceans. We find that we can account for this missing source by including in the model the photolysis of particulate nitrate on sea salt aerosols at rates inferred from laboratory studies and field observations of nitrous acid (HONO) over the Atlantic. The median PSS-inferred tropospheric NO2 column density for the ATom campaign is 1.7g±g0.44g×g1014gmolec.gcm-2, and the NO2 column density simulated by the four models is in the range of 1.4-2.4g×g1014gmolec.gcm-2, implying that the uncertainty from using modeled NO2 tropospheric columns over clean areas in the retrievals for stratosphere-troposphere separation is about 1g×g1014gmolec.gcm-2. We find from GEOS-Chem that lightning is the main primary NOx source in the free troposphere over the tropics and southern midlatitudes, but aircraft emissions dominate at northern midlatitudes in winter and in summer over the oceans. Particulate nitrate photolysis increases ozone concentrations by up to 5gppbv (parts per billion by volume) in the free troposphere in the northern extratropics in the model, which would largely correct the low model bias relative to ozonesonde observations. Global tropospheric OH concentrations increase by 19g%. The contribution of the free tropospheric background to the tropospheric NO2 columns observed by satellites over the contiguous U.S. increases from 25g±g11g% in winter to 65g±g9g% in summer, according to the GEOS-Chem vertical profiles. This needs to be accounted for when deriving NOx emissions from satellite NO2 column measurements