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In the UK, the public health threat of heatwaves is exacerbated by the co-occurrence of ozone pollution episodes. Here we present retrieved vertical profiles of nitrogen dioxide (NO2) and formaldehyde (HCHO) over Central London from a newly installed long-term Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) instrument on a 60-m altitude rooftop site during two of three heatwaves in the hottest summer on record. We evaluate routine space-based sensor observations of air pollutant precursors over London and enhance the permanent air quality monitor network. Daily mean tropospheric column densities of NO2 and HCHO from the TROPOspheric Monitoring Instrument (TROPOMI) are consistent with those from the MAX-DOAS (both R = 0.71) after accounting for different vertical sensitivities. As expected, TROPOMI NO2 is 27-31% less than MAX-DOAS NO2, due to horizontal dilution of local sources of road-traffic NO2. TROPOMI HCHO is 20% more than MAX-DOAS HCHO; a larger difference than in past validation studies, but within the range of systematic retrieval errors. MAX-DOAS lowest layer (0-110 m) retrievals have analogous day-to-day variability to surface site NO2 (R ≥ 0.7). Surface site measurements of isoprene, which rapidly oxidises to HCHO with a high yield, also have similar diurnal variations to MAX-DOAS HCHO (R > 0.6). Generally, daytime ozone production, which is diagnosed with MAX-DOAS HCHO:NO2 tropospheric column ratios, is limited by the availability of volatile organic compounds. During heatwaves, ozone production shifts from NOx-saturated to NOx-limited as biogenic emissions of isoprene increase exponentially with temperature, leading to non-compliance with ozone regulatory standards. In ongoing work, we are assessing the ability to retrieve the NOx (NO + NO2) reservoir compound nitrous acid (HONO) to address uncertainties in HONO chemistry. This will aid understanding of the oxidation capacity of the atmosphere and ozone budget in centres of megacities such as London.

Polar stratospheric clouds (PSCs) are an important component of the ozone stratospheric chemistry in polar regions. Ground-based spectroscopic measurements can be taken for detecting PSCs in various weather conditions using the so-called colour index (CI) and are a valuable complement to other PSC data sets such as satellite observations.In this study, continuous long-term measurements from two DOAS (Differential Optical Absorption Spectroscopy) instruments at Kiruna, Sweden (68° N, 20° E), and at the German research station Neumayer, Antarctica (70° S, 8° W) are analysed. In 20 years of measurements, no significant trend is detected for either measurement station. However, the years with preceding large volcanic eruptions show unexpectedly high occurrences of PSC-like signatures during springtime which suggests the influence of volcanic aerosol. This is likewise indicated by enhanced aerosol extinction during these time periods as seen from OMPS (Ozone Mapping and Profiler Suite) data, but is not captured by other PSC climatologies. The observed springtime signal looks very similar to the CI of PSCs and can only be distinguished by other proxy data such as temperature. This ambiguity needs to be considered in the interpretation of colour index data. The potential importance of our results to stratospheric ozone chemistry is not yet clear.

We introduce the new Global Ozone Monitoring Experiment-2 (GOME-2) daily and monthly level-3 product of total column ozone (O3), total and tropospheric column nitrogen dioxide (NO2), total column water vapour, total column bromine oxide (BrO), total column formaldehyde (HCHO), and total column sulfur dioxide (SO2) (daily products https://doi.org/10.15770/EUM_SAF_AC_0048, AC SAF, 2023a; monthly products https://doi.org/10.15770/EUM_SAF_AC_0049, AC SAF, 2023b). The GOME-2 level-3 products aim to provide easily translatable and user-friendly data sets to the scientific community for scientific progress as well as to satisfy public interest. The purpose of this paper is to present the theoretical basis as well as the verification and validation of the GOME-2 daily and monthly level-3 products. The GOME-2 level-3 products are produced using the overlapping area-weighting method. Details of the gridding algorithm are presented. The spatial resolution of the GOME-2 level-3 products is selected based on the sensitivity study. The consistency of the resulting level-3 products among three GOME-2 sensors is investigated through time series of global averages, zonal averages, and bias. The accuracy of the products is validated by comparison to ground-based observations. The verification and validation results show that the GOME-2 level-3 products are consistent with the level-2 data. Small discrepancies are found among three GOME-2 sensors, which are mainly caused by the differences in the instrument characteristic and level-2 processor. The comparison of GOME-2 level-3 products to ground-based observations in general shows very good agreement, indicating that the products are consistent and fulfil the requirements to serve the scientific community and general public.

The presence of reactive bromine in polar regions is a widespread phenomenon that plays an important role in the photochemistry of the Arctic and Antarctic lower troposphere, including the destruction of ozone, the disturbance of radical cycles, and the oxidation of gaseous elemental mercury. The chemical mechanisms leading to the heterogeneous release of gaseous bromine compounds from saline surfaces are in principle well understood. There are, however, substantial uncertainties about the contribution of different potential sources to the release of reactive bromine, such as sea ice, brine, aerosols, and the snow surface, as well as about the seasonal and diurnal variation and the vertical distribution of reactive bromine. Here we use continuous long-term measurements of the vertical distribution of bromine monoxide (BrO) and aerosols at the two Antarctic sites Neumayer (NM) and Arrival Heights (AH), covering the periods of 2003–2021 and 2012–2021, respectively, to investigate how chemical and physical parameters affect the abundance of BrO. We find the strongest correlation between BrO and aerosol extinction (R=0.56 for NM and R=0.28 for AH during spring), suggesting that the heterogeneous release of Br2 from saline airborne particles (blowing snow and aerosols) is a dominant source for reactive bromine. Positive correlations between BrO and contact time of air masses, both with sea ice and the Antarctic ice sheet, suggest that reactive bromine is not only emitted by the sea ice surface but by the snowpack on the ice shelf and in the coastal regions of Antarctica. In addition, the open ocean appears to represent a source for reactive bromine during late summer and autumn when the sea ice extent is at its minimum. A source–receptor analysis based on back trajectories and sea ice maps shows that main source regions for BrO at NM are the Weddell Sea and the Filchner–Ronne Ice Shelf, as well as coastal polynyas where sea ice is newly formed. A strong morning peak in BrO frequently occurring during summer and that is particularly strong during autumn suggests a night-time build-up of Br2 by heterogeneous reaction of ozone on the saline snowpack in the vicinity of the measurement sites. We furthermore show that BrO can be sustained for at least 3 d while travelling across the Antarctic continent in the absence of any saline surfaces that could serve as a source for reactive bromine.

Ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) is a widely used measurement technique for the remote detection of atmospheric aerosol and trace gases. The technique relies on the analysis of ultra-violet and visible radiation spectra of scattered sunlight (skylight) to obtain information on different atmospheric parameters. From an appropriate set of spectra recorded under different viewing directions (typically a group of observations at different elevation angles) the retrieval of aerosol and trace gas vertical distributions is achieved through numerical inversion methods. It is well known that the polarisation state of skylight is particularly sensitive to atmospheric aerosol content as well as aerosol properties and that polarimetric measurements could therefore provide additional information for MAX-DOAS profile retrievals; however, such measurements have not yet been used for this purpose. To address this issue, we have developed the RAPSODI (Retrieval of Atmospheric Parameters from Spectroscopic Observations using DOAS Instruments) algorithm. In contrast to existing MAX-DOAS algorithms, it can process polarimetric information, and it can retrieve simultaneously profiles of aerosols and various trace gases at multiple wavelengths in a single retrieval step; a further advantage is that it contains a Mie scattering model, allowing for the retrieval of aerosol microphysical properties. The forward-model component in RAPSODI is based on a linearised vector radiative transfer model with Jacobian facilities, and we have used this model to create a database of synthetic measurements in order to carry out sensitivity analyses aimed at assessing the potential of polarimetric MAX-DOAS observations. We find that multispectral polarimetry significantly enhances the sensitivity, particularly to aerosol-related quantities. Assuming typical viewing geometries, the degrees of freedom for signal (DOFS) increase by about 50 % and 70 % for aerosol vertical distributions and aerosol properties, respectively, and by approximately 10 % for trace gas vertical profiles. For an idealised scenario with a horizontally homogeneous atmosphere, our findings predict an improvement in the inversion results' accuracy (root-mean-square deviations to the true values) of about 60 % for aerosol vertical column densities (VCDs) as well as for aerosol surface concentrations and by 40 % for aerosol properties. For trace gas VCDs, very little improvement is apparent, although the accuracy of trace gas surface concentrations improves by about 50 %.

Here we present a new retrieval algorithm of the slant column densities (SCDs) of chlorine dioxide (OClO) by differential optical absorption spectroscopy (DOAS) from measurements performed by TROPOspheric Monitoring Instrument (TROPOMI) on board of Sentinel-5P satellite. To achieve a substantially improved accuracy, which is especially important for OClO observations, accounting for absorber and pseudo absorber structures in optical depth even of the order of 10−4 is important. Therefore, in comparison to existing retrievals, we include several additional fit parameters by accounting for spectral effects like the temperature dependency of the Ring effect and Ring absorption effects, a higher-order term for the OClO SCD dependency on wavelength and accounting for the BrO absorption. We investigate the performance of different retrieval settings by an error analysis with respect to random variations, large-scale systematic variations as a function of solar zenith angle and also more localized systematic variations by a novel application of an autocorrelation analysis. The retrieved TROPOMI OClO SCDs show a very good agreement with ground-based zenith sky measurements and are correlated well with preliminary data of the operational TROPOMI OClO retrieval algorithm currently being developed as part of ESA's Sentinel-5P+ Innovation project.

The presence of reactive bromine in polar regions is a widespread phenomenon that plays an important role in the photochemistry of the Arctic and Antarctic lower troposphere, including the destruction of ozone, the disturbance of radical cycles, and the oxidation of gaseous elemental mercury. The chemical mechanisms leading to the heterogeneous release of gaseous bromine compounds from saline surfaces are in principle well understood. There are, however, substantial uncertainties about the contribution of different potential sources to the release of reactive bromine, such as sea ice, brine, aerosols, and the snow surface, as well as about the seasonal and diurnal variation and the vertical distribution of reactive bromine. Here we use continuous long-term measurements of the vertical distribution of bromine monoxide (BrO) and aerosols at the two Antarctic sites Neumayer (NM) and Arrival Heights (AH), covering the periods of 2003–2021 and 2012–2021, respectively, to investigate how chemical and physical parameters affect the abundance of BrO. We find the strongest correlation between BrO and aerosol extinction (R=0.56 for NM and R=0.28 for AH during spring), suggesting that the heterogeneous release of Br2 from saline airborne particles (blowing snow and aerosols) is a dominant source for reactive bromine. Positive correlations between BrO and contact time of air masses, both with sea ice and the Antarctic ice sheet, suggest that reactive bromine is not only emitted by the sea ice surface but by the snowpack on the ice shelf and in the coastal regions of Antarctica. In addition, the open ocean appears to represent a source for reactive bromine during late summer and autumn when the sea ice extent is at its minimum. A source–receptor analysis based on back trajectories and sea ice maps shows that main source regions for BrO at NM are the Weddell Sea and the Filchner–Ronne Ice Shelf, as well as coastal polynyas where sea ice is newly formed. A strong morning peak in BrO frequently occurring during summer and that is particularly strong during autumn suggests a night-time build-up of Br2 by heterogeneous reaction of ozone on the saline snowpack in the vicinity of the measurement sites. We furthermore show that BrO can be sustained for at least 3 d while travelling across the Antarctic continent in the absence of any saline surfaces that could serve as a source for reactive bromine.

We introduce the new GOME-2 daily and monthly level 3 product of total column ozone (O3), total and tropospheric column nitrogen dioxide (NO2), total column water vapour, total column bromine oxide (BrO), total column formaldehyde (HCHO) and total column sulphur dioxide (SO2). The GOME-2 level 3 products are aimed to provide easily translatable and user-friendly data sets to the scientific community for scientific progress as well as satisfying public interest. The purpose of this paper is to present the theoretical basis as well as the verification and validation of the GOME-2 daily and monthly level 3 products. The GOME-2 level 3 products are produced using the overlapping area weighting method. Details of the gridding algorithm are presented. The spatial resolution of the GOME-2 level 3 products is selected based on sensitivity study. The consistency of the resulting level 3 products among three GOME-2 sensors is investigated through time series of global averages, zonal averages, and bias. The accuracy of the products is validated by comparing to ground-based observations. The verification and validation results show that the GOME-2 level 3 products are consistent with the level 2 data. Small discrepancies are found among three GOME-2 sensors, which are mainly caused by the differences in instrument characteristic and level 2 processor. The comparison of GOME-2 level 3 products to ground-based observations in general shows very good agreement, indicating the products are consistent and fulfil the requirements to serve the scientific community and general public.

Polar stratospheric clouds (PSCs) are an important component of ozone chemistry in polar regions. Studying the ozone-depleting processes requires a precise description of PSCs on a long-term basis. Although satellite observations already yield high spatial coverage, continuous ground-based measurements covering long time periods can be a valuable complement. In this study, differential optical absorption spectroscopy (DOAS) instruments are used to investigate the occurrence of PSCs based on the so-called colour index (CI), i.e. the colour of the zenith sky. Defined as the ratio between the observed intensities of scattered sunlight at two wavelengths, it provides a method to detect PSCs during twilight even in the presence of tropospheric clouds. We present data from instruments at the German research station Neumayer, Antarctica (71∘ S, 8∘ W), as well as Kiruna, Sweden (68∘ N, 20∘ E), which have been in operation for more than 20 years. For a comprehensive interpretation of the measurement data, the well-established radiative transfer model McArtim is used and radiances of scattered sunlight are simulated at several wavelengths for different solar zenith angles and various atmospheric conditions. The aim is to improve and evaluate the potential of this method. It is then used to infer the seasonal cycle and the variability of PSC occurrence throughout the time series measured in both hemispheres. A good agreement is found to satellite retrievals with deviations particularly in spring. The unexpectedly high signal observed in the DOAS data during springtime suggests the influence of volcanic aerosol. This is also indicated by enhanced aerosol extinction as seen from OMPS (Ozone Mapping and Profiler Suite) data but is not captured by other PSC climatologies. The presented approach allows the detection of PSCs for various atmospheric conditions not only for individual case studies but over entire time series, which is a decisive advance compared to previous work on the PSC detection by ground-based instruments. Apart from the interannual variability, no significant trend is detected for either measurement station. The averaged PSC relative frequency amounts to about 37 % above the Neumayer station and about 18 % above Kiruna.

Polar stratospheric clouds (PSCs) are an important component of the ozone stratospheric chemistry in polar regions. Although satellite observations nowadays provide high spatial coverage, continuous long-term spectroscopic measurements from the ground with high temporal resolution remain a valuable complement. Moreover, the presented method allows the detection of PSCs even in the presence of tropospheric clouds, while this is not possible with ground-based lidar measurements in such cases.For a comprehensive interpretation of measurement data, the well-established radiative transfer model McArtim is used and spectra of scattered sunlight at different solar zenith angles are simulated for various atmospheric conditions. Investigating the ratio between observed intensities at two wavelengths, i.e. the so-called colour index (CI), enables the detection of PSCs during twilight. Due to the wavelength variability of scattering processes, the choice of the wavelength pair is determining the effect which PSCs exhibit in the spectra. Likewise, the optical properties, altitude and extent of the PSC layer are decisive parameters that are investigated in detail with the help of 3D simulations. In these, the PSC layer is not simulated as horizontally extended, but as a confined area with different sizes.The findings are then compared to measured spectra from a MAX-DOAS (Multi AXis-Differential Optical Absorption Spectroscopy) instrument, which has been operating at the German research station Neumayer (70° S, 8° W) in Antarctica since 1999. While the simulations already provide insight into the sensitivity of ground-based spectroscopic measurements for the detection of PSCs, the comparison to measurement data confirms the good applicability of this method.

Multi-axis differential optical absorption spectroscopy (MAX-DOAS) and direct sun NO2 vertical column network data are used to investigate the accuracy of tropospheric NO2 column measurements of the GOME-2 instrument on the MetOp-A satellite platform and the OMI instrument on Aura. The study is based on 23 MAX-DOAS and 16 direct sun instruments at stations distributed worldwide. A method to quantify and correct for horizontal dilution effects in heterogeneous NO2 field conditions is proposed. After systematic application of this correction to urban sites, satellite measurements are found to present smaller biases compared to ground-based reference data in almost all cases. We investigate the seasonal dependence of the validation results as well as the impact of using different approaches to select satellite ground pixels in coincidence with ground-based data. In optimal comparison conditions (satellite pixels containing the station) the median bias between satellite tropospheric NO2 column measurements and the ensemble of MAX-DOAS and direct sun measurements is found to be significant and equal to −34 % for GOME-2A and −24 % for OMI. These biases are further reduced to −24 % and −18 % respectively, after application of the dilution correction. Comparisons with the QA4ECV satellite product for both GOME-2A and OMI are also performed, showing less scatter but also a slightly larger median tropospheric NO2 column bias with respect to the ensemble of MAX-DOAS and direct sun measurements.

In September 2016, 36 spectrometers from 24 institutes measured a number of key atmospheric pollutants for a period of 17 d during the Second Cabauw Intercomparison campaign for Nitrogen Dioxide measuring Instruments (CINDI-2) that took place at Cabauw, the Netherlands (51.97∘ N, 4.93∘ E). We report on the outcome of the formal semi-blind intercomparison exercise, which was held under the umbrella of the Network for the Detection of Atmospheric Composition Change (NDACC) and the European Space Agency (ESA). The three major goals of CINDI-2 were (1) to characterise and better understand the differences between a large number of multi-axis differential optical absorption spectroscopy (MAX-DOAS) and zenith-sky DOAS instruments and analysis methods, (2) to define a robust methodology for performance assessment of all participating instruments, and (3) to contribute to a harmonisation of the measurement settings and retrieval methods. This, in turn, creates the capability to produce consistent high-quality ground-based data sets, which are an essential requirement to generate reliable long-term measurement time series suitable for trend analysis and satellite data validation. The data products investigated during the semi-blind intercomparison are slant columns of nitrogen dioxide (NO2), the oxygen collision complex (O4) and ozone (O3) measured in the UV and visible wavelength region, formaldehyde (HCHO) in the UV spectral region, and NO2 in an additional (smaller) wavelength range in the visible region. The campaign design and implementation processes are discussed in detail including the measurement protocol, calibration procedures and slant column retrieval settings. Strong emphasis was put on the careful alignment and synchronisation of the measurement systems, resulting in a unique set of measurements made under highly comparable air mass conditions. The CINDI-2 data sets were investigated using a regression analysis of the slant columns measured by each instrument and for each of the target data products. The slope and intercept of the regression analysis respectively quantify the mean systematic bias and offset of the individual data sets against the selected reference (which is obtained from the median of either all data sets or a subset), and the rms error provides an estimate of the measurement noise or dispersion. These three criteria are examined and for each of the parameters and each of the data products, performance thresholds are set and applied to all the measurements. The approach presented here has been developed based on heritage from previous intercomparison exercises. It introduces a quantitative assessment of the consistency between all the participating instruments for the MAX-DOAS and zenith-sky DOAS techniques.

Vertical profiles of aerosols, NO2, and SO2 were retrieved from Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements at a field site in northern Alberta, Canada, during August and September 2013. The site is approximately 16 km north of two mining operations that are major sources of industrial pollution in the Athabasca oil sands region. Pollution conditions during the study ranged from atmospheric background conditions to heavily polluted with elevated plumes, according to the meteorology. This study aimed to evaluate the performance of the aerosol and trace gas retrievals through comparison with data from a suite of other instruments. Comparisons of aerosol optical depths (AODs) from MAX-DOAS aerosol retrievals, lidar vertical profiles of aerosol extinction, and the AERONET sun photometer indicate good performance by the MAX-DOAS retrievals. These comparisons and modelling of the lidar S ratio highlight the need for accurate knowledge of the temporal variation in the S ratio when comparing MAX-DOAS and lidar data. Comparisons of MAX-DOAS NO2 and SO2 retrievals to Pandora spectral sun photometer vertical column densities (VCDs) and active DOAS mixing ratios indicate good performance of the retrievals, except when vertical profiles of pollutants within the boundary layer varied rapidly, temporally, and spatially. Near-surface retrievals tended to overestimate active DOAS mixing ratios. The MAX-DOAS observed elevated pollution plumes not observed by the active DOAS, highlighting one of the instrument's main advantages. Aircraft measurements of SO2 were used to validate retrieved vertical profiles of SO2. Advantages of the MAX-DOAS instrument include increasing sensitivity towards the surface and the ability to simultaneously retrieve vertical profiles of aerosols and trace gases without requiring additional parameters, such as the S ratio. This complex dataset provided a rare opportunity to evaluate the performance of the MAX-DOAS retrievals under varying atmospheric conditions.

This paper reports on ground-based validation of the atmospheric OClO data record produced in the framework of EUMETSAT’s Satellite Application Facility on Atmospheric Chemistry Monitoring (AC SAF) using the GOME2-A and -B instruments over the 2007–2016 and 2013–2016 periods, respectively. OClO slant column densities are compared to correlative measurements collected from 9 NDACC Zenith-Scattered-Light DOAS (ZSL-DOAS) instruments distributed in both the Arctic and Antarctic. Sensitivity tests are performed on the ground-based data to estimate the impact of the different OClO DOAS analysis settings. On this basis, we infer systematic uncertainties of about 25 % between the different ground-based data analysis, reaching total uncertainties ranging from about 26 % to 33 % for the different stations. Time-series at the different sites show good agreement between satellite and ground-based data, both for the inter-annual variability and the overall OClO seasonal behavior. GOME-2A results are found to be nosier than those of GOME-2B, especially after 2011, probably due to instrumental degradation effects. Daily linear regression analysis for OClO activated periods yield correlation coefficients of 0.8 for GOME-2A and 0.87 for GOME-2B, with slopes of 0.64 and 0.72, respectively. Biases are within 8 x 1013 molec/cm2 with some differences between GOME-2A and GOME-2B, depending on the station. Overall, considering all the stations, a median bias of about −2.2 x 1013 molec/cm2 is found for both GOME-2 instruments.

Ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a widely-used measurement technique for the remote detection of atmospheric aerosol and trace gases. The technique relies on the analysis ultra-violet and visible radiation spectra of scattered sunlight (skylight) to obtain information on different atmospheric parameters. From an appropriate set of spectra recorded under different viewing directions (typically a group of observations at different elevation angles) the retrieval of aerosol and trace gas vertical distributions is achieved through numerical inversion methods. It is well known that the polarisation state of skylight is particularly sensitive to atmospheric aerosol content as well as aerosol properties, and that polarimetric measurement could therefore provide additional information for MAX-DOAS profile retrievals; however such measurement have not yet been used for this purpose. To address this issue, we have developed the RAPSODI (Retrieval of Atmospheric Parameters from Spectroscopic Observations using DOAS Instruments) algorithm. In contrast to existing MAX-DOAS algorithms, it can process polarimetric information, and it can retrieve simultaneously profiles of aerosols and various trace gases at multiple wavelengths in a single retrieval step; a further advantage is that it contains a Mie scattering model, allowing for the retrieval aerosol microphysical properties. The forward model component in RAPSODI is based on a linearized vector radiative transfer model with Jacobian facilities, and we have used this model to create a data base of synthetic measurements in order to carry out sensitivity analyses aimed at assessing the potential of polarimetric MAX-DOAS observations. We find that multispectral polarimetry significantly enhances the sensitivity, particularly to aerosol related quantities. Assuming typical viewing geometries, the degree of freedom for signal (DFS) increases by about 50 % and 70 % for aerosol vertical distributions and aerosol properties, respectively, and by approximately 10 % for trace gas vertical profiles. For an idealised scenario with a horizontally homogeneous atmosphere, our findings predict an improvement in the inversions results' accuracy (root-mean-square deviations to the true values) of about 60 % for aerosol VCDs as well as for aerosol surface concentrations, and by 40 % for aerosol properties. For trace gas VCDs, very little improvement is apparent, although the accuracy of trace gas surface concentrations improves by about 50 %.

The TROPOspheric Monitoring Instrument (TROPOMI) is a UV-VIS-NIR-SWIR instrument on board of Sentinel-5P satellite developed for monitoring the Earth’s atmosphere. It was launched on 13 October 2017 in a near polar orbit. It measures spectrally resolved earthshine radiances at an unprecedented spatial resolution of around 3.5 x 7.2 km² (3.5 x 5.6 km² starting from 6 Aug 2019) (near nadir) with a total swath width of ~ 2600 km on the Earth's surface providing daily global coverage. From the measured spectra high resolved trace gas distributions can be retrieved by means of differential optical absorption spectroscopy (DOAS). Chlorine dioxide (OClO) is a by-product of the ozone depleting halogen chemistry in the stratosphere. Although being rapidly photolysed at low solar zenith angles (SZAs) it plays an important role as an indicator of the chlorine activation in polar regions during polar winter and spring at twilight conditions because of the nearly linear dependence of its formation to chlorine oxide (ClO). Here we present a new retrieval algorithm of the slant column densities (SCDs) of chlorine dioxide (OClO) by DOAS. To achieve a substantially improved accuracy, which is especially important for OClO observations, accounting for absorber and pseudo absorber structures in optical depth even of the order of 10−4 is important. Therefore in comparison to existing retrievals, we include several additional fit parameters accounting for spectral effects like the temperature dependency of the Ring effect and Ring absorption effects, higher order term for the OClO SCD dependency on wavelength and account for the BrO absorption. We investigate the performance of different retrieval settings by an error analysis with respect to random variations, large scale systematic variations as function of solar zenith angle and also more localised systematic variations by a novel application of an autocorrelation analysis. The retrieved TROPOMI OClO SCDs show a very good agreement with ground based zenith sky measurements and are correlated well with preliminary data of the opeartional TROPOMI OClO retrieval algorithm currently being developed as part of ESA's S5p+I project.

In this study the consistency between MAX-DOAS measurements and radiative transfer simulations of the atmospheric O4 absorption is investigated on 2 mainly cloud-free days during the MAD-CAT campaign in Mainz, Germany, in summer 2013. In recent years several studies indicated that measurements and radiative transfer simulations of the atmospheric O4 absorption can only be brought into agreement if a so-called scaling factor (

Polar Stratospheric Clouds (PSCs) favour heterogeneous reactions and thus are an important component of ozone depletion processes in polar regions. Although satellite observations already yield high spatial coverage, the sampling frequency of a specific air volume depends on the measurement method. Here, continuous ground-based measurements with high temporal resolution can be a valuable complement.Since 1999, a MAX-DOAS (Multi AXis-Differential Optical Absorption Spectroscopy) instrument has been operating at the German research station Neumayer (70° S, 8° W), Antarctica. Primarily, slant column densities of trace gases such as NO2, BrO and OClO are retrieved. However, in this study the so-called colour index (CI), i.e. the colour of the zenith sky, is investigated. Defined as the ratio between the observed intensities of scattered sun light at two wavelengths, it enables to monitor the occurrence of polar stratospheric clouds during twilight even in the presence of tropospheric clouds.Using the radiative transfer model McArtim, the CI changes in the presence of polar stratospheric clouds can be analysed. Especially the height of the PSC layer affects the retrieved signal, but also the choice of the wavelengths has a strong impact. Here, it is advantageous that measurements are available in the UV and visible spectral range which allows a more extensive comparison of different CI choices. In order to assess the application of the colour index method, meteorological data are used to identify PSC cases in the data set.The aim is to improve and evaluate the potential of this method. It is then used to infer the occurrence of PSCs throughout the measurement time series of more than 20 years.

Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a well-established measurement technique for the detection of atmospheric aerosol and trace gases: ultra-violet and visible radiation spectra of skylight are analyzed to obtain information on different atmospheric parameters. An appropriate set of spectra recorded under different viewing geometries ("Multi-Axis") allows retrieval of aerosol and trace gas vertical distributions as well as aerosol properties by applying numerical inversion methods. Currently one of the method’s major limitations in ground-based applications is the limited information contained in the measurements that reduces the sensitivity, particularly at higher altitudes.It is well known but not yet used in MAX-DOAS profile retrievals that measuring skylight of different polarisation directions provides additional information: The degree of polarisation for instance strongly depends on the atmospheric aerosol content and the aerosol properties and – since the light path differs for the light of different polarisation - the set of geometries available for the inversion is extended. We present a novel polarization-sensitive MAX-DOAS instrument (PMAX-DOAS) and a corresponding inversion algorithm, capable of using polarimetric information to significantly extend the information content of the measurements. The improvement over conventional “unpolarised” MAX-DOAS approaches will be discussed, based on both, synthetic data and real measurements.

Compared to ground-based or satellite measurements, atmospheric observations based on aircraft missions have many advantages, such as the potential to observe a large atmospheric volume using remote sensing measurements, among which Differential Optical Absorption Spectroscopy (DOAS) is a well established method for the observation of integrated trace gas concentrations along the light path. However, the interpretation of remote spectroscopic measurements using scattered sunlight is complicated due to the lack of prior knowledge on the light paths between sun and detector, and thus on the observed air volume. Using radiative transfer calculations, quantities commonly derived from DOAS measurements are integrated vertical columns of various trace gases, providing no information about their vertical distribution.On the ground, tomographic approaches have been used to reconstruct the spatial distribution of trace gases by using multiple viewing directions and detectors. HAIDI, the Heidelberg Airborne Imaging DOAS Instrument, was designed to transfer this concept to the air. In addition to its excellent temporal and spatial resolution (40 m x 40 m at 1.5 km flight altitude, 266 m x 266 m at 10 km flight altitude, at 10 ms temporal resolution), HAIDI uses three separate scanning telescopes aimed at +/-45° forward- and backward looking angles and the nadir direction. In combination with a 3D radiative transfer model, this allows a reconstruction of the 3D distribution of the detected trace gases in the vicinity of the flight track.HAIDI joined the EMerGe (Effect of Megacities on the Transport and transformation of Pollutants on the Regional to Global Scales) missions on HALO, the High Altitude and LOng range research aircraft based at DLR (German Aerospace Center) in Oberpfaffenhofen, Germany. The EMerGe missions targeted the emission outflows of megacities to investigate their compositions and the atmospheric impact of urban pollution in Europe (July 2017) and Asia (March 2018). HAIDI observed a number of trace gases such as NO2, SO2 and HCHO. For NO2 and SO2 in particular, strong plumes originating from power plants and ships were found, which were then used for inversion of the 3D distribution of the plume and emission estimation. Here we present the method and results of the HAIDI measurements during the EMeRGe missions.

MAX-DOAS and direct sun NO2 vertical column network data are used to investigate the accuracy of tropospheric NO2 column measurements of the GOME-2 instrument on the MetOP-A satellite platform and the OMI instrument on Aura. The study is based on 23 MAX-DOAS and 16 direct sun instruments at stations distributed worldwide. A method to quantify and correct for horizontal dilution effects in heterogeneous NO2 field conditions is proposed. After systematic application of this correction to urban sites, satellite measurements are found to present smaller biases compared to ground-based reference data in almost all cases. We investigate the seasonal dependence of the validation results, as well as the impact of using different approaches to select satellite ground pixels in coincidence with ground-based data. In optimal comparison conditions (satellite pixels containing the station) the median bias between satellite tropospheric NO2 column measurements and the ensemble of MAX-DOAS and direct sun measurements is found to be significant and equal to −36 % for GOME-2A and −20 % for OMI. These biases are further reduced to −24 % and −8 % respectively, after application of the dilution correction. Comparisons with the QA4ECV satellite product for both GOME-2A and OMI is also performed, showing less scatter but also a slightly larger median tropospheric NO2 column bias with respect to the ensemble of MAX-DOAS and direct sun measurements.

In many spectroscopic applications, optical fibres are an essential part of the instrumental setup. One of such applications is Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS), which is a remote sensing technique to detect atmospheric aerosol and trace gases by analysing spectra of scattered skylight. Typically, multi-mode quartz fibres (MMQF) of > 100 µm in diameter are used in MAX-DOAS instruments to interconnect the telescope unit with a grating spectrometer. Besides acting as a light guide giving more freedom in the spatial arrangement of telescope and spectrometer and homogenizing the illumination of the spectrometer entrance slit, such fibres have depolarising properties originating predominantly from manufacturing induced birefringent effects. This property is particularly desirable in MAX-DOAS applications, since the incoming partially polarized skylight should ideally be depolarized before entering the polarisation sensitive spectrometer. The behaviour of polarised light in mono-mode fibres is well investigated and even utilized in telecommunications, whereas equivalent literature on multi-mode fibres is scarce.We measured the depolarisation capabilities of a set of 20 MMQF of different age, length and diameter as typically used in MAX-DOAS applications. Independent of the fibre diameter, we found that in some recently manufactured (in 2018) fibres polarisation is well preserved with a decrease in total degree of polarisation (DOLP) to 1/e of its initial value at a fibre length Le > 10 m at 450 nm, while in older fibres (> 10 years in age) Le ≈ 1 m was found. This is probably due to improvements in the manufacturing process in the recent years. We further investigated the dependence of Le on wavelength, on the polarisation orientation of the ingoing light and on additional birefringence induced by applying external strain (bending) to the fibres. The results are presented and discussed with regard to their implications for MAX-DOAS observations.

Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a well-established ground-based measurement technique for the detection of atmospheric aerosol and trace gases: ultra-violet and visible radiation spectra of skylight are analyzed to obtain information on different atmospheric parameters. An appropriate set of spectra recorded under different viewing geometries ("Multi-Axis") allows retrieval of aerosol and trace gas vertical distributions by applying numerical inversion methods. Currently one of the method’s major limitations is the limited information content in the measurements that reduces the sensitivity particularly at higher altitudes.It is well known but not yet used in MAX-DOAS profile retrievals that measuring skylight of different polarisation directions provides additional information: the degree of polarisation for instance strongly depends on the atmospheric aerosol content and the aerosol properties and – since the light path (?) differs for light of different polarisation - the set of geometries available for the inversion is extended. We present a novel polarization-sensitive MAX-DOAS instrument and a corresponding inversion algorithm, capable of using polarization information. Further, in contrast to existing MAX-DOAS algorithms consisting of separate aerosol and trace gas retrieval modules, our novel inversion scheme simultaneously retrieves aerosol and trace gas profiles of several species in a single step. The improvement over “unpolarised” MAX-DOAS approaches will be discussed, based on both, synthetic data and real measurements.

Bromine explosions and corresponding ozone depletion events (ODEs) are common in the Arctic spring. The snowpack on sea ice and sea salt aerosols (SSA) are both thought to release bromine, but the relative contribution of each source is not yet known. Furthermore, the role of atmospheric conditions is not fully understood. Long-term measurements of bromine monoxide (BrO) provide useful insight into the underlying processes of bromine activation. Here we present a four-year dataset (2016-2019) of springtime BrO partial columns retrieved from Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements in Eureka, Canada (80.1° N, 86.4° W). Due to the altitude of the measurement site (610 m), the measurements often represent BrO above the shallow boundary layer, and the strength of the temperature inversion has limited impact on the BrO partial columns. When the boundary layer is deep, however, the effects of the enhanced vertical mixing manifest as an increase in the minimum BrO values (and reduced ODE frequency) for wind speeds of ~8 m/s or greater. We find that BrO events show two modes differentiated by local wind direction and air mass history. Longer time spent in first-year sea ice areas corresponds to increased BrO for one of these modes only. We argue that snow on multi-year ice (salted and acidified by Arctic haze) might also contribute to bromine release. The MAX-DOAS measurements show that high aerosol optical depth is required to maintain lofted BrO. In situ measurements indicate that accumulation mode aerosols (mostly Arctic haze) have no direct correlation with BrO. The presence of coarse mode aerosols, however, is a necessary and sufficient condition for observing enhanced BrO at Eureka. The measurements of coarse mode aerosols are consistent with SSA generated from blowing snow. The good correlation between BrO and coarse mode aerosols (R2 up to 0.57) supports the view that SSA is a direct source of bromine to the polar troposphere.

Today, the majority of humanity lives in urban areas. Accordingly, major population centers are a significant source of a multitude of atmospheric emissions from human activity such as traffic, heating, industry or power generation. Pollutants directly impact the local inhabitant's health, but are also transported to neighbouring areas, undergo chemical evolution and can have an impact on climate. To understand and assess effective measures for reducing the effects, it is important to determine the source locations and strengths as well as relevant chemical processes.Aircraft-based measurements can cover the gap between long-term ground-based and globe-covering satellite instruments with its high temporal and spatial coverage during flight time. Remote sensing methods in particular allow a fast and wide-spread probing of atmospheric trace gas distributions. Within this context, the HAIDI (Heidelberg Airborne Imaging DOAS Instrument) instrument was designed to provide data of extremely high temporal and spatial resolution (40m x 40m at 1.5 km flight altitude, 266m x 266m at 10 km flight altitude, at 10 ms temporal resolution) in 2D and 3D during overflight.During the EMeRGe (Effect of Megacities on the Transport and Transformation of Pollutants on the Regional to Global Scales) missions HAIDI was part of the comprehensive set of measurement equipment installed on the research airplane HALO (High Altitude and LOng range re-search aircraft) of the DLR (German Aerospace Center). The EMeRGe missions targeted the emission outflows of megacities to investigate their compositions and the atmospheric impact of urban pollution. One mission part was conducted in Europe (July 2017) and aimed at areas around Paris, London, the Po area, Madrid and the Benelux area. For contrast, the second mission part was based in Taiwan (March 2018) and investigated Taiwan cities, Bangkok, Manila, Japanese cities and China outflow.HAIDI derived a number of trace gases such as NO2, SO2, BrO and HCHO. Due to large concentrations present, for NO2 and SO2 in particular it was possible to obtain high-resolution 2D data of megacity areas and plumes of megacities, powerplants and ships and estimate their emissions. We will present results of the HAIDI measurements during the EMeRGe mission.

Retrieving the column of an absorbing trace gas from spectral data requires that all absorbers in the corresponding wavelength range are sufficiently well known. This is especially important for the retrieval of weak absorbers, whose absorptions are often in the 10−4 range. Previous publications on the absorptions of the oxygen dimer O2–O2 (or short: O4) list absorption peaks at 328 and 419 nm, for which no spectrally resolved literature cross sections are available. As these absorptions potentially influence the spectral retrieval of various trace gases, such as HCHO, BrO, OClO and IO, their shape and magnitude need to be quantified. We assume that the shape of the absorption peaks at 328 and 419 nm can be approximated by their respective neighbouring absorption peaks. Using this approach we obtain estimates for the wavelength of the absorption and its magnitude. Using long-path differential optical absorption spectroscopy (LP-DOAS) observations and multi-axis DOAS (MAX-DOAS) observations, we estimate the peak absorption cross sections of O4 to be (1.96 ± 0.20) × 10−47 cm5 molec−2 and determine the wavelength of its maximum at 328.59 ± 0.15 nm. For the absorption at 419.13 ± 0.42 nm a peak O4 cross-section value is determined to be (5.0 ± 3.5) × 10−48 cm5 molec−2.

Heterogeneous photochemistry converts bromide (Br−) to reactive bromine species (Br atoms and bromine monoxide, BrO) that dominate Arctic springtime chemistry. This phenomenon has many impacts such as boundary-layer ozone depletion, mercury oxidation and deposition, and modification of the fate of hydrocarbon species. To study environmental controls on reactive bromine events, the BRomine, Ozone, and Mercury EXperiment (BROMEX) was carried out from early March to mid-April 2012 near Barrow (Utqiaġvik), Alaska. We measured horizontal and vertical gradients in BrO with multiple-axis differential optical absorption spectroscopy (MAX-DOAS) instrumentation at three sites, two mobile and one fixed. During the campaign, a large crack in the sea ice (an open lead) formed pushing one instrument package ∼ 250 km downwind from Barrow (Utqiaġvik). Convection associated with the open lead converted the BrO vertical structure from a surface-based event to a lofted event downwind of the lead influence. The column abundance of BrO downwind of the re-freezing lead was comparable to upwind amounts, indicating direct reactions on frost flowers or open seawater was not a major reactive bromine source. When these three sites were separated by ∼ 30 km length scales of unbroken sea ice, the BrO amount and vertical distributions were highly correlated for most of the time, indicating the horizontal length scales of BrO events were typically larger than ∼ 30 km in the absence of sea ice features. Although BrO amount and vertical distribution were similar between sites most of the time, rapid changes in BrO with edges significantly smaller than this ∼ 30 km length scale episodically transported between the sites, indicating BrO events were large but with sharp edge contrasts. BrO was often found in shallow layers that recycled reactive bromine via heterogeneous reactions on snowpack. Episodically, these surface-based events propagated aloft when aerosol extinction was higher (> 0.1 km−1); however, the presence of aerosol particles aloft was not sufficient to produce BrO aloft. Highly depleted ozone (

In September 2016, 36 spectrometers from 24 institutes measured a number of key atmospheric pollutants for a period of 17 days during the Second Cabauw Intercomparison campaign for Nitrogen Dioxide measuring Instruments (CINDI-2) that took place at Cabauw, The Netherlands (51.97° N, 4.93° E). We report on the outcome of the formal semi-blind intercomparison exercise, which was held under the umbrella of the Network for the Detection of Atmospheric Composition Change (NDACC) and the European Space Agency (ESA). The three major goals of CINDI-2 were to characterise and better understand the differences between a large number of Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) and zenith-sky DOAS instruments and analysis methods, to discuss the performance of the various types of instruments and to contribute to a harmonisation of the measurement settings and retrieval methods. This, in turn, creates the capability to produce consistent high-quality ground-based data sets, which are an essential requirement to generate reliable long-term measurement time series suitable for trend analysis and satellite data validation. The data products investigated during the semi-blind intercomparison are slant columns of nitrogen dioxide (NO2), the oxygen dimer (O4) and ozone (O3) measured in the UV and visible wavelength region, formaldehyde (HCHO) in the UV spectral region and NO2 in an additional (smaller) wavelength range in the visible. The campaign design and implementation processes are discussed in detail including the measurement protocol, calibration procedures and slant column retrieval settings. Strong emphasis was put on the careful alignment and synchronisation of the measurement systems, resulting in an unprecedented set of measurements made under highly comparable air mass conditions. The CINDI-2 data sets were investigated using a regression analysis of the slant columns measured by each instrument and for each of the target data products. The slope and intercept of the regression analysis respectively quantify the mean systematic bias and offset of the individual data sets against the reference, and the RMS error provides an estimate of the measurement noise or dispersion. These three criteria are examined and for each of the parameters and each of the data products, performance thresholds are set and applied to all the measurements. The approach presented here has been developed based on heritage from previous intercomparison exercises. It introduces a quantitative assessment of the measurement performance of all the participating instruments for the MAX-DOAS and zenith-sky DOAS techniques.

Over the last few decades, differential optical absorption spectroscopy (DOAS) has been used as a common technique to simultaneously measure abundances of a variety of atmospheric trace gases. Exploiting the unique differential absorption cross section of trace-gas molecules, mixing ratios can be derived by measuring the optical density along a defined light path and by applying the Beer–Lambert law. Active long-path (LP-DOAS) instruments can detect trace gases along a light path of a few hundred metres up to 20 km, with sensitivities for mixing ratios down to ppbv and pptv levels, depending on the trace-gas species. To achieve high measurement accuracy and low detection limits, it is crucial to reduce instrumental artefacts that lead to systematic structures in the residual spectra of the analysis. Spectral residual structures can be introduced by most components of a LP-DOAS measurement system, namely by the light source, in the transmission of the measurement signal between the system components or at the level of spectrometer and detector. This article focuses on recent improvements by the first application of a new type of light source and consequent changes to the optical setup to improve measurement accuracy. Most state-of-the-art LP-DOAS instruments are based on fibre optics and use xenon arc lamps or light-emitting diodes (LEDs) as light sources. Here we present the application of a laser-driven light source (LDLS), which significantly improves the measurement quality compared to conventional light sources. In addition, the lifetime of LDLS is about an order of magnitude higher than of typical Xe arc lamps. The small and very stable plasma discharge spot of the LDLS allows the application of a modified fibre configuration. This enables a better light coupling with higher light throughput, higher transmission homogeneity, and a better suppression of light from disturbing wavelength regions. Furthermore, the mode-mixing properties of the optical fibre are enhanced by an improved mechanical treatment. The combined effects lead to spectral residual structures in the range of 5-10×10-5 root mean square (rms; in units of optical density). This represents a reduction of detection limits of typical trace-gas species by a factor of 3–4 compared to previous setups. High temporal stability and reduced operational complexity of this new setup allow the operation of low-maintenance, automated LP-DOAS systems, as demonstrated here by more than 2 years of continuous observations in Antarctica.

Multi-axis differential optical absorption spectroscopy (MAX-DOAS) is a widely used measurement technique for the detection of a variety of atmospheric trace gases. Using inverse modelling, the observation of trace gas column densities along different lines of sight enables the retrieval of aerosol and trace gas vertical profiles in the atmospheric boundary layer using appropriate retrieval algorithms. In this study, the ability of eight profile retrieval algorithms to reconstruct vertical profiles is assessed on the basis of synthetic measurements. Five of the algorithms are based on the optimal estimation method, two on parametrised approaches, and one using an analytical approach without involving any radiative transfer modelling. The synthetic measurements consist of the median of simulated slant column densities of O4 at 360 and 477 nm, as well as of HCHO at 343 nm and NO2 at 477 nm, from seven datasets simulated by five different radiative transfer models. Simulations are performed for a combination of 10 trace gas and 11 aerosol profiles, as well as 11 elevation angles, three solar zenith, and three relative azimuth angles. Overall, the results from the different algorithms show moderate to good performance for the retrieval of vertical profiles, surface concentrations, and total columns. Except for some outliers, the root-mean-square difference between the true and retrieved state ranges between (0.05–0.1) km−1 for aerosol extinction and (2.5–5.0) ×1010 molec cm−3 for HCHO and NO2 concentrations.

We use ground-based spectroscopic remote sensing measurements of the stratospheric trace gases O3, HCl, ClO, BrO, HNO3, NO2, OClO, ClONO2, N2O and HF, along with radiosonde profiles of temperature ...

In this study the consistency between MAX-DOAS measurements and radiative transfer simulations of the atmospheric O 4 absorption is investigated on two mainly clear days during the MAD-CAT campaign in Mainz, Germany, in Summer 2013. In recent years several studies indicated that measurements and radiative transfer simulations of the atmospheric O 4 absorption can only be brought into agreement if a so-called scaling factor ( 4 absorption. However, many studies, in particular based on direct sun light measurements, came to the opposite conclusion, that there is no need for a scaling factor. Up to now, there is no explanation for the observed discrepancies between measurements and simulations. Previous studies infered the need for a scaling factor from the comparison of the aerosol optical depth derived from MAX-DOAS O 4 measurements with that derived from coincident sun photometer measurements. In this study a different approach is chosen: the measured O 4 absorption at 360 nm is directly compared to the O 4 absorption obtained from radiative transfer simulations. The atmospheric conditions used as input for the radiative transfer simulations were taken from independent data sets, in particular from sun photometer and ceilometer measurements at the measurement site. The comparisons are performed for two selected clear days with similar aerosol optical depth but very different aerosol properties. For both days not only the O 4 absorptions are compared, but also all relevant error sources of the spectral analysis, the radiative transfer simulations as well as the extraction of the input parameters used for the radiative transfer simulations are quantified. One important result obtained from the analysis of synthetic spectra is that the O 4 absorptions derived from the spectral analysis agree within 1 % with the corresponding radiative transfer simulations. The performed tests and sensitivity studies might be useful for the analysis and interpretation of O 4 MAX-DOAS measurements in future studies. Different comparison results are found for both days: On 18 June, measurements and simulations agree within their (rather large) errors (the ratio of simulated and measured O 4 absorptions is found to be 1.01 ± 0.16). In contrast, on 8 July measurements and simulations significantly disagree: For the middle period of that day the ratio of simulated and measured O 4 absorptions is found to be 0.71 ± 0.12, which differs significantly from unity. Thus for that day a scaling factor is needed to bring measurements and simulations into agreement. One possible reason for the comparison results on 18 June is the rather large aerosol extinction (and its large uncertainty) close to the surface, which has a large effect on the radiative transfer simulations. Besides the inconsistent comparison results for both days, also no explanation for a O 4 scaling factor could be derived in this study. Thus similar, but more extended future studies should be performed, which preferably include more measurement days, more instruments and should be supported by more detailed independent aerosol measurements. Also additional wavelengths should be included. The MAX-DOAS measurements collected during the recent CINDI-2 campaign are probably well suited for that purpose.

Polar stratospheric clouds (PSCs) are often observed in the Kiruna region in northern Sweden, east of the Scandinavian mountain range, during wintertime. PSC occurrence can be detected by ground-based optical instruments. Most of these require clear tropospheric weather. By applying the zenith-sky colour index technique, which works under most weather conditions, the data availability can be extended. The observations suggest that PSC events, especially of type II (water PSCs) may indeed more common than predicted by synoptic models, which is expected because of the frequent presence of mountain-induced leewaves. However, it will be of importance to increase the density of independent observations.Key words. Atmospheric composition and structure (aerosols and particles · cloud physics and chemistry) · Meteorology and atmospheric dynamics (mesoscale meteorology)

Toxic nitrogen oxides produced by high temperature combustion are prevalent in urban environments, contributing to a significant health burden. Nitrogen oxides such as NO2 and HONO in pollution are important for hydroxyl radical (OH) production and overall oxidative capacity in urban environments; however, current mechanisms cannot explain high daytime levels of HONO observed in many urban and rural locations around the world. Here we present HONO, NO2 and aerosol extinction vertical distributions retrieved from multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements in suburban Melbourne, which are the first MAX-DOAS results from the Australian continent. Using the optimal estimation algorithm HEIPRO we show that vertical profiles for NO2 and HONO can be calculated with a low dependence on the retrieval forward model and a priori parameters, despite a lack of independent co-located aerosol or trace gas measurements. Between December 2016 and April 2017 average peak NO2 values of 8±2 ppb indicated moderate traffic pollution levels, and high daytime peak values of HONO were frequently detected, averaging 220±30 ppt in the middle of the day. HONO levels measured in Melbourne were typically lower than those recorded in the morning in other places around the world, indicating minimal overnight accumulation, but peaked in the middle of the day to be commensurate with midday concentrations in locations with much higher NO2 pollution. Regular midday peaks in the diurnal cycle of HONO surface concentrations have only previously been reported in rural locations. The HONO measured implies a daytime source term 1 ppb h−1 above the predicted photostationary state (PSS) concentration and represents an OH radical source up to 4 times stronger than from ozone photolysis alone in the lowest 500 m of the troposphere. The dependence of the high midday HONO levels on soil moisture, combined with the observed diurnal and vertical profiles, provides evidence for a strong photoactivated and ground-based daytime HONO source.