Axis Differential Optical Absorption Spectroscopy Research Articles

Research and application of differential optical absorption two-dimensional detection system for rotorcraft unmanned aerial vehicle

In order to meet the technical requirements for miniaturization, multi-angle, multi-altitude, and fast simultaneous acquisition of atmospheric pollutants, this study develops an integrated, lightweight, and cost-effective airborne differential optical absorption spectroscopy (DOAS) system. This system is designed in order to be used on a rotorcraft unmanned aerial vehicle (UAV) platform for monitoring atmospheric pollutants. The compositions of the hexacopter UAV platform and the airborne DOAS system are detailed in this work. The system includes a multi axis differential optical absorption spectroscopy (MAX-DOAS) spectral acquisition system, a control system, and a flight environment monitoring system. Commands are sent from a computer via serial communication to drive a gimbal, controlling the azimuth angle and elevation angle of the telescope, with a camera recording the light obstruction. The sunlight scattered by the atmosphere is collected by the telescope and transmitted via fiber optics to the spectrometer, which then transmits the data to the control computer. Additionally, the system captures data of altitude, temperature, humidity, and GPS location during flight, and filters out spectral data obtained under abnormal flight conditions. Stability studies indicate that the mean angular deviations for yaw, roll, and pitch are 0.07°, –0.13°, and –0.12° respectively, which meet the requirements for monitoring stability. Comparative experiments with a commercial ground-based DOAS system show that the correlation coefficients between the monitoring data of both systems are both greater than 0.92, confirming the reliability of the airborne system. In field flight experiments, the airborne DOAS system conducts observations at altitudes of 30 m, 60 m, and 90 m, with the elevation angle set at 0° and the azimuth angle measured every 30° from 0° to 360°. The system successfully obtains the concentration distributions of NO<sub>2</sub>, SO<sub>2</sub>, and HCHO at different azimuth angles and altitudes. The results indicate that the concentrations of these three gases decrease with altitude increasing, with higher concentrations observed in the southeast direction, indicating the presence of pollution sources in that direction. Further analysis with considering altitude changes indicates that the rate of decrease in NO<sub>2</sub> concentration and SO<sub>2</sub> concentration slow down with altitude increasing, while the rate of decrease in HCHO remains relatively constant. These findings indicate that this system effectively meets the technical requirements for simultaneous, rapid, multi-angle, and multi-altitude detection of atmospheric pollutants, providing essential support for the detailed monitoring of complex urban micro-environments.

Ground-Based MAX-DOAS Observation of Trace Gases from 2019 to 2021 in Huaibei, China

With the spread of the COVID-19 pandemic and the implementation of closure measures in 2020, population mobility and human activities have decreased, which has seriously impacted atmospheric quality. Huaibei City is an important coal and chemical production base in East China, which faces increasing environmental problems. The impact of anthropogenic activities on air quality in this area was investigated by comparing the COVID-19 lockdown in 2020 with the normal situation in 2021. Tropospheric NO2, HCHO and SO2 column densities were observed by ground-based multiple axis differential optical absorption spectroscopy (MAX-DOAS). In situ measurements for PM2.5, NO2, SO2 and O3 were also taken. The observation period was divided into four phases, the pre-lockdown period, phase 1 lockdown, phase 2 lockdown and the post-lockdown period. Ground-based MAX-DOAS results showed that tropospheric NO2, HCHO and SO2 column densities increased by 41, 14 and 14%, respectively, during phase 1 in 2021 vs. 2020. In situ results showed that NO2 and SO2 increased by 59 and 11%, respectively, during phase 1 in 2021 vs. 2020, but PM2.5 and O3 decreased by 15 and 17%, respectively. In the phase 2 period, due to the partial lifting of control measures, the concentration of pollutants did not significantly change. The weekly MAX-DOAS results showed that there was no obvious weekend effect of pollutants in the Huaibei area, and NO2, HCHO and SO2 had obvious diurnal variation characteristics. In addition, the relationship between the column densities and wind speed and direction in 2020 and 2021 was studied. The results showed that, in the absence of traffic control in 2021, elevated sources in the Eastern part of the city emitted large amounts of NO2. The observed ratios of HCHO to NO2 suggested that tropospheric ozone production involved NOX-limited scenarios. The correlation analysis between HCHO and different gases showed that HCHO mainly originated from primary emission sources related to SO2.

MAX-DOAS and in-situ measurements of aerosols and trace gases over Dongying, China: Insight into ozone formation sensitivity based on secondary HCHO

This study presents a comprehensive overview of the atmospheric pollutants including Sulfur dioxide (SO2), Nitrogen dioxide (NO2), Formaldehyde (HCHO), Particulate Matter PM; PM10: diameter ≤ 10 µm, and PM2.5: diameter ≤ 2.5 µm), and Ozone (O3), over Dongying (Shandong Province) from March-April 2018 and September-October 2019 by employing ground-based Multiple Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations along with the in-situ measurements attained by the national air quality monitoring platform. The concentrations of SO2 and NO2 were under the acceptable level, while both PM2.5, and PM10 were higher than the safe levels as prescribed by national and international air quality standards. The results depict that 21% of the total observation days were found to be complex polluted days (PM2.5 > 35 µg/m3 and O3 > 160 µg/m3). The secondary HCHO was used for accurate analysis of O3 sensitivity. A difference of 11.40% and 10% during March-April 2018 and September-October 2019 respectively in O3 sensitivity was found between HCHOtotal/NO2 and HCHOsec/NO2. The results indicate that primary HCHO have significant contribution in HCHO. O3 formation predominantly remained to be in VOC-limited and transitional regime during March-April 2018 and September-October 2019 in Dongying. These results imply that concurrent control of both NOx and VOCs would benefit in ozone reductions. Additionally, the criteria pollutants (PM, SO2, and NO2) depicted strong correlations with each other except for O3 for which weak correlation coefficient was obtained with all the species. This study will prove to be baseline for designing of air pollution control strategies.

Relating atmospheric aerosol amounts to blue to red ratio and grayscale contrast fluctuations using digitalization of routine webcam photographs taken in the urban environment of Vienna

Visibility and visual contrast depend on several factors such as aerosol concentration, fog attenuation and humidity, viewing direction relative to the sun as well as trace gas amounts. Usually, visibility is determined by observers or by measurement devices such as visiometers. Only few studies reported on the relationship between visibility and aerosol concentration using digital cameras. In the present study, we develop and test new algorithms for the determination of the amount of aerosols by using digital photography. Two years of routine webcam recordings are used for this study. The digital images are taken at the meteorological measurement platform, which is situated on the roof of a university campus building and overlooking the city of Vienna. The photographs are automatically taken as part of Multi AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements, which are continuously performed during daytime at different azimuthal and elevation angles. Two new algorithms for estimating aerosol amounts by quantifying digital images taken on cloud-free days are introduced within the scope of the present study: (1) the ratio of the blue to red (B/R) RGB channels within a vertical pixel line and (2) the grayscale contrast fluctuations (GCF) within a horizontal pixel line. Vertical B/R and horizontal GCF are compared against ground-based aerosol optical depth (AOD) and in-situ particulate matter (PM) < 10 μm (PM10) measurements. Interestingly, the comparisons show promising results for specific azimuthal directions and during certain daytime periods, e.g. for constellations when the sun is to the side or directly behind the camera. In terms of coefficients of determination (R2), B/R correlates best with AOD during summer, while GCF shows better performance in winter. R2 values of up to 0.86 and 0.73 are found between horizontal line GCF and in-situ PM10 and between vertical line B/R and AOD, respectively. For a better interpretation of the correlations, simultaneous measurements of relative humidity (RH) are included in the analyses.

Seasonal Investigation of MAX-DOAS and In Situ Measurements of Aerosols and Trace Gases over Suburban Site of Megacity Shanghai, China

Shanghai has gained much attention in terms of air quality research owing to its importance to economic capital and its huge population. This study utilizes ground-based remote sensing instrument observations, namely by Multiple AXis Differential Optical Absorption Spectroscopy (MAX-DOAS), and in situ measurements from the national air quality monitoring platform for various atmospheric trace gases including Nitrogen dioxide (NO2), Sulfur dioxide (SO2), Ozone (O3), Formaldehyde (HCHO), and Particulate Matter (PM; PM10: diameter ≤ 10 µm, and PM2.5: diameter ≤ 2.5 µm) over Shanghai from June 2020 to May 2021. The results depict definite diurnal patterns and strong seasonality in HCHO, NO2, and SO2 concentrations with maximum concentrations during winter for NO2 and SO2 and in summer for HCHO. The impact of meteorology and biogenic emissions on pollutant concentrations was also studied. HCHO emissions are positively correlated with temperature, relative humidity, and the enhanced vegetation index (EVI), while both NO2 and SO2 depicted a negative correlation to all these parameters. The results from diurnal to seasonal cycles consistently suggest the mainly anthropogenic origin of NO2 and SO2, while the secondary formation from the photo-oxidation of volatile organic compounds (VOCs) and substantial contribution of biogenic emissions for HCHO. Further, the sensitivity of O3 formation to its precursor species (NOx and VOCs) was also determined by employing HCHO and NO2 as tracers. The sensitivity analysis depicted that O3 formation in Shanghai is predominantly VOC-limited except for summer, where a significant percentage of O3 formation lies in the transition regime. It is worth mentioning that seasonal variation of O3 is also categorized by maxima in summer. The interdependence of criteria pollutants (O3, SO2, NO2, and PM) was studied by employing the Pearson’s correlation coefficient, and the results suggested complex interdependence among the pollutant species in different seasons. Lastly, potential source contribution function (PSCF) analysis was performed to have an understanding of the contribution of different source areas towards atmospheric pollution. PSCF analysis indicated a strong contribution of local sources on Shanghai’s air quality compared to regional sources. This study will help policymakers and stakeholders understand the complex interactions among the atmospheric pollutants and provide a baseline for designing effective control strategies to combat air pollution in Shanghai.

Evaluation of the coupled high-resolution atmospheric chemistry model system MECO(n) using in situ and MAX-DOAS NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; measurements

Abstract. We present high spatial resolution (up to 2.2×2.2 km2) simulations focussed over south-west Germany using the online coupled regional atmospheric chemistry model system MECO(n) (MESSy-fied ECHAM and COSMO models nested n times). Numerical simulation of nitrogen dioxide (NO2) surface volume mixing ratios (VMRs) are compared to in situ measurements from a network with 193 locations including background, traffic-adjacent and industrial stations to investigate the model's performance in simulating the spatial and temporal variability of short-lived chemical species. We show that the use of a high-resolution and up-to-date emission inventory is crucial for reproducing the spatial variability and resulted in good agreement with the measured VMRs at the background and industrial locations with an overall bias of less than 10 %. We introduce a computationally efficient approach that simulates diurnal and daily variability in monthly-resolved anthropogenic emissions to resolve the temporal variability of NO2. MAX-DOAS (Multiple AXis Differential Optical Absorption Spectroscopy) measurements performed at Mainz (49.99∘ N, 8.23∘ E) were used to evaluate the simulated tropospheric vertical column densities (VCDs) of NO2. We propose a consistent and robust approach to evaluate the vertical distribution of NO2 in the boundary layer by comparing the individual differential slant column densities (dSCDs) at various elevation angles. This approach considers details of the spatial heterogeneity and sensitivity volume of the MAX-DOAS measurements while comparing the measured and simulated dSCDs. The effects of clouds on the agreement between MAX-DOAS measurements and simulations have also been investigated. For low elevation angles (≤8∘), small biases in the range of −14 % to +7 % and Pearson correlation coefficients in the range of 0.5 to 0.8 were achieved for different azimuth directions in the cloud-free cases, indicating good model performance in the layers close to the surface. Accounting for diurnal and daily variability in the monthly-resolved anthropogenic emissions was found to be crucial for the accurate representation of time series of measured NO2 VMR and dSCDs and is particularly critical when vertical mixing is suppressed, and the atmospheric lifetime of NO2 is relatively long.

Effect of Boundary Layer Evolution on Nitrogen Dioxide (NO2) and Formaldehyde (HCHO) Concentrations at a High-altitude Observatory in Western India

ABSTRACT Nitrogen dioxide (NO2) and formaldehyde (HCHO) are some of the most important trace gases in the atmosphere, acting as precursors for ozone formation and as pollutants at high concentrations. Although several observations of these species have been reported in the boundary layer, observations at high altitude sites are limited, especially in India. This study reports observations of NO2 and HCHO using the Multi AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) technique at the High Altitude Cloud Physics Laboratory (HACPL), Mahabaleshwar in the rural Western Ghats region of India. Measurements were conducted during the pre-monsoon season between 25th April and 30th May 2018. The average NO2 mixing ratio was 0.19 ± 0.06 ppb (range: 0.03 ppb to 0.69 ppb). Typically, NO2 mixing ratios were found to increase from early in morning and reached a maximum in the afternoon, contrary to an expected diurnal profile dominated by photochemistry. The average HCHO mixing ratio was 1.6 ± 0.61 ppb (range: 0.16 ppb–4.5 ppb). HCHO mixing ratios also showed an increase from early in the morning and reach a maximum at ~3 pm in the afternoon after which a decrease was observed, peaking much later than expected from a photochemistry dominated profile. Using observations of the boundary layer height, back trajectories and the known photochemistry, we conclude that the observed diurnal variation in these two species is dominated by the mixing of emissions from the base of the mountain, resulting from the evolution of the boundary layer at the HACPL site.

Spatiotemporal Variations of NO&lt;sub&gt;2&lt;/sub&gt; over Fukuoka Japan, Observed by Multiple MAX-DOAS and 3-D Coherent Doppler Lidar

To clarify three-dimensional (3-D) spatiotemporal variations and horizontal–vertical transport processes in nitrogen dioxide (NO2) over urban areas, combined NO2 profile observations by multiple Multi AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) and direct wind observations by 3-D coherent Doppler lidar were made over an urban area in Japan. MAX-DOAS measurements were conducted at Yakuin and Fukuoka University in Fukuoka urban area with high temporal resolution of four minutes.Enhanced NO2 concentrations were often observed over the city center. We conducted a case study on 29 November 2018 under clear sky conditions and NO2 profiles were well retrieved. Using MAX-DOAS at two locations, high NO2 concentrations were observed near the surface of the city center in the morning. Higher NO2 concentrations appearing gradually at higher altitudes over the city center and disappearing in the afternoon are explained using direct evidence of a 3-D wind field: an airmass with a high NO2 concentration was transported upward over the city center. Then, the airmass was advected landward by a sea breeze. Multiple MAX-DOAS combined with 3-D Doppler lidar constitutes a powerful tool for elucidating horizontal–vertical transport processes. It can contribute to the improvement of data retrieval by satellites for urban areas.

Evaluating different methods for elevation calibration of MAX-DOAS (Multi AXis Differential Optical Absorption Spectroscopy) instruments during the CINDI-2 campaign

Abstract. We present different methods for in-field elevation calibration of MAX-DOAS (Multi AXis Differential Optical Absorption Spectroscopy) instruments that were applied and inter-compared during the second Cabauw Intercomparison campaign for Nitrogen Dioxide measuring Instruments (CINDI-2). One necessary prerequisite of consistent MAX-DOAS retrievals is a precise and accurate calibration of the elevation angles of the different measuring systems. Therefore, different methods for this calibration were applied to several instruments during the campaign, and the results were inter-compared. This work first introduces and explains the different methods, namely far- and near-lamp measurements, white-stripe scans, horizon scans and sun scans, using data and results for only one (mainly the Max Planck Institute for Chemistry) instrument. In the second part, the far-lamp measurements and the horizon scans are examined for all participating groups. Here, the results for both methods are first inter-compared for the different instruments; secondly, the two methods are compared amongst each other. All methods turned out to be well-suited for the calibration of the elevation angles of MAX-DOAS systems, with each of them having individual advantages and drawbacks. Considering the results of this study, the systematic uncertainties of the methods can be estimated as ±0.05∘ for the far-lamp measurements and the sun scans, ±0.25∘ for the horizon scans, and around ±0.1∘ for the white-stripe and near-lamp measurements. When comparing the results of far-lamp and horizon-scan measurements, a spread of around 0.9∘ in the elevation calibrations is found between the participating instruments for both methods. This spread is of the order of a typical field of view (FOV) of a MAX-DOAS instrument and therefore affecting the retrieval results. Further, consistent (wavelength dependent) offsets of 0.32∘ and 0.40∘ between far-lamp measurements and horizon scans are found, which can be explained by the fact that, despite the flat topography around the measurement site, obstacles such as trees might mark the visible horizon during daytime. The observed wavelength dependence can be explained by surface albedo effects. Lastly, the results are discussed and recommendations for future campaigns are given.

Investigating the impact of Glyoxal retrieval from MAX-DOAS observations during haze and non-haze conditions in Beijing

This study presents the Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements for Glyoxal (CHOCHO) in Beijing, China (39.95°N, 116.32°E). CHOCHO is the smallest compound of di-carbonyl group. As a primary sink of CHOCHO, its photolysis with NOx (oxides of nitrogen) results in the production of tropospheric ozone. Therefore, the focus of CHOCHO DOAS measurements is increasing in trend. We did the measurements from 09 May 2017 to 09 September 2017. The study was conducted to compare different retrieval settings in order to reveal best DOAS fit settings for CHOCHO; furthermore, effect of haze and non-haze days on CHOCHO concentration was examined. The root mean square of residual and Differential Slant Column density (dSCD) error was reduced when measurements were done with lower wavelength limit around 432–438 nm and upper intervals around 455–460 nm. Thus, lower wavelength intervals around 432–438 nm and upper intervals around 457–460 nm were best for the retrieval of dSCDs for CHOCHO. Meteorological conditions like haze or non-haze days did not have significant effect on DOAS fit parameters. The CHOCHO vertical column densities range from 1.33E+14 to 9.77E+14 molecules/cm2 during the study period with average of 6.16E+14 molecules/cm2. The results indicated that during haze days CHOCHO concentration was higher because of lower rate of photolysis and atmospheric oxidation potential. Our results did not show any significant weekend effect on CHOCHO atmospheric concentration.

Simultaneous Observations of Nitrogen Dioxide, Formaldehyde and Ozone in the Indo-Gangetic Plain

ABSTRACT This study reports the concentrations of nitrogen dioxide (NO2) and formaldehyde (HCHO), retrieved using the Multi AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) technique and collocated observations of surface ozone (O3) conducted over the Indo-Gangetic Plain (IGP) during the 2014 monsoon period as part of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX). The average daytime NO2 mixing ratio was 0.81 ± 0.20 ppbv (parts per billion by volume) (range: 0.08–6.06 ppbv). NO2 was observed to decrease during the morning between 06:00 and 09:00 local time and then stabilise for the rest of the day. The average daytime HCHO mixing ratio was 1.93 ± 0.60 ppbv (range: 0.32–8.81 ppbv). Unlike NO2, HCHO, driven by daytime photochemical formation from hydrocarbon precursors, increased during the early morning. The average O3 mixing ratio was 30.0 ± 13.0 ppbv (range: 2.7–81.9 ppbv) during the daytime and 22.5 ± 10.2 ppbv (range: 1–63 ppbv) during the nighttime. Analyses of the back trajectories indicatedfound that the NO2 mixing ratios during CAIPEEX-2014 were affected by long-range transport from thermal power plants situated about 110 km to the south but the HCHO mixing ratios and O3 production were influenced by local emissions. These observations suggest that in rural IGP, ozone concentrations are affected by local emission rather than by long-range transport.

MAX-DOAS measurements and vertical profiles of glyoxal and formaldehyde in Madrid, Spain

Glyoxal (CHOCHO) and formaldehyde (HCHO) are organic trace gases that play an important role in tropospheric chemistry as oxidation products of a number of volatile organic compounds (VOCs). In this study, we report year-round daytime measurements of glyoxal and formaldehyde in the urban atmosphere of Madrid, Spain. Their vertical concentration profiles were retrieved using the Multi AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) technique and a Radiative Transfer Model (RTM) that simulates solar photon paths through the atmosphere. The diurnal variations of HCHO show two distinct peaks during the day, in the early morning and late afternoon in spring and summer, while the second peak is shifted towards noon in autumn and winter, due to lower photolysis rates and more effective boundary layer accumulation of HCHO in those seasons. The HCHO surface mixing ratios range from 6 ppbv to 27 ppbv in spring-summer and from 10 ppbv to 30 ppbv in autumn-winter. Monthly hourly-averaged glyoxal surface mixing ratios in the early morning show higher values during winter, 2 ppbv, than in summer, 0.7 ppbv. We also evaluated the ratio between glyoxal and formaldehyde (RGF) surface mixing ratios, as an indicator of the nature of VOCs precursors. The RGF was also correlated with the measured NO2, which represents a direct signal of anthropogenic emissions, along with the VOCs emission inventories in Madrid. The RGF results yielded higher ratios in spring, 0.1–0.13, than in winter and autumn (in the range of 0.02–0.07) when NO2 levels were higher.

Vertical Profiles of Tropospheric Ozone From MAX‐DOAS Measurements During the CINDI‐2 Campaign: Part 1—Development of a New Retrieval Algorithm

AbstractGround‐based measurements of tropospheric ozone (O3) are valuable for studies of atmospheric chemistry, air pollution, climate change, and for satellite validation. The Multi Axis Differential Optical Absorption Spectroscopy (MAX‐DOAS) technique has been widely used to derive vertical profiles of trace gases and aerosols in the troposphere. However, tropospheric O3 has not yet been satisfactorily derived from MAX‐DOAS measurements due to the influence of stratospheric O3 absorption. In this study, we developed two new retrieval approaches for tropospheric O3 from MAX‐DOAS measurements. In method 1, stratospheric O3 profiles from external data sources are considered in the retrieval. In method 2, stratospheric and tropospheric O3 are separated based on the temperature‐dependent differences between tropospheric and stratospheric O3 absorption structures in the UV spectral range. The feasibility of both methods is first verified by applying them to synthetic spectra. Then they are applied to real MAX‐DOAS measurements recorded during the CINDI‐2 campaign in Cabauw, the Netherlands (September 2016). The obtained results are compared with independent O3 measurements and global chemical transport model simulations. Good agreement of the near‐surface O3 concentrations with the independent data sets is found for both methods. However, tropospheric O3 profiles are only reasonably derived using method 1, while they are significantly overestimated at altitudes above 1 km using method 2, probably due to the approximation of the ring spectra used to correct the rotational Raman scattering structures in the DOAS fit. Advantages and disadvantages of both methods are discussed and improvement directions are suggested for further studies.

The MAX-DOAS network in Mexico City to measure atmospheric pollutants

An instrument based on the Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) technique was designed and constructed to measure scattered sunlight in the UV-visible region at different elevation angles. Slant column densities (SCDs) of specific gas absorbers such as nitrogen dioxide (NO2) and form-aldehyde (HCHO) are derived from the measured spectra. In this contribution, the technical characteristics and performance of the instruments, their deployment in a newly formed observational network within the metropolitan area of Mexico City, and some results of the retrieved NO2 and HCHO SCDs are presented. These measurements provide more insight on the vertical and spatial distribution of these key atmospheric pollutants and their temporal variability, which also serve as a basis for present and future satellite validation studies.

Variability of bromine monoxide at Barrow, Alaska, over four halogen activation (March–May) seasons and at two on‐ice locations

AbstractReactive halogens profoundly influence springtime Arctic atmospheric chemistry, but their relationship to sea ice and environmental conditions is not well understood. Multiple axis differential optical absorption spectroscopy measured bromine monoxide (BrO) at Barrow, Alaska, and at two Arctic Ocean buoys. For each season of Barrow measurements, we examined the air mass histories using back trajectory modeling and ice coverage maps. We find a weak positive linear correlation (R = 0.38) between half‐hourly BrO lower tropospheric vertical column densities (LT‐VCD) and time in first year sea ice (FYI) areas. These data show evidence of a nonlinear increase of LT‐VCD BrO with low‐average column in the absence of ice contact, with the column increasing and saturating at ice contact longer than ≈1.5 days. We find that trajectories arriving at Barrow are dominated by FYI area influence with little multiyear ice (MYI) area contact; therefore, this study cannot make any conclusions regarding MYI area influences on reactive halogen production. Contact with calculated potential frost flower influence is not correlated with BrO column (R = 0.04). At Barrow, annual averages of BrO column over the halogen activation season and time in FYI areas are highly correlated (R = 0.93, significant at 90% confidence), which is interpreted as an effect of interannual transport variability. At on‐ice locations, we observe a wide range of BrO LT‐VCDs, suggesting that while an air mass spending time in sea ice areas is required to observe significant BrO, sea ice contact alone does not imply high BrO, and other environmental controls are important.

Comparison and validation of band residual difference algorithm and principal component analysis algorithm for retrievals of atmospheric SO2 columns from satellite observations

Remote sensing technology provides an unprecedented tool for the continuous and real-time monitoring of atmospheric SO2 from volcanic eruption and anthropogenic emission. The Global Ozone Monitoring Experiment (GOME), SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY), and Ozone Monitoring Instrument (OMI) have high SO2 monitoring capability. The OMI, which was launched on the EOS/Aura platform in July 2004, has the same hyperspectral measurements as the GOME and SCIAMACHY, but offers the improved spatial resolution at nadir (1324 km2) and daily global coverage for short-lifetime SO2. For OMI operational SO2 planetary boundary layer (PBL) retrieval, the previous band residual difference (BRD) algorithm has been replaced by principal component analysis (PCA) algorithm, which effectively reduces the systematic biases in SO2 column retrievals. However, there are few studies on the evaluations and validations of PCA SO2 retrievals over China, and the long-term comparisons with BRD SO2 retrievals also need to be conducted. In this study, the accuracies of PCA and BRD SO2 retrievals are validated by using ground-based multi axis differential optical absorption spectroscopy (MAX-DOAS) located in Beijing, and regional atmospheric modeling system, community multi-scale air quality (RAMS-CMAQ) modeling system model which can simulate the vertical distribution of atmospheric SO2. Moreover, BRD and PCA SO2 retrievals from oceanic area, eastern China and Reunion volcanic eruption are compared to find the long-term trend and spatiotemporal differences between SO2 columns. Finally, the uncertainty of SO2 retrieval, caused by measurement errors, band selection and input parameter errors in radiative transfer model, are analysed to understand the limitations of BRD and PCA algorithms. Results show that both PCA and BRD SO2 retrievals over Beijing are lower than ground-based MAX-DOAS measurements of SO2. PCA and BRD SO2 retrievals over eastern China are lower than the simulated SO2 columns from RAMS-CMAQ in winter 2008, but in July and August BRD SO2 columns are higher than RAMS-CMAQ simulations. The values of SO2 columns from BRD over China are more consistent with those from ground-based MAX-DOAS and RAMS-CMAQ model than from PCA. Although PCA algorithm effectively reduces the noise in SO2 column retrieval, SO2 columns from PCA over China are lower than those from BRD. For oceanic area where SO2 amount is nearly zero, the standard deviation of PCA results is lower than that of BRD, but the absolute value of averaged PCA SO2 column is larger than that of BRD. In the case of Reunion volcanic eruption with SO2 columns larger than 25 DU, the BRD SO2 columns are lower than PCA retrievals. Meanwhile, with the increase of SO2 column, the difference between BRD and PCA SO2 retrievals increases. Detailed uncertainty analysis shows the influences of measurement errors, band selection and inputs of radiative transfer model on the retrieval results. This study is important for developing the retrieval algorithm, and can also improve the application of OMI SO2 products.

Tropospheric NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; vertical column densities over Beijing: results of the first three years of ground-based MAX-DOAS measurements (2008–2011) and satellite validation

Abstract. Ground-based measurements of scattered sunlight by the Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) have been carried out at an urban site (39.95° N, 116.32° E) in Beijing megacity since 6 August 2008. In this study, we retrieved the tropospheric NO2 vertical column densities (VCDs) over Beijing from these MAX-DOAS observations from August 2008 to September 2011. Over this period, the daytime (08:00–17:00 Beijing Time (BJT, which equals UTC + 8)) mean tropospheric NO2 VCDs varied from 0.5 to 13.3 with an average of 3.6 during summertime, and from 0.2 to 16.8 with an average of 5.8 during wintertime, all in units of 1016 molecules cm−2. The average diurnal variation patterns of tropospheric NO2 over Beijing appeared to be rather different from one season to another, indicating differences in the emission strength and atmospheric lifetime. In contrast to previous studies, we find a small weekly cycle of the tropospheric NO2 VCD over Beijing. The NO2 VCD in the late afternoon was the largest on Saturday and the lowest on Sunday, and in the morning it reached a clear maximum on Wednesday. We also find a post-Olympic Games effect, with 39–54% decrease in the tropospheric NO2 VCD over Beijing estimated for August of 2008, compared to the following years. The tropospheric NO2 VCDs derived by our ground MAX-DOAS measurements show a good correlation with SCIAMACHY and OMI satellite data. However, compared with the MAX-DOAS measurements, the satellite observations underestimate the tropospheric NO2 VCDs over Beijing systematically, by 43% for SCIAMACHY and 26–38% for OMI (DOMINO v2.0 and DOMINO v1.02). Based on radiative transfer simulations, we show that the aerosol shielding effect can explain this underestimation, while the gradient smoothing effect caused by the coarse spatial resolution of the satellite observations could play an additional role.

Validation of total ozone data between satellite and ground-based measurements at Zhongshan and Syowa stations in Antarctica

We present validation between total ozone from satellite and ground-based observations of the Dobson and Brewer spectrometers and ozone radiosonde at Zhongshan and Syowa Antarctic research stations, for September 2004 to March 2009. Results show that mean bias error between Zhongshan (Syowa) and Ozone Monitor Instrument Total Ozone Mapping Spectrometer (OMI-TOMS) data are -0.06%±3.32% (-0.44%±2.41%); between it and OMI Multi Axis Differential Optical Absorption Spectroscopy (OMI-DOAS) data, the error is -0.34%±4.99% (-0.22%±4.85%). Mean absolute bias error values of OMI-TOMS data are less than those of OMI-DOAS. This means that total ozone of OMI-TOMS is closer to ground-based observation than that of OMI-DOAS. Comparison between direct observational total ozone of ground-based and integrated ozone from the ozone profile measured by ozone radiosonde shows that ozone amount calculated with the Solar Backscatter Ultraviolet (SBUV) method above balloon burst height is similar to corresponding Microwave Limb Sounder (MLS) data. Therefore, MLS data can be substituted with SBUV data to estimate ozone amount above that level. Mean bias error of the MLS ozone column is 2% compared with the ozonesonde column, with standard deviation within 9.5%. Comparison of different layers from ozone profiler and MLS data indicates that at the 215 hPa layer, the MLS ozone value is high, with relative deviation more than 20%. At the 100 hPa and 68 hPa layers, the MLS ozone value is also high. This deviation is mainly in spring, during Antarctic ozone hole appearance. In this period, at the height of severe ozone loss, relative deviation of MLS ozone values is especially large.

The NIMO Monte Carlo model for box-air-mass factor and radiance calculations

A new fully spherical multiple scattering Monte Carlo radiative transfer model named NIMO (NIWA Monte Carlo model) is presented. The ray tracing algorithm is described in detail along with the treatment of scattering and absorption, and the simulation of backward adjoint trajectories. The primary application of NIMO is the calculation of box-air-mass factors (box-AMFs), which are used to convert slant column densities (SCDs) of trace gases, derived from UV–visible multiple axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements, into vertical column densities (VCDs). Box-AMFs are also employed as weighting functions for optimal estimation retrievals of vertical trace gas profiles from SCDs. Monte Carlo models are well suited to AMF calculations at high solar zenith angles (SZA) and at low viewing elevation angles where multiple scattering is important. Additionally, the object-oriented structure of NIMO makes it easily extensible to new applications by plugging in objects for new absorbing or scattering species. Box-AMFs and radiances, calculated for various wavelengths, SZAs, viewing elevation and azimuth angles and aerosol scenarios, are compared with results from nine other models using a set of exercises from a recent radiative transfer model intercomparison. NIMO results for these simulations are well within the range of variability of the other models.

Determination of tropospheric NO2 by airborne multi axis differential optical absorption spectroscopy

The airborne multi axis differential optical absorption spectroscopy (AMAXDOAS) is employed to retrieve tropospheric vertical columns of NO2. The differential slant columns of NO2 are retrieved combined with the spectra in different viewing directions by the DOAS method and the vertical columns are calculated with the radiative transfer model SCIATRAN. The dependences of AMF on altitude, surface albedo, and solar zenith angle are studied. The measurement campaign in pearl region delta region on December 10 in 2008 is reported. The NO2 vertical column distribution is obtained in zenith and nadir directions. The results from airborne multi axis DOAS are compared with those from a ground-based multi axis DOAS in Zhuhai, with a deviation of 0.08. The results show that AMAXDOAS can be used to measure the tropospheric NO2 in a large area within a short time.