Influence of the wintertime North Atlantic Oscillation on European tropospheric composition: an observational and modelling study

We have used satellite observations and a simulation from the TOMCAT chemistry transport model (CTM) to investigate the influence of the well-known wintertime North Atlantic Oscillation (NAO) on European tropospheric composition. Under the positive phase of the NAO (NAO-high), strong westerlies tend to enhance transport of European pollution (e.g. nitrogen oxides, NOx; carbon monoxide, CO) away from anthropogenic source regions. In contrast, during the negative phase of the NAO (NAO-low), more stable meteorological conditions lead to a build-up of pollutants over these regions relative to the wintertime average pollution levels. However, the secondary pollutant ozone shows the opposite signal of larger values during NAO-high. NAO-high introduces Atlantic ozone-enriched air into Europe, while under NAO-low westerly transport of ozone is reduced, yielding lower values over Europe. Furthermore, ozone concentrations are also decreased by chemical loss through the reaction with accumulated primary pollutants such as nitric oxide (NO) in NAO-low. Peroxyacetyl nitrate (PAN) in the upper troposphere–lower stratosphere (UTLS) peaks over Iceland and southern Greenland in NAO-low, between 200 and 100 hPa, consistent with the trapping by an anticyclone at this altitude. Model simulations show that enhanced PAN over Iceland and southern Greenland in NAO-low is associated with vertical transport of polluted air from the mid-troposphere into the UTLS. Overall, this work shows that NAO circulation patterns are an important governing factor for European wintertime composition and air pollution

Validation of SCIAMACHY top-of-atmosphere reflectance for aerosol remote sensing using MERIS L1 data

Aerosol remote sensing is very much dependent on the accurate knowledge of the top-of-atmosphere (TOA) reflectance measured by a particular instrument. The status of the calibration of such an instrument is reflected in the quality of the aerosol retrieval. Current data of the SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) instrument (operated with the data processor version 5 and earlier) give too small values of the TOA reflectance, compared e.g. to data from MERIS (Medium Resolution Imaging Spectrometer), both operating on ENVISAT (ENVIronmental SATellite). This effect causes retrievals of wrong aerosol optical thickness and disables the processing of aerosol parameters. <br><br> From an inter-comparison of MERIS and SCIAMACHY TOA reflectance, for collocated scenes correction factors are derived to improve the insufficient SCIAMACHY L1 data calibration for data obtained with the processor 5 for the purpose of aerosol remote sensing. The corrected reflectance has been used for tests of remote sensing of the aerosol optical thickness by the BAER (Bremen AErosol Retrieval) approach using SCIAMACHY data

The TOMCAT global chemical transport model v1.6: description of chemical mechanism and model evaluation

This paper documents the tropospheric chemical mechanism scheme used in the TOMCAT 3-D chemical transport model. The current scheme includes a more detailed representation of hydrocarbon chemistry than previously included in the model, with the inclusion of the emission and oxidation of ethene, propene, butane, toluene and monoterpenes. The model is evaluated against a range of surface, balloon, aircraft and satellite measurements. The model is generally able to capture the main spatial and seasonal features of high and low concentrations of carbon monoxide (CO), ozone (O3), volatile organic compounds (VOCs) and reactive nitrogen. However, model biases are found in some species, some of which are common to chemistry models and some that are specific to TOMCAT and warrant further investigation. The most notable of these biases are (1) a negative bias in Northern Hemisphere (NH) winter and spring CO and a positive bias in Southern Hemisphere (SH) CO throughout the year, (2) a positive bias in NH O3 in summer and a negative bias at high latitudes during SH winter and (3) a negative bias in NH winter C2 and C3 alkanes and alkenes. TOMCAT global mean tropospheric hydroxyl radical (OH) concentrations are higher than estimates inferred from observations of methyl chloroform but similar to, or lower than, multi-model mean concentrations reported in recent model intercomparison studies. TOMCAT shows peak OH concentrations in the tropical lower troposphere, unlike other models which show peak concentrations in the tropical upper troposphere. This is likely to affect the lifetime and transport of important trace gases and warrants further investigation

A new scheme for sulphur dioxide retrieval from IASI measurements: application to the Eyjafjallajökull eruption of April and May 2010

Abstract. A new optimal estimation algorithm for the retrieval of sulphur dioxide (SO2) has been developed for the Infrared Atmospheric Sounding Interferometer (IASI) using the channels between 1000–1200 and 1300–1410 cm−1. These regions include the two SO2 absorption bands centred at about 8.7 and 7.3 μm (the ν1 and ν3 bands respectively). The retrieval assumes a Gaussian SO2 profile and returns the SO2 column amount in Dobson units and the altitude of the plume in millibars (mb). Forward modelled spectra (against which the measurements are compared) are based on the Radiative Transfer for TOVS (RTTOV) code. In our implementation RTTOV uses atmospheric profiles from European Centre for Medium-Range Weather Forecasts (ECMWF) meteorological data. The retrieval includes a comprehensive error budget for every pixel derived from an error covariance matrix that is based on the SO2-free climatology of the differences between the IASI and forward modelled spectra. The IASI forward model includes the ability to simulate a cloud or ash layer in the atmosphere. This feature is used to illustrate that: (1) the SO2 retrieval is not affected by underlying cloud but is affected if the SO2 is within or below a cloud layer; (2) it is possible to discern if ash (or other atmospheric constituents not considered in the error covariance matrix) affects the retrieval using quality control based on the fit of the measured spectrum by the forward modelled spectrum. In this work, the algorithm is applied to follow the behaviour of SO2 plumes from the Eyjafjallajökull eruption during April and May 2010. From 14 April to 4 May (during Phase I and II of the eruption) the total amount of SO2 present in the atmosphere, estimated by IASI measurements, is generally below 0.02 Tg. During the last part of the eruption (Phase III) the values are an order of magnitude higher, with a maximum of 0.18 Tg measured on the afternoon of 7 May. </jats:p

Aerosol dynamic processes in the Hunga plume in January 2022: does water vapor accelerate aerosol aging?

The 2022 Hunga eruption injected an unprecedented 150 Tg of water vapor into the stratosphere, accelerating SO2 oxidation and sulfate aerosol formation. Despite releasing less ash than previous eruptions of similar magnitude, the role of ash in the early plume and its rapid removal remain unclear. We performed experiments with the ICOsahedral Nonhydrostatic model with Aerosols and Reactive Trace gases (ICON-ART) to better understand the role of water vapor, SO2 and ash emissions, the aerosol–radiation interaction, and aerosol dynamical processes (nucleation, condensation, and coagulation) in the Hunga plume in the first week after the eruption. Furthermore, we compared our results with satellite observations to validate SO2 oxidation and aerosol dynamical processes. Our results show that about 1.2 Tg of SO2 emission, along with water vapor emission, is necessary to explain both the SO2 column loadings and sulfate aerosol optical depth during the first week after the eruption. Although the model reproduces the development of SO2 and sulfate aerosols well, the aerosol dynamics alone cannot explain the ash removal after the eruption, as was seen in satellite images. However, some of the ash might not be detected due to the exceptionally strong coating of the ash particles. Both the strong coating and a doubling of the sulfate effective radii within 1 week occur only when water vapor emission is included in the chemistry. Furthermore, the aerosol–radiation interaction warms the plume and reduces or, depending on the experiment, even reverses the descent of the water vapor plume that would otherwise occur due to radiative cooling.</p

Diagnosing air quality changes in the UK during the COVID-19 lockdown using TROPOMI and GEOS-Chem

The dramatic and sudden reduction in anthropogenic activity due to lockdown measures in the UK in response to the COVID-19 outbreak has resulted in a concerted effort to estimate local and regional changes in air quality, though changes in underlying emissions remain uncertain. Here we combine satellite observations of tropospheric NO_{2} from TROPOspheric Monitoring Instrument and the Goddard Earth Observing System (GEOS)-Chem 3D chemical transport model to estimate that NO_{x} emissions declined nationwide by ~20% during the lockdown (23 March to 31 May 2020). Regionally, these range from 22% to 23% in the western portion of the country to 29% in the southeast and Manchester, and >40% in London. We apply a uniform 20% lockdown period emission reduction to GEOS-Chem anthropogenic emissions over the UK to determine that decline in lockdown emissions led to a national decline in PM_{2.5} of 1.1 μg m^{−3}, ranging from 0.6 μg m^{−3} in Scotland to 2 μg m^{−3} in the southwest. The decline in emissions in cities (>40%) is greater than the national average and causes an increase in ozone of ~2 ppbv in London and Manchester. The change in ozone and PM_{2.5} concentrations due to emission reductions alone is about half the total change from 2019 to 2020. This emphasizes the need to account for emissions and other factors, in particular meteorology, in future air pollution abatement strategies and regulatory action

Exploiting airborne far-infrared measurements to optimise an ice cloud retrieval

Studies have indicated that far-infrared radiances hold significant information about the microphysics of ice clouds, particularly the ice crystal habit. In support of the European Space Agency's Far-Infrared Outgoing Radiation Understanding and Monitoring mission, we perform the first retrieval on an observation of coincident upwelling far- and mid-infrared radiances taken from an aircraft above a cirrus cloud layer. Four retrievals are performed: including and neglecting the far-infrared portion of the spectrum and assuming two different habit mixes. Results are compared to in situ measurements of the cloud optical thickness, cloud top height, cloud effective radius, and habit distributions. We find that despite the known limitations of ice cloud optical property models, all the retrievals show agreement within the in situ measurements of the cloud optical thickness, cloud top height, and cloud effective radius. However, the inclusion of the far-infrared enables a distinction between two different habits that is not possible using only mid-infrared channels. Furthermore, in this case study, the uncertainty in the retrieval of cloud top height and cloud optical thickness halves with the inclusion of the far-infrared. As with other studies, we also see an additional degree of freedom for the temperature and water vapour retrievals. Our study highlights the need for the improvement of current ice cloud optical models, with the radiance residuals from the converged retrievals still exceeding the instrument uncertainty within the far-infrared. However, it provides observational support for the theoretical improvement that far-infrared observations could bring to retrievals of ice cloud properties.</p

Retrievals of water vapour and temperature exploiting the far-infrared: application to aircraft observations in preparation for the FORUM mission

We present the extension of the Rutherford Appleton Laboratory (RAL) Infrared Microwave Sounding (IMS) optimal estimation retrieval scheme to include the use of far-infrared channels in preparation for the upcoming Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission. The IMS code has been previously applied to mid-infrared spectral radiances measured by the Infrared Atmospheric Sounding Instrument (IASI) to retrieve temperature and water vapour. Given this, the evolution and evaluation of the extended scheme is performed in two steps. First, clear-sky retrievals of temperature and water vapour are performed on IASI and FORUM simulations. Comparable retrieval biases are observed for retrievals of temperature and water vapour; however, there is an increase of ∼ 1 degree of freedom for water vapour and temperature for the FORUM configuration. Secondly, radiances observed from an aircraft flight in the upper troposphere are modified to match the FORUM spectral characteristics. Retrievals from these radiances using the modified code show a strong agreement with contemporaneous in situ measurements of the atmospheric state, reducing the root-mean-square error (RMSE) by 18 % for water vapour from the a priori, giving confidence in its performance. The extended IMS scheme is now available for use on FORUM observations and can be easily adapted to other far- and mid-infrared instrument configurations.</p