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We investigate the ability of column-integrated trace gas measurements from a geostationary satellite to constrain surface fluxes at regional scale. The proposed GEOCARB instrument measures CO2, CO and CH4 at a maximum resolution of 3 km east–west × 2.7 km north–south. Precisions are 3 ppm for CO2, 10 ppb for CO and 18 ppb for CH4. Sampling frequency is flexible. Here we sample a region at the location of Shanghai every 2 daylight hours for 6 days in June. We test the observing system by calculating the posterior uncertainty covariance of fluxes. We are able to constrain urban emissions at 3 km resolution including an isolated power plant. The CO measurement plays the strongest role; without it our effective resolution falls to 5 km. Methane fluxes are similarly well estimated at 5 km resolution. Estimating the errors for a full year suggests such an instrument would be a useful tool for both science and policy applications.

This study aims to investigate the response of simulated tropical cyclone formation to specific climate conditions, using an idealized aquaplanet framework of an ∼40-km-horizontal-resolution atmospheric general circulation model. Two sets of idealized model experiments have been performed, one with a set of uniformly distributed constant global sea surface temperatures (SSTs) and another in which varying meridional SST gradients are imposed. The results show that the strongest relationship between climate and tropical cyclone formation is with vertical static stability: increased static stability is strongly associated with decreased tropical cyclone formation. Vertical wind shear and midtropospheric vertical velocity also appear to be related to tropical cyclone formation, although below a threshold value of wind shear there appears to be little relationship. The relationship of tropical cyclone formation with maximum potential intensity and mean sea surface temperature is weak and not monotonic. These simulations strongly suggest that vertical static stability should be part of any climate theory of tropical cyclone formation.

Recent global climate models with sufficient resolution and physics offer a promising approach for simulating real-world tropical cyclone (TC) statistics and their changing relationship with climate. In the first part of this study, we examine the performance of a high-resolution (∼40-km horizontal grid) global climate model, the atmospheric component of the Australian Community Climate and Earth System Simulator (ACCESS) based on the Met Office Unified Model (UM8.5) Global Atmosphere (GA6.0). The atmospheric model is forced with observed sea surface temperature, and 20 years of integrations (1990–2009) are analyzed for evaluating the simulated TC statistics compared with observations. The model reproduces the observed climatology, geographical distribution, and interhemispheric asymmetry of global TC formation rates reasonably well. The annual cycle of regional TC formation rates over most basins is also well captured. However, there are some regional biases in the geographical distribution of TC formation rates. To identify the sources of these biases, a suite of model-simulated large-scale climate conditions that critically modulate TC formation rates are further evaluated, including the assessment of a multivariate genesis potential index. Results indicate that the model TC genesis biases correspond well to the inherent biases in the simulated large-scale climatic states, although the relative effects on TC genesis of some variables differs between basins. This highlights the model’s mean-state dependency in simulating accurate TC formation rates.

Gas-phase ethene ozonolysis experiments were conducted at room temperature to determine formic acid yields as a function of relative humidity (RH) using the integrated EXTreme RAnge chamber-Chemical Ionisation Mass Spectrometry technique, employing a CH3I ionisation scheme. RHs studied were <1, 11, 21, 27, 30 % and formic acid yields of (0.07±0.01) and (0.41±0.07) were determined at <1 % RH and 30 % RH respectively, showing a strong water dependence. It has been possible to estimate the ratio of the rate coefficient for the reaction of the Criegee biradical, CH2OO with water compared with decomposition. This analysis suggests that the rate of reaction with water ranges between 1×10−12–1×10−15 cm3 molecule−1 s−1 and will therefore dominate its loss with respect to bimolecular processes in the atmosphere. Global model integrations suggest that this reaction between CH2OO and water may dominate the production of HC(O)OH in the atmosphere.

Recently reported model-measurement discrepancies for the concentrations of the HOx radical species (OH and HO2 ) in locations characterized by high emission rates of isoprene have indicated possible deficiencies in the representation of OH recycling and formation in isoprene mechanisms currently employed in numerical models; particularly at low levels of NOx . Using version 3.1 of the Master Chemical Mechanism (MCM v3.1) as a base mechanism, the sensitivity of the system to a number of detailed mechanistic changes is examined for a wide range of NOx levels, using a simple box model. The studies consider sensitivity tests in relation to three general areas for which experimental and/or theoretical evidence has been reported in the peer-reviewed literature, as follows: (1) implementation of propagating channels for the reactions of HO2 with acyl and β-oxo peroxy radicals with HO2 , with support from a number of studies; (2) implementation of the OH-catalysed conversion of isoprene-derived hydroperoxides to isomeric epoxydiols, as characterised by Paulot et al.~(2009a); and (3) implementation of a mechanism involving respective 1,5 and 1,6 H atom shift isomerisation reactions of the β-hydroxyalkenyl and cis-δ-hydroxyalkenyl peroxy radical isomers, formed from the sequential addition of OH and O2 to isoprene, based on the theoretical study of Peeters et al. (2009). All the considered mechanistic changes lead to simulated increases in the concentrations of OH, with (1) and (2) resulting in respective increases of up to about 7% and 16%, depending on the level of NOx . (3) is found to have potentially much greater impacts, with enhancements in OH concentrations of up to a factor of about 3.3, depending on the level of NOx , provided the (crucial) rapid photolysis of the hydroperoxy-methyl-butenal products of the cis-δ-hydroxyalkenyl peroxy radical isomerisation reactions is represented, as also postulated by Peeters et al.~(2009). Additional tests suggest that the mechanism with the reported parameters cannot be fully reconciled with atmospheric observations and existing laboratory data without some degree of parameter refinement and optimisation which would probably include a reduction in the peroxy radical isomerisation rates and a consequent reduction in the OH enhancement propensity. However, an order of magntitude reduction in the isomerisation rates is still found to yield notable enhancements in OH concentrations of up to a factor of about 2, with the maximum impact at the low end of the considered NOx range. A parameterized representation of the mechanistic changes is optimized and implemented into a reduced variant of the Common Representative Intermediates mechanism (CRI v2-R5), for use in the STOCHEM global chemistry-transport model. The impacts of the modified chemistry in the global model are shown to be consistent with those observed in the box model sensitivity studies, and the results are illustrated and discussed with a particular focus on the tropical forested regions of the Amazon and Borneo where unexpectedly elevated concentrations of OH have recently been reported.

We document the implementation of the Common Representative Intermediates Mechanism version 2, reduction 5 into the United Kingdom Chemistry and Aerosol model (UKCA) version 10.9. The mechanism is merged with the stratospheric chemistry already used by the StratTrop mechanism, as used in UKCA and the UK Earth System Model, to create a new CRI‐Strat mechanism. CRI‐Strat simulates a more comprehensive treatment of non‐methane volatile organic compounds (NMVOCs) and provides traceability with the Master Chemical Mechanism. In total, CRI‐Strat simulates the chemistry of 233 species competing in 613 reactions (compared to 87 species and 305 reactions in the existing StratTrop mechanism). However, while more than twice as complex than StratTrop, the new mechanism is only 75% more computationally expensive. CRI‐Strat is evaluated against an array of in situ and remote sensing observations and simulations using the StratTrop mechanism in the UKCA model. It is found to increase production of ozone near the surface, leading to higher ozone concentrations compared to surface observations. However, ozone loss is also greater in CRI‐Strat, leading to less ozone away from emission sources and a similar tropospheric ozone burden compared to StratTrop. CRI‐Strat also produces more carbon monoxide than StratTrop, particularly downwind of biogenic VOC emission sources, but has lower burdens of nitrogen oxides as more is converted into reservoir species. The changes to tropospheric ozone and nitrogen budgets are sensitive to the treatment of NMVOC emissions, highlighting the need to reduce uncertainty in these emissions to improve representation of tropospheric chemical composition. Plain Language Summary To understand the climate and predict how it will change in the future, we need to understand its chemical composition—the trace gases and small particles that exist in tiny quantities in the atmosphere. A key tool we use to do this are computer models which simulate the atmosphere and processes within it. Key processes include the formation of ozone, a harmful pollutant and greenhouse gas in the lower atmosphere. However, the chemistry involved in forming ozone is very complicated, so computer simulations of the atmosphere must greatly simplify the chemistry. These simple schemes may introduce errors in the model. We also have much more complex chemical mechanisms which simulate our best understanding of all chemical reactions, but these complex schemes require too much computational power to be used when simulating the whole atmosphere. In this paper, we describe the implementation of a chemical mechanism that sits between these levels of complexity, realistically simulating the formation and destruction of ozone without being too slow to run. We compare this new mechanism against measurements taken of the atmosphere and the preexisting, simpler chemical mechanism and show that the new mechanism greatly enhances the amount of ozone that is produced. Key Points The CRI‐Strat mechanism has been integrated into the United Kingdom Chemistry and Aerosol model, greatly increasing the complexity of volatile organic compound chemistry compared to StratTrop CRI‐Strat simulates higher surface ozone compared to StratTrop due to greater production, but tropospheric ozone burden is similar The ozone and oxidized nitrogen budgets when running with the CRI‐Strat mechanism show high sensitivity to the input non‐methane volatile organic compound emissions

Chemical modelling studies have been conducted over north-western Europe in summer conditions, showing that night-time dinitrogen pentoxide (N2O5) heterogeneous reactive uptake is important regionally in modulating particulate nitrate and has a~modest influence on oxidative chemistry. Results from Weather Research and Forecasting model with Chemistry (WRF-Chem) model simulations, run with a detailed volatile organic compound (VOC) gas-phase chemistry scheme and the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) sectional aerosol scheme, were compared with a series of airborne gas and particulate measurements made over the UK in July 2010. Modelled mixing ratios of key gas-phase species were reasonably accurate (correlations with measurements of 0.7–0.9 for NO2 and O3). However modelled loadings of particulate species were less accurate (correlation with measurements for particulate sulfate and ammonium were between 0.0 and 0.6). Sulfate mass loadings were particularly low (modelled means of 0.5–0.7 μg kg−1air, compared with measurements of 1.0–1.5 μg kg−1air). Two flights from the campaign were used as test cases – one with low relative humidity (RH) (60–70%), the other with high RH (80–90%). N2O5 heterogeneous chemistry was found to not be important in the low-RH test case; but in the high-RH test case it had a strong effect and significantly improved the agreement between modelled and measured NO3 and N2O5. When the model failed to capture atmospheric RH correctly, the modelled NO3 and N2O5 mixing ratios for these flights differed significantly from the measurements. This demonstrates that, for regional modelling which involves heterogeneous processes, it is essential to capture the ambient temperature and water vapour profiles. The night-time NO3 oxidation of VOCs across the whole region was found to be 100–300 times slower than the daytime OH oxidation of these compounds. The difference in contribution was less for alkenes (× 80) and comparable for dimethylsulfide (DMS). However the suppression of NO3 mixing ratios across the domain by N2O5 heterogeneous chemistry has only a very slight, negative, influence on this oxidative capacity. The influence on regional particulate nitrate mass loadings is stronger. Night-time N2O5 heterogeneous chemistry maintains the production of particulate nitrate within polluted regions: when this process is taken into consideration, the daytime peak (for the 95th percentile) of PM10 nitrate mass loadings remains around 5.6 μg kg−1air, but the night-time minimum increases from 3.5 to 4.6 μg kg−1air. The sustaining of higher particulate mass loadings through the night by this process improves model skill at matching measured aerosol nitrate diurnal cycles and will negatively impact on regional air quality, requiring this process to be included in regional models.

Calculations of the absorptive partitioning of secondary organic aerosol components were carried out using a number of methods to estimate vapour pressure and non-ideality. The sensitivity of predicted condensed component masses, volatility, O:C ratio, molar mass and functionality distribution to the choice of estimation methods was investigated in mixtures of around 2700 compounds generated by a near explicit mechanism of atmospheric VOC degradation. The sensitivities in terms of all metrics were comparable to those previously reported (using 10 000 semi-randomly generated compounds). In addition, the change in predicted aerosol properties and composition with changing VOC emission scenario was investigated showing key dependencies on relative anthropogenic and biogenic contributions. Finally, the contribution of non-ideality to the changing distribution of condensed components was explored in terms of the shift in effective volatility by virtue of component activity coefficients, clearly demonstrating both enhancement and reduction of component masses associated with negative and positive deviations from ideality.

A global chemistry transport model is employed to investigate the impact of recent laboratory determinations of photolysis parameters for formaldehyde on concentrations of tropospheric trace gases. Using the new laboratory data, the photolysis of formaldehyde is a more significant removal pathway. HOx levels are increased with the greatest changes towards the top of the troposphere and the poles, making formaldehyde a more significant source of upper tropospheric HOx than previously thought. Global totals of ozone and secondary organic aerosol increase with the rise in ozone being more significant at higher solar zenith angles. Copyright © 2010 Royal Meteorological Society

Coral reefs have been found to produce the sulfur compound dimethyl sulfide (DMS), a climatically relevant aerosol precursor predominantly associated with phytoplankton. Until recently, the role of coral-reef-derived DMS within the climate system had not been quantified. A study preceding the present work found that DMS produced by corals had negligible long-term climatic forcing at the global–regional scale. However, at sub-daily timescales more typically associated with aerosol and cloud formation, the influence of coral-reef-derived DMS on local aerosol radiative effects remains unquantified. The Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) has been used in this work to study the role of coral-reef-derived DMS at sub-daily timescales for the first time. WRF-Chem was run to coincide with an October 2016 field campaign over the Great Barrier Reef, Queensland, Australia, against which the model was evaluated. After updating and scaling the DMS surface water climatology, the model reproduced DMS and sulfur concentrations well. The inclusion of coral-reef-derived DMS resulted in no significant change in sulfate aerosol mass or total aerosol number. Subsequently, no direct or indirect aerosol effects were detected. The results suggest that the co-location of the Great Barrier Reef with significant anthropogenic aerosol sources along the Queensland coast prevents coral-reef-derived aerosol from having a modulating influence on local aerosol burdens in the current climate.