Atmosphere - Quality

A comparative analysis of satellite and ground-based spectroscopic measurements of CO and CH₄ total content (CO TC) in the atmosphere in the background and polluted conditions (stations of OIAP RAS and NDACC) for the 2010-2015 time-period.

The significant correlation between satellite and ground-based CO TC data for all satellite sensors in background conditions was obtained. Also the empirical private transient relationships between satellite CO MOPITT v6 Joint, AIRS v6, IASI MeTop-A products and the data of solar-tracking ground-based spectrometers are analyzed.

The satellite IASI and AIRS v6 CO TC products underestimate CO from 1.5 to 3.8 times in polluted conditions (wild fires and Beijing megapolis). The correlation of daily operating satellite data with ground-based CO ones increases significantly under excluding of days with low height of planetary boundary layer PBL (less than 500 meters for middle latitudes sites and 200 m for high altitudes) from the comparison. This result was obtained for both gases, for all of the selected sensors and measuring sites. To improve the quality and representativeness of  satellite data on atmospheric pollution CO, we recommend excluding of days with a low PBL height from analysis of spatial and temporal variations and subsequent calculations, especially for assessing the emissions from high-power surface sources (as example, wild fires and urban emissions).

Best correlation (R²≥0.5) in diurnal CH₄ TC with ground-based data was found for AIRS v6.

This work has supported by the Russian Scientific Foundation under grant №14-47-00049 and partially by the Russian Foundation for Basic Research (grant № 13-05-41395).

Error estimates of measurements of atmospheric state variables are essential to judge whether differences between estimates of the atmospheric state can be explained or if they hint at unknown problems. While many recipes exist to calculate the error budget of an observation, the data user is faced with the problem that errors reported by various instrument groups are rarely consistent. In this talk, the following problems will be tackled: (a) which error components should be included in the error budget; (b) how should the content of prior information in the retrievals be dealt with; (c) how can retrievals be characterized which were not produced via an optimal-estimation-like formalism and thus do not provide the usual diagnostics; (d) how can error estimates which were generated using different approaches be made comparable, and (e) what is to be considered when comparing with direct (non-remote) measurements. An action plan towards unified error reporting will be suggested.

Since more than fifteen years, total and tropospheric NO₂ columns have been retrieved from nadir space-borne sensors such as SCIAMACHY on ENVISAT, OMI on AURA and GOME-2 on MetOp platforms. The NO₂ data products are generally retrieved in three main steps: (1) a DOAS spectral analysis yielding the total column amount of NO₂ along the slant optical path, (2) an estimation of the stratospheric NO₂ column, to be subtracted from the total column to derive the tropospheric contribution, and (3) a conversion of the total and tropospheric slant columns into vertical columns based on airmass factor calculations which require a-priori knowledge of the NO₂ vertical distribution and surface albedo, as well as information on cloud cover and height.

In this study we combine correlative measurements available from several ground-based remote sensing networks to address the validation of (1) the GOME-2 GDP 4.8 NO₂ products generated within the EUMETSAT O3M-SAF project, and (2) the SCIAMACHY, OMI and GOME-2 TEMIS product. Zenith-sky DOAS/SAOZ measurements from the NDACC network are used to assess the stratospheric NO₂ columns retrieved from the satellite, while the consistency of the total and tropospheric NO₂ columns in urban, sub-urban and back-ground conditions is investigated using direct-sun Pandora and multi-axis MAXDOAS data sets from about 40 stations. Where available, vertical profile information from MAXDOAS measurements is used to assess the reliability of the different satellite a-priori profile shapes.

Results are discussed in terms of observed biases between satellite and ground-based data sets, their dependence on location, season and cloud conditions. For stratospheric columns, the uncertainty related to the correction applied for ensuring the photochemical matching between satellite and ground-based observations is also evaluated. The satellite pixels resolution effect is statistically explored in relation to the typical extent of the emission sources at urban site locations, using data from SCIAMACHY 60x30km², GOME-2 40x80km² and OMI 13x24km².

Nitrogen oxides, NO_x (NO_x = NO + NO₂) play a key role in tropospheric chemistry. In addition to their directly harmful effects on the respiratory system of living organisms, they influence the levels of tropospheric ozone and contribute to acid rain and eutrophication of ecosystems. As they are produced in combustion processes, they can serve as an indicator for anthropogenic air pollution.

In the late summers of 2014 and 2015, two large measurement campaigns were conducted in Romania by several European research institutes, with financial support from ESA. The AROMAT / AROMAT-2 campaigns (Airborne ROmanian Measurements of Aerosols and Trace gases) were dedicated to measurements of air quality parameters utilizing newly developed instrumentation at state-of-the-art. The experiences gained, will help to calibrate and validate the measurements taken by the upcoming Sentinel-S5p mission in 2017.

The IUP Bremen contributed to these campaigns with its airborne imaging DOAS (Differential Optical Absorption Spectroscopy) instrument: AirMAP. AirMAP stands for Airborne Measurements of Atmospheric Pollutants. AirMAP allows retrieving spatial distributions of trace gas columns densities in a stripe below the aircraft. The measurements have a high spatial resolution of approximately 30 x 80 m² at a typical flight altitude of 3000 m. Due to the instrumental setup and the large swath, gapless maps of trace gas distributions above a large city, like Bucharest or Berlin, can be acquired within a time window of approximately two hours. These properties make AirMAP a valuable tool for the validation of trace gas measurements from space.

In addition to to the urban NO₂ distributions, NO₂ and SO₂ were measured in the plume of the Turceni power plant, which is an isolated point source in a rural part of Romania.

In this work the results of the research flights will be presented. The study will focus on the validation of AirMAP’s measurements themselves by comparison to other ground-based platforms like (mobile) MAX-DOAS measurements. Conclusions will be drawn on the quality of the measurements, their applicability for satellite data validation and possible improvements for future measurements.

Total column water vapour (TCWV), or its equivalent product of wet tropospheric correction (WTC), is a very important product not only from the meteorological point of view but also from satellite measurements point of view. It is needed for the correction of satellite products that are affected by the existence of the water vapour in the atmosphere like radar altimetry measurements. Furthermore, it can be used as a diagnostic to improve the performances of the various sensors to enable the provision of high quality data products.

Microwave Radiometer (MWR), which is part of the altimetry suite onboard ERS-1, ERS-2, Envisat and the forthcoming Sentinel-3 provides long-term continuous observation time series TCWV (and cloud liquid water content). Similarly, ERS-1/2 ATSR and Envisat AATSR can provide a long-term time series of (infrared) observations of TCWV which can be even extended with Sentinel-3. ESA is carrying out several projects, namely ESA LTDP EMiR and ESA LTDP ALTS projects, to provide high quality TCWV data sets from both types of instruments. The product, which was generated using pure physical retrieval algorithm without any empirical adjustment, is of high quality. Being reliable within 1 km from land, the product suits coastal zone applications.

Once the data sets are made available by ESA, they will be assessed and their characteristics will be studied and highlighted.