Oleg Postylyakov

On the possibility of estimating the volume of NO2 emissions in cities using zenith spectral observations of diffuse solar radiation near 450 nm

Methodological questions are considered for using the measurements of the integral content of NO2 in the atmospheric boundary layer, carried out at a fixed station, to analyze NO2 sources in a city. These measurements are carried out regularly at the IAP RAS network in Moscow. The experimental estimates of NO2 content are compared with calculations of pollutant transport by the ISCST3 model on the basis of the emission inventory, which shows a good agreement between these two approaches. We suggested a technique for estimating NO2 emissions in the urban environment with the use of the NO2 integral content measurements by the zenith spectral method. During the observation period of 2010–2011, vehicular emissions of NO2 in Moscow amounted to 88 kt/year.

First measurements of formaldehyde integral content in the atmosphere using MAX-DOAS in the Moscow region

We present the first observations of formaldehyde (HCHO) atmospheric column performed for Zvenigorod, Moscow region, Russia. The data were retrieved from UV spectra of the scattered solar radiation measured by the multi-axis differential optical absorption spectroscopy (MAX-DOAS) instrument developed by the Japan Agency for Marine-Earth Science and Technology. We developed an algorithm for the HCHO retrieval from these spectra. For retrieval of the HCHO differential slant column densities, we used the DOAS settings used as baseline in the Cabauw Intercomparison Campaign of Nitrogen Dioxide Measuring Instruments (CINDI). The slant column densities of HCHO were converted to vertical column densities (VCDs) using the air mass factors calculated by a radiative transfer model. The determination of HCHO in the reference spectrum used multi-axis measurements. The variability of the HCHO vertical column in 2010 is analysed. The HCHO vertical column density is larger during east wind directions than during non-east wind directions. This can be associated with the Moscow Megacity influence on air quality at Zvenigorod. The estimation of the Moscow Megacity influence on HCHO abundance at Zvenigorod is around 2.5 × 1014 molec cm−2 per 1 km length of trajectory path inside the Moscow Ring Road. A temperature effect is noticeable in the HCHO VCD. Our data show a statistically significant positive temperature effect in HCHO for the background condition for temperatures from −5°C to +33°C. The temperature trend in HCHO data at Zvenigorod Scientific Station is about (8.9 ± 2.3) × 1014 molec cm−2 (°C)−1. The increase of the HCHO VCD during increase of the air temperature can be explained by the HCHO formation from non-methane biogenic volatile organic compounds (e.g. isoprene) for which more emission is expected at higher temperatures, and by growth of areas of forest and turf fires.

On cloud bottom boundary determination by digital stereo photography from the Earth’s surface

In this paper, we studied the method for measuring the cloud bottom boundary altitude using the stereo pair of cloud images obtained using two digital photo cameras. We suggested a method for determining the camera orientation parameters using the nighttime images of star sky. The range to the cloud is calculated using the shift of the image of a cloud fragment as a whole. A given fragment on the photographs is identified using the methods of morphological image analysis. When the stereo base is 60 m and images are taken with a resolution of 1200 pixels within a field of view of 60°, the uncertainty does not exceed 10% when cloud altitude is less than 4 km. Optimizing the parameters of photography and increasing the stereo base may substantially improve the accuracy of the cloud base altitude estimation. Examples are presented of cloud bottom boundary determination using a prototype of the experimental setup as compared to data of a laser range finder.

Measurements of formaldehyde total content using DOAS technique: a new retrieval method for overcast

Formaldehyde (HCHO) is a significant constituent of the atmospheric chemistry involved in a lot of chemical reactions, which principal global source is the intermediate oxidation of volatile organic compounds (VOCs). Taking into account that HCHO basically undergo by photolysis and reaction with hydroxyl radical within a few hours, isoprene together with other short-lived VOCs and direct HCHO emissions can cause local HCHO enhancement over certain areas, and, hence, cases with HCHO, that exceed some background level, can be examined as local pollution of the atmosphere by VOCs. HCHO has significant specific features in spectral structure of UV absorption cross-section to be measured using the differential optical absorption spectroscopy (DOAS) technique. Several retrieval algorithms applicable for DOAS measurements in cloudless were previously developed. A new algorithm applicable for overcast and cloudless sky and its error analysis is briefly presented in this paper. In case we know the cloud base height, but don’t know cloud optical depth, the error of HCHO total content retrieval is about 10-20% for winter season, about 5% for summer season, and about 30-40% for transition season when the ABL is below the cloud base. In case we know both the cloud base height and cloud optical depth, the error is about 5-10% for winter season, less than 5% for summer season, and about 25-35% for transition season when the ABL is below the cloud base. The errors dramatically increase when clouds penetrate into ABL in both cases.

On determination of formaldehyde content in atmospheric boundary layer for overcast using DOAS technique

Formaldehyde (HCHO) is involved in a lot of chemical reactions in the atmosphere. Taking into account that HCHO basically undergo by photolysis and reaction with hydroxyl radical within a few hours, short-lived VOCs and direct HCHO emissions can cause local HCHO enhancement over certain areas, and, hence, exceeding background level of HCHO can be examined as a local pollution of the atmosphere by VOCs or existence of a local HCHO source. Several retrieval algorithms applicable for DOAS measurements in cloudless were previously developed. A new algorithm applicable for overcast and cloudless sky and its error analysis is briefly introduced by this paper. Analysis of our HCHO VCD retrieval for overcast shows that when one know the cloud base height, but doesn’t know cloud optical depth, the typical errors of HCHO total content retrieval are less than 10% for snow season, less than 5% for snow-free seasons, and reaches 40-45% for season with non-stable snow cover. In case one knows both the cloud base height and the cloud optical depth, the typical errors are about 5% for snow season, less than 2.5% for snow-free seasons, and are within about 10–30% for season with non-stable snow cover. Given above error estimations are valid if the HCHO layer is below the cloud base. The errors dramatically increase when HCHO layer penetrates into clouds in both cases. The first preliminary results of HCHO VCD retrieval for overcast are shown. The average difference of the HCHO VCDs for wind from Moscow megapolis and wind from few urbanized areas is about 0.8×1016 mol×cm-2 and approximately corresponds to estimates of influence of Moscow megapolis observed in clear-sky conditions.

Measurements of formaldehyde total content in troposphere using DOAS technique in Moscow Region: preliminary results of three year observations

Measurements of the formaldehyde (HCHO) atmospheric column are performed at Zvenigorod Scientific Station, Moscow Region, Russia since 2008 by the MAX-DOAS instrument. A previously developed algorithm for the formaldehyde retrieval was updated by adding an availability to use information on the surface albedo and the height of the atmospheric boundary layer provided by other measurements and/or modeling. We present preliminary results of the analysis of observations performed in 2010-2012. The obtained data allow quantifying the Moscow megapolis influence on air quality at Zvenigorod. The average HCHO vertical column density observed at the east winds is larger than one at the west winds. The Moscow influence causes the difference of about 0.85×1016 mol cm-2 between these values. This difference slightly depends on the air temperature and the season. A temperature effect is noticeable in the formaldehyde atmospheric column. Our data show statistically significant positive temperature effect in formaldehyde for the background and polluted conditions for temperatures from –5°C to +35°C. The temperature trend in formaldehyde data at Zvenigorod varies between 7.5×1014 and 9.3×1014 mol cm-2 °C-1 for all wind directions. The increase of the formaldehyde atmospheric column with the increase of the air temperature can be caused by the HCHO formation from non-methane biogenic volatile organic compounds (mainly - isoprene) for which more emission is expected at higher temperatures, and by growth of areas of forest and turf fires.

Selection of optical model of stereophotography experiment for determination the cloud base height as a problem of testing of statistical hypotheses

Earlier, we developed a method for estimating the height and speed of clouds from cloud images obtained by a pair of digital cameras. The shift of a fragment of the cloud in the right frame relative to its position in the left frame is used to estimate the height of the cloud and its velocity. This shift is estimated by the method of the morphological analysis of images. However, this method requires that the axes of the cameras are parallel. Instead of real adjustment of the axes, we use virtual camera adjustment, namely, a transformation of a real frame, the result of which could be obtained if all the axes were perfectly adjusted. For such adjustment, images of stars as infinitely distant objects were used: on perfectly aligned cameras, images on both the right and left frames should be identical. In this paper, we investigate in more detail possible mathematical models of cloud image deformations caused by the misalignment of the axes of two cameras, as well as their lens aberration. The simplest model follows the paraxial approximation of lens (without lens aberrations) and reduces to an affine transformation of the coordinates of one of the frames. The other two models take into account the lens distortion of the 3rd and 3rd and 5th orders respectively. It is shown that the models differ significantly when converting coordinates near the edges of the frame. Strict statistical criteria allow choosing the most reliable model, which is as much as possible consistent with the measurement data. Further, each of these three models was used to determine parameters of the image deformations. These parameters are used to provide cloud images to mean what they would have when measured using an ideal setup, and then the distance to cloud is calculated. The results were compared with data of a laser range finder.

Effects of aerosol phase function and other atmospheric parameters in radiometric calibration of hyperspectral visible/NIR satellite instruments above test sites of different altitudes

To verify data obtained by a satellite instrument a systematic calibration of the instrument is carried out. In addition to an internal calibration using on–board lamp or reflected solar radiation, the external calibration based on a comparison of radiance measurements above special ground test sites and calculated radiances is often employed. Radiances at the top of the atmosphere can be calculated using a radiative transfer model basing on measurement of the atmospheric properties and surface characteristics at the test sites. External calibration of hyperspectral instrument is sensitive to the spectral structure of absorbing and scattering of atmospheric species and, as a consequence, has a specific spectral structure of errors. We compared theoretical errors of a satellite hyperspectral instrument radiometric calibration using two test sites one of which is located in downcountry at 200 m a.s.l. and another one in highlands at 2000 m a.s.l. We suppose that both stations are equipped by the same set of instruments for measurements of the properties of the atmosphere and surface reflectance. The aerosol vertical profile and the aerosol phase function are supposed as not measured characteristics. The analysis is performed for an instrument with the spectral resolution of 1-8 nm which is typical for special regime of payload GSA of Russian satellite Resurs-P. The errors related with the atmospheric composition (including possible scenarios of the aerosol phase function and the aerosol vertical profile) and albedo measurement errors were theoretically examined. The errors strongly depend on aerosol loading. In case of low aerosol loading (corresponding to aerosol optical depth of 0.1 at 0 m a.s.l.) errors are less than 10% at both sites for all the wavelengths between 400 nm and 1000 nm with the exception of the absorption band of water vapor about 950 nm, where errors reach 35% at downcountry and 14% at highlands. For aerosol optical depth of 1 at 0 m a.s.l. the errors can reach 45% at downcountry and 18% at highlands.

First experiment on retrieval of tropospheric NO2 over polluted areas with 2.4-km spatial resolution basing on satellite spectral measurements

Three satellites of the Resurs-P series (№1, №2, №3) aimed for remote sensing of the Earth began to operate in Russia in 2013-2016. Hyperspectral instruments GSA onboard Resurs-P perform routine imaging of the Earth surface in the spectral range of 400-1000 nm with the spectral resolution better than 10 nm and the spatial resolution of 30 m. In a special regime the GSA/Resurs-P may reach higher spectral resolution with the spatial resolution of 120 m and be used for retrieval of the tropospheric NO2 spatial distribution. We developed the first GSA/Resurs-P algorithm for the tropospheric NO2 retrieval and shortly analyze the first results for the most polluted Hebei province of China. The developed GSA/Resurs-P algorithm shows the spatial resolution of about 2.4 km for tropospheric NO2 pollution what significantly exceed resolution of other available now satellite instruments and considered as a target for future geostationary (GEO) missions for monitoring of tropospheric NO2 pollution. Differ to the currently operated low-Earth orbit (LEO) instruments, which may provide global distribution of NO2 every one or two days, GSA performs NO2 measurement on request. The precision of the NO2 measurements with 2.4 km resolution is about 2.5x1015 mol/cm2 (for DSCD) therefore it is recommended to use it for investigation of the tropospheric NO2 in polluted areas. Thus GSA/Resurs-P is the interesting and unique tool for NO2 pollution investigations and testing methods of interpretation of future high-resolution satellite data on pollutions and their emissions.

Study of different operational modes of the IAP 2-port-DOAS instrument for atmospheric trace gases investigation during CINDI-2 campaign basing on residual noise analysis

An instrument for measuring atmospheric trace gases by DOAS method using scattered solar radiation was developed in A.M.Obukhov IAP RAS. The instrument layout is based on the lab Shamrock 303i spectrograph supplemented by 2-port radiation input system employing optical fiber. Optical ports may be used with a telescope with fixed field of view or with a scanning MAX-DOAS unit. MAX-DOAS unit port will be used for investigation of gas contents and profiles in the low troposphere. In September 2016 the IAP instrument participated in the CINDI-2 campaign, held in the Netherlands. CINDI 2 (2nd Cabauw Intercomparison of Nitrogen Dioxide Measuring Instruments) involves about 40 instruments quasi-synchronously performing DOAS measurements of NO2 and other trace gases. During the campaign the instrument ports had telescopes A and B with similar field of view of about 0.3°. Telescope A was always directed to the zenith. Telescope B was directed at 5° elevation angle. Two gratings were installed in the spectrometer. They provide different spectral resolution (FWHM ~0.4 and 0.8 nm respectively) and spectral window width (~70 and ~140 nm respectively). During CINDI-2 campaign we performed test measurements in UV and visible wavelength ranges to investigate instrument stability and retrieval errors of NO2 and HCHO contents. We perform the preliminary error analysis of retrieval of the NO2 and HCHO differential slant column densities using spectra measured in four modes of the instrument basing on residual noise analysis in this paper. It was found that rotation of grating turret does not significantly affected on quality of NO2 DSCD retrieval from spectra which measured in visible spectral region. Influence of grating turret rotation is much more significant for gas DSCD retrieval from spectra which measured in UV spectral region. Standard deviation of retrieval error points to presence of some systematic error.