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We present the inter-comparison of delta slant column densities (SCDs) and vertical profiles of nitrous acid (HONO) derived from measurements of different multiaxis differential optical absorption spectroscopy (MAXDOAS) instruments and using different inversion algorithms during the Second Cabauw Inter-comparison campaign for Nitrogen Dioxide measuring Instruments (CINDI- 2) in September 2016 at Cabauw, the Netherlands (51.97° N, 4.93° E). The HONO vertical profiles, vertical column densities (VCDs), and near-surface volume mixing ratios are compared between different MAX-DOAS instruments and profile inversion algorithms for the first time. Systematic and random discrepancies of the HONO results are derived from the comparisons of all data sets against their median values. Systematic discrepancies of HONO delta SCDs are observed in the range of ±0:3×1015 molec. cm2, which is half of the typical random discrepancy of 0:6× 1015 molec. cm2. For a typical high HONO delta SCD of 2×1015 molec. cm2, the relative systematic and random discrepancies are about 15% and 30 %, respectively. The inter-comparison of HONO profiles shows that both systematic and random discrepancies of HONO VCDs and nearsurface volume mixing ratios (VMRs) are mostly in the range of ∼ ±0:5×1014 molec. cm2 and ∼ ±0:1 ppb (typically ∼ 20 %). Further we find that the discrepancies of the retrieved HONO profiles are dominated by discrepancies of the HONO delta SCDs. The profile retrievals only contribute to the discrepancies of the HONO profiles by ∼ 5 %. However, some data sets with substantially larger discrepancies than the typical values indicate that inappropriate implementations of profile inversion algorithms and configurations of radiative transfer models in the profile retrievals can also be an important uncertainty source. In addition, estimations of measurement uncertainties of HONO dSCDs, which can significantly impact profile retrievals using the optimal estimation method, need to consider n

Aerosol observations above the Southern Ocean and Antarctic sea ice are scarce. Measurements of aerosols and atmospheric composition were made in East Antarctic pack ice on-board the Australian icebreaker Aurora Australis during the spring of 2012. One particle formation event was observed during the 32 days of observations. This event occurred on the only day to exhibit extended periods of global irradiance in excess of 600 W m−2. Within the single air-mass influencing the measurements, number concentrations of particles larger than 3 nm (CN3) reached almost 7700 cm−3 within a few hours of clouds clearing, and grew at rates of 5.6 nm h−1. Formation rates of 3 nm particles were in the range of those measured at other Antarctic locations at 0.2–1.1 ± 0.1 cm−3 s−1. Our investigations into the nucleation chemistry found that there were insufficient precursor concentrations for known halogen or organic chemistry to explain the nucleation event. Modelling studies utilising known sulfuric acid nucleation schemes could not simultaneously reproduce both particle formation or growth rates. Surprising correlations with Total Gaseous Mercury (TGM) were found that, together with other data, suggest a mercury driven photochemical nucleation mechanism may be responsible for aerosol nucleation. Given the very low vapour pressures of the mercury species involved, this nucleation chemistry is likely only possible where pre-existing aerosol concentrations are low and both TGM concentrations and solar radiation levels are relatively high (~ 1.5 ng m−3 and ≥ 600 W m−2, respectively), such as those observed in the Antarctic sea ice boundary layer in this study or in the global free-troposphere, particularly in the Northern Hemisphere.

Aerosol observations above the Southern Ocean and Antarctic sea ice are scarce. Measurements of aerosols and atmospheric composition were made in East Antarctic pack ice on board the Australian icebreaker Aurora Australis during the spring of 2012. One particle formation event was observed during the 32 days of observations. This event occurred on the only day to exhibit extended periods of global irradiance in excess of 600 W m−2. Within the single air mass influencing the measurements, number concentrations of particles larger than 3 nm (CN3) reached almost 7700 cm−3 within a few hours of clouds clearing, and grew at rates of 5.6 nm h−1. Formation rates of 3 nm particles were in the range of those measured at other Antarctic locations at 0.2-1.1 ± 0.1 cm−3 s−1. Our investigations into the nucleation chemistry found that there were insufficient precursor concentrations for known halogen or organic chemistry to explain the nucleation event. Modelling studies utilising known sulfuric acid nucleation schemes could not simultaneously reproduce both particle formation or growth rates. Surprising correlations with total gaseous mercury (TGM) were found that, together with other data, suggest a mercury-driven photochemical nucleation mechanism may be responsible for aerosol nucleation. Given the very low vapour pressures of the mercury species involved, this nucleation chemistry is likely only possible where pre-existing aerosol concentrations are low and both TGM concentrations and solar radiation levels are relatively high (∼ 1.5 ng m−3 and ≥ 600 W m−2, respectively), such as those observed in the Antarctic sea ice boundary layer in this study or in the global free troposphere, particularly in the Northern Hemisphere.

Glyoxal is an important intermediate species formed by the oxidation of common biogenic and anthropogenic volatile organic compounds such as isoprene, toluene, and acetylene. Although glyoxal has been shown to play an important role in urban and forested environments, its role in the open ocean environment is still not well understood, with only a few observations showing evidence for its presence in the open ocean marine boundary layer (MBL). In this study, we report observations of glyoxal from 10 field campaigns in different parts of the world's oceans. These observations together represent the largest database of glyoxal in the MBL. The measurements are made with similar instruments that have been used in the past, although the open ocean values reported here, average of about 25 parts per trillion by volume (pptv) with an upper limit of 40 pptv, are much lower than previously reported observations that were consistently higher than 40 pptv and had an upper limit of 140 pptv, highlighting the uncertainties in the differential optical absorption spectroscopy method for the retrieval of glyoxal. Despite retrieval uncertainties, the results reported in this work support previous suggestions that the currently known sources of glyoxal are insufficient to explain the average MBL concentrations. This suggests that there is an additional missing source, more than a magnitude larger than currently known sources, which is necessary to account for the observed atmospheric levels of glyoxal. Therefore, it could play a more important role in the MBL than previously considered. Key Points Largest database of glyoxal in the MBL Levels of glyoxal in the MBL are lower than previously reported Additional source of glyoxal needed to explain observations ©2014. American Geophysical Union. All Rights Reserved.

The trend in stratospheric NO2 column at the NDACC (Network for the Detection of Atmospheric Composition Change) station of Jungfraujoch (46.5° N, 8.0° E) is assessed using ground-based FTIR and zenith-scattered visible sunlight SAOZ measurements over the period 1990 to 2009 as well as a composite satellite nadir data set constructed from ERS-2/GOME, ENVISAT/SCIAMACHY, and METOP-A/GOME-2 observations over the 1996–2009 period. To calculate the trends, a linear least squares regression model including explanatory variables for a linear trend, the mean annual cycle, the quasi-biennial oscillation (QBO), solar activity, and stratospheric aerosol loading is used. For the 1990–2009 period, statistically indistinguishable trends of -3.7 ± 1.1% decade-1 and -3.6 ± 0.9% decade-1 are derived for the SAOZ and FTIR NO2 column time series, respectively. SAOZ, FTIR, and satellite nadir data sets show a similar decrease over the 1996–2009 period, with trends of -2.4 ± 1.1% decade-1, -4.3 ± 1.4% decade-1, and -3.6 ± 2.2% decade-1, respectively. The fact that these declines are opposite in sign to the globally observed +2.5% decade-1 trend in N2O, suggests that factors other than N2O are driving the evolution of stratospheric NO2 at northern mid-latitudes. Possible causes of the decrease in stratospheric NO2 columns have been investigated. The most likely cause is a change in the NO2/NO partitioning in favor of NO, due to a possible stratospheric cooling and a decrease in stratospheric chlorine content, the latter being further confirmed by the negative trend in the ClONO2 column derived from FTIR observations at Jungfraujoch. Decreasing ClO concentrations slows the NO + ClO ¿ NO2 + Cl reaction and a stratospheric cooling slows the NO + O3 ¿ NO2 + O2 reaction, leaving more NOx in the form of NO. The slightly positive trends in ozone estimated from ground- and satellite-based data sets are also consistent with the decrease of NO2 through the NO2 + O3 ¿ NO3 + O2 reaction. Finally, we

From June to July 2009 more than thirty different in-situ and remote sensing instruments from all over the world participated in the Cabauw Intercomparison campaign for Nitrogen Dioxide measuring Instruments (CINDI). The campaign took place at KNMI's Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands. Its main objectives were to determine the accuracy of state-of-the-art ground-based measurement techniques for the detection of atmospheric nitrogen dioxide (both in-situ and remote sensing), and to investigate their usability in satellite data validation. The expected outcomes are recommendations regarding the operation and calibration of such instruments, retrieval settings, and observation strategies for the use in ground-based networks for air quality monitoring and satellite data validation. Twenty-four optical spectrometers participated in the campaign, of which twenty-one had the capability to scan different elevation angles consecutively, the so-called Multi-axis DOAS systems, thereby collecting vertical profile information, in particular for nitrogen dioxide and aerosol. Various in-situ samplers and lidar instruments simultaneously characterized the variability of atmospheric trace gases and the physical properties of aerosol particles. A large data set of continuous measurements of these atmospheric constituents has been collected under various meteorological conditions and air pollution levels. Together with the permanent measurement capability at the CESAR site characterizing the meteorological state of the atmosphere, the CINDI campaign provided a comprehensive observational data set of atmospheric constituents in a highly polluted region of the world during summertime. First detailed comparisons performed with the CINDI data show that slant column measurements of NO2, O4 and HCHO with MAX-DOAS agree within 5 to 15%, vertical profiles of NO2 derived from several independent instruments agree wit

In June 2009, 22 spectrometers from 14 institutes measured tropospheric and stratospheric NO2 from the ground for more than 11 days during the Cabauw Intercomparison campaign of Nitrogen Dioxide measuring Instruments (CINDI), at Cabauw, NL (51.97° N, 4.93° E). All visible instruments used a common wavelength range and set of cross sections for the spectral analysis. Most of the instruments were of the multi-axis design with analysis by differential spectroscopy software (MAX-DOAS), whose non-zenith slant columns were compared by examining slopes of their least-squares straight line fits to mean values of a selection of instruments, after taking 30-min averages. Zenith slant columns near twilight were compared by fits to interpolated values of a reference instrument, then normalised by the mean of the slopes of the best instruments. For visible MAX-DOAS instruments, the means of the fitted slopes for NO2 and O4 of all except one instrument were within 10% of unity at almost all non-zenith elevations, and most were within 5%. Values for UV MAX-DOAS instruments were almost as good, being 12% and 7%, respectively. For visible instruments at zenith near twilight, the means of the fitted slopes of all instruments were within 5% of unity. This level of agreement is as good as that of previous intercomparisons, despite the site not being ideal for zenith twilight measurements. It bodes well for the future of measurements of tropospheric NO2, as previous intercomparisons were only for zenith instruments focussing on stratospheric NO2, with their longer heritage.

The 2006 WMO/UNEP Scientific Assessment of Ozone Depletion identified halogenated very short-lived substances (VSLS) as contributors to the atmospheric budget of halogens. As well, it raised a question regarding the extent of the contribution of halogenated VSLS to atmospheric Bry and Iy. Traditionally, scientists have been more concerned in determining the anthropogenic budget of halogenated compounds while nature is the major producer of such species. In order to have a complete atmospheric budget of halogenated VSLS, it is important to have a better understanding of what species are biogenically produced as well as their respective degradation pathways. Oceanic emissions of halocarbons may be a new link between climate change and the composition of the global atmosphere. The rates of halocarbon emissions are sensitive to sea-surface temperatures (SSTs), nutrient supply and upwelling; all of which are to be affected by climate change. Therefore, increases in SSTs will increase emission rates. On the one hand, seaweed has been identified as a major producer of biogenic polyhalogenated VSLS. Marine macroalgae (kelp) and phytoplankton emit halogen containing gases into the marine boundary layer, constituting 90 to 95% of the total global flux of volatile halocarbons to the atmosphere. On the other hand, the possibility of industrial scale marine kelp farming as a means of carbon sequestration (i.e. marine analogy of the Kyoto Protocol forest) is being pondered by countries with long coastlines and little land suitable for forestation. Would a Kyoto Protocol forest analog be the right strategy for climate change mitigation?With the use of a portable Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) spec- trometer, studies have been performed in the coast of New Zealand in order to determine the presence of BrO and IO during the spring and summer months of the Southern Hemisphere. MAX-DOAS uses scattered sunlight received from multiple viewing directio

NIWA operates a network of zenith-sky viewing DOAS (Differential Optical Absorption Spec- troscopy) instruments to measure NO2 and BrO. The longest existing time series (1981 present) of NO2 has been measured at Lauder (45oS), New Zealand and the trend of this long- term data set has been studied extensively. Here we present a summary of stratospheric NO2 trends observed at several Northern and Southern Hemisphere stations (including Lauder) and an update of our understanding of the observed hemispheric asymmetry. These trends provide an important anchor for the interpretation of NO2 trends measured by satellites.BrO observations are currently made by NIWA at two Southern Hemisphere sites, Lauder and Arrival Heights (78oS) with each data set spanning more than 15 years. The zenith sky BrO observations are complemented with direct sun observations at Lauder since 2001and with MAX-DOAS (Multi-axis Differential Optical Absorption Spectroscopy) observations at Arrival Heights (78oS) since 1998. A retrieval technique to separate the tropospheric and stratospheric partial columns of BrO was developed for the combination of zenith sky and direct sun measurements with the zenith sky observations providing predominantly the information on the stratospheric partial column and the direct sun observations providing the tropospheric contribution. This retrieval has now been applied to Lauder BrO UV-visible measurements for the whole time period (2001 - present) and the updated results including an upper limit of BrO in the troposphere and the stratospheric bromine loading will be presented.The retrieval method has now also been extended so that it can be applied to zenith sky data only. Furthermore, an independent retrieval algorithm has been developed including a forward model capable of dealing with multiple scattering (Monte Carlo radiative transfer model) to enable us to retrieve altitude information in the boundary layer and lower troposphere. This retrieval method has been

We will present a newly developed algorithm for the retrieval of tropospheric trace gas profiles from MAX-DOAS measurements. A Monte Carlo radiative transfer model, NIMO (NIWA Monte Carlo model) is used to calculate the weighting functions and forward model DSCDs (Differential Slant Column Densities). NIMO uses the local estimation technique to substantially speed up the determination of DSCDs for any given set of measurement ge- ometries, enabling use of the model online rather than using pre-calculated lookup tables. The optimal estimation method is used to retrieve profiles for either single or multiple scan sequences or over prescribed time intervals. This inversion method is used to derive NO2 profiles from MAX-DOAS measurements made during the CINDI campaign at Cabauw, Netherlands, in June/July 2009. BrO profiles retrieved from sea-ice MAX-DOAS measure- ments, made during two Antarctic springtime campaigns in 2006 and 2007, are also presented.

As noted in the WMO/UNEP Scientific Assessment of Ozone Depletion: 2006, halogenated very short-lived substances (VSLS) contribute to the atmospheric budget of halogens and thereby lead to substantial decreases in ozone and increases in surface UV radiation in the tropics and mid-latitudes. Halogenated VSLS are primarily of natural origin; oceanic emissions constitute the largest source providing 90-95% of the total global flux to the atmosphere. Macro algae in the ocean appear to be an important source of polyhalogenated VSLS. Oxidation of halogenated VSLS in the atmosphere (i.e. photolysis and reactions with OH) produces halogen oxide radicals (e.g. ClO, BrO, IO) which have been suggested as the main component of gas-phase halogens. Countries with long coastlines and little land suitable for forestation are investigating the possibility of industrial scale marine kelp farming as a means of carbon sequestration. This marine analogy of the Kyoto Protocol forest has been thought as a means to contribute to climate change mitigation. Knowledge of how natural emissions of VSLS will respond to both the drivers of climate change (e.g. changes in CO2 and land use) and to the consequences of climate change (e.g. changes in sea surface temperature and wind stress) is very limited. As a result, it is imperative that observational studies are performed to quantify the contributions of these natural VSLS to halogen loading in the troposphere and, subsequently, in the stratosphere. For this, transport and degradation processes of the source gases and product gases need to be studied and quantified. A key question surfacing from the WMO Assessment is to what extent halogenated VSLS contribute to atmospheric Bry and Iy. During a field campaign conducted during the spring of 2009, measurements of BrO and IO were made along the coastline of the South Island of New Zealand using a portable Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) spectrometer with the aim o

Spectroscopic measurements of BrO using direct sun and zenith sky viewinggeometries are combined in an optimal estimation retrieval algorithm to obtaintropospheric and stratospheric columns of BrO. Twenty-two twilight periods areinvestigated over Arrival Heights, Antarctica (77.8 S, 166.7 E) during the polar springperiod of 2002. This paper presents the first tropospheric and stratospheric BrO columnretrievals from UV-visible ground-based measurements for a polar location. A directcomparison is made between stratospheric columns retrieved at 80 , 84 , and 88 solarzenith angles (SZA) from the spectroscopic measurements and those calculated by theSLIMCAT three-dimensional chemical transport model. The ground-based column BrOobservations are consistent with a SLIMCAT stratospheric Bry loading of 21.2 parts pertrillion at 20 km. SLIMCAT reproduces the observed sunrise column BrO increase butdoes not match the sunset observations, which display less variation. The significantwarming of the Antarctic polar stratosphere in 2002 led to highly variable stratosphericcolumns being observed. The observed column BrO decreased with the transition fromvortex to extravortex air on 21 September but did not change much following the return ofthe vortex on 12 October. For the tropospheric column, an almost normal distributionconsistent with a background of 0.3 ± 0.3 1013 molecules cm 2 is observed from theground (80 , 84 , and 88 for both sunrise and sunset). A statistically significant bromineexplosion event (at the 2s level) was detected at the end of October with a troposphericcolumn of 1.8 ± 0.1 1013 molecules cm 2. The measured tropospheric columns arecompared with the tropospheric Model of Atmospheric Transport and ChemistryMaxPlanck Institute for Chemistry version model. The tropospheric BrO sunrise columnobservations can only be explained with an additional bromine source other thandecomposition of CH3Br and downward transport of long-lived bromine from thestratosphere. A comp