Five Years of Spatially Resolved Ground-Based MAX-DOAS Measurements of Nitrogen Dioxide in the Urban Area of Athens: Synergies with In Situ Measurements and Model Simulations

Long-term nitrogen dioxide (NO2) slant column density measurements using the MAX-DOAS (multi-axis differential optical absorption spectroscopy) technique were analyzed in order to demonstrate the temporal and horizontal variability of the trace gas in Athens for the period October 2012–July 2017. The synergy with in situ measurements and model simulations was exploited for verifying the MAX-DOAS technique and its ability to assess the spatiotemporal characteristics of NO2 pollution in the city. Tropospheric NO2 columns derived from ground-based MAX-DOAS observations in two horizontal and five vertical viewing directions were compared with in situ chemiluminescence measurements representative of urban, urban background and suburban conditions; a satisfactory correlation was found for the urban (r ≈ 0.55) and remote areas (r ≈ 0.40). Mean tropospheric slant columns retrieved from measurements at the lowest elevation over the urban area ranged from 0.1 to 32 × 1016 molec cm−2. The interannual variability showed a rate of increase of 0.3 × 1016 molec cm−2 per year since 2012 in the urban area, leading to a total increase of 20%. The retrieved annual cycles captured the seasonal variability with lower NO2 levels in summer, highly correlated (r ≈ 0.85) with the urban background and suburban in situ observations. The NO2 diurnal variation for different seasons exhibited varied patterns, indicating the different role of photochemistry and anthropogenic activities in the different seasons. Compared to in situ observations, the MAX-DOAS NO2 morning peak occurred with a one-hour delay and decayed less steeply in winter. Measurements at different elevation angles are shown as a primary indicator of the vertical distribution of NO2 at the urban environment; the vertical convection of the polluted air masses and the enhanced NO2 near-surface concentrations are demonstrated by this analysis. The inhomogeneity of the NO2 spatial distribution was shown using a relevant inhomogeneity index; greater variability was found during the summer period. Comparisons with city-scale model simulations demonstrated that the horizontal light path length of MAX-DOAS covered a distance of 15 km. An estimation of urban sources’ contribution was also made by applying two simple methodologies on the MAX-DOAS measurements. The results were compared to NO2 predictions from the high resolution air quality model to infer the importance of vehicle emissions for the urban NO2 levels; 20–35% of the urban NO2 was found to be associated with road transport

Εφαρμογή μεθόδων τηλεπισκόπισης για τη μελέτη της χωροχρονικής κατανομής ατμοσφαιρικών ρύπων στο λεκανοπέδιο Αττικής

The adverse health effects of urban air pollution on a large proportion of the world population living in urban environments, along with its impact on the climate, results to continuous, low-cost air monitoring and measurement validations being a necessity. This study is focused on the investigation of the temporal and spatial variation of certain atmospheric pollutants in the greater area of Athens, mainly using ground-based remote sensing techniques and in particular, the differential optical absorption spectroscopy (DOAS) technique. The visible and near-UV molecular absorption bands were used in this study for the DOAS measurements. The atmospheric pollutants that were measured with a Multi-Axis (MAX) DOAS instrument, are: nitrogen dioxide (NO2), formaldehyde (HCHO) and glyoxal (CHOCHO) and a detailed temporal and spatial distribution of these trace gases is provided; a consistent picture of the daily, weekly and seasonal variations is given. Oxygen dimer (O4) measurements were also used as input information in a retrieval algorithm for the calculation of the vertical distribution of the aerosol extinction. Lidar and sun-photometer measurements were used for the evaluation of the retrieved profiles. The MAX-DOAS technique proved to be a reliable method for measuring aerosol levels and their vertical distribution in the urban environment of Athens, thus, new perspectives have opened up for assessing urban aerosol pollution on a long term-basis in Athens from continuous and uninterrupted MAX-DOAS measurements. During the present study, the need to investigate the effect of the economic crisis to the air quality of the city has also risen: the increased price of heating oil resulted in an increase of biomass combustion for heating purposes, which in turn has led to severe smog episodes. Therefore, the combined effect of the reduced fossil fuel consumption and the increasing biomass combustion, as inferred from long-term carbon monoxide (CO) measurements and in conjunction with black carbon measurements, was examined. The importance of this study lies on the fact that it quantifies the diachronic changes in air pollution in Athens and associates it with several socioeconomic factors (e.g. measures, economic crisis, behavioural changes), while it can also be considered as a reference in the future for assessing and evaluating expected mitigation measures.Οι δυσμενείς επιπτώσεις της αστικής ατμοσφαιρικής ρύπανσης στην υγεία σε μεγάλο ποσοστό του παγκόσμιου πληθυσμού που ζει σε αστικά περιβάλλοντα, σε συνδυασμό με τις κλιματικές επιπτώσεις της ατμοσφαιρικής ρύπανσης, καθιστούν αναγκαία τη συνεχή, χαμηλού κόστους παρακολούθηση της ποιότητας της ατμόσφαιρας. Η παρούσα διατριβή επικεντρώνεται στην εφαρμογή επίγειων μεθόδων τηλεπισκόπησης για τη μελέτη της χωροχρονικής κατανομής ατμοσφαιρικών ρύπων στο λεκανοπέδιο της Αττικής και έχει βασιστεί κατά κύριο λόγο στη διαφορική οπτική φασματοσκοπία απορρόφησης (DOAS-Differential Optical Absorption Spectroscopy) διάχυτης ηλιακής ακτινοβολίας. Η συγκεκριμένη τεχνική βασίζεται στις μοριακές δομές απορρόφησης στο ορατό και εγγύς υπεριώδες του ηλεκτρομαγνητικού φάσματος. Οι αέριοι ρύποι που μελετήθηκαν στα πλαίσια της εργασίας με το σύστημα DOAS πολλαπλών διευθύνσεων (Multi Axis, MAX-DOAS) είναι το διοξείδιο του αζώτου (ΝΟ2), η φορμαλδεϋδη (HCHO) και η γλυοξάλη (CHOCHO). Έγινε επίσης υπολογισμός της κατακόρυφης κατανομής των αιωρούμενων σωματιδίων με χρήση αλγορίθμου και μετρήσεις διμερούς του οξυγόνου (Ο4), καθώς και εκτίμηση της αξιοπιστίας των υπολογισμών αυτών μέσω σύγκρισής τους με αντίστοιχα αποτελέσματα συστήματος lidar και ηλιακού φωτομέτρου. Συνολικά, δείχθηκε ότι το επίγειο σύστημα MAX-DOAS είναι ικανό να παρέχει αυτόνομες μετρήσεις ρουτίνας για τη μελέτη της χωροχρονικής κατανομής αερολυμάτων και αερίων ρύπων που επηρεάζουν την αστική φωτοχημεία.Τέλος, διερευνήθηκε η επίδραση της οικονομικής κρίσης των τελευταίων ετών στα επίπεδα της αστικής ρύπανσης: μελετήθηκε η επίπτωση της μειωμένης χρήσης ορυκτών καυσίμων και η παράλληλη αύξηση της χρήσης βιομάζας σε μακροχρόνιες μετρήσεις μονοξειδίου του άνθρακα (CO) σε συνδυασμό με μετρήσεις μαύρου άνθρακα

Five Years of Spatially Resolved Ground-Based MAX-DOAS Measurements of Nitrogen Dioxide in the Urban Area of Athens: Synergies with In Situ Measurements and Model Simulations

Long-term nitrogen dioxide (NO2) slant column density measurements using the MAX-DOAS (multi-axis differential optical absorption spectroscopy) technique were analyzed in order to demonstrate the temporal and horizontal variability of the trace gas in Athens for the period October 2012–July 2017. The synergy with in situ measurements and model simulations was exploited for verifying the MAX-DOAS technique and its ability to assess the spatiotemporal characteristics of NO2 pollution in the city. Tropospheric NO2 columns derived from ground-based MAX-DOAS observations in two horizontal and five vertical viewing directions were compared with in situ chemiluminescence measurements representative of urban, urban background and suburban conditions; a satisfactory correlation was found for the urban (r ≈ 0.55) and remote areas (r ≈ 0.40). Mean tropospheric slant columns retrieved from measurements at the lowest elevation over the urban area ranged from 0.1 to 32 × 1016 molec cm−2. The interannual variability showed a rate of increase of 0.3 × 1016 molec cm−2 per year since 2012 in the urban area, leading to a total increase of 20%. The retrieved annual cycles captured the seasonal variability with lower NO2 levels in summer, highly correlated (r ≈ 0.85) with the urban background and suburban in situ observations. The NO2 diurnal variation for different seasons exhibited varied patterns, indicating the different role of photochemistry and anthropogenic activities in the different seasons. Compared to in situ observations, the MAX-DOAS NO2 morning peak occurred with a one-hour delay and decayed less steeply in winter. Measurements at different elevation angles are shown as a primary indicator of the vertical distribution of NO2 at the urban environment; the vertical convection of the polluted air masses and the enhanced NO2 near-surface concentrations are demonstrated by this analysis. The inhomogeneity of the NO2 spatial distribution was shown using a relevant inhomogeneity index; greater variability was found during the summer period. Comparisons with city-scale model simulations demonstrated that the horizontal light path length of MAX-DOAS covered a distance of 15 km. An estimation of urban sources’ contribution was also made by applying two simple methodologies on the MAX-DOAS measurements. The results were compared to NO2 predictions from the high resolution air quality model to infer the importance of vehicle emissions for the urban NO2 levels; 20–35% of the urban NO2 was found to be associated with road transport

Ground-based validation of the Copernicus Sentinel-5P TROPOMI NO2 measurements with the NDACC ZSL-DOAS, MAX-DOAS and Pandonia global networks

This paper reports on consolidated ground-based validation results of the atmospheric NO2 data produced operationally since April 2018 by the TROPOspheric Monitoring Instrument (TROPOMI) on board of the ESA/EU Copernicus Sentinel-5 Precursor (S5P) satellite. Tropospheric, stratospheric, and total NO2 column data from S5P are compared to correlative measurements collected from, respectively, 19 Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS), 26 Network for the Detection of Atmospheric Composition Change (NDACC) Zenith-Scattered-Light DOAS (ZSL-DOAS), and 25 Pandonia Global Network (PGN)/Pandora instruments distributed globally. The validation methodology gives special care to minimizing mismatch errors due to imperfect spatio-temporal co-location of the satellite and correlative data, e.g. by using tailored observation operators to account for differences in smoothing and in sampling of atmospheric structures and variability and photochemical modelling to reduce diurnal cycle effects. Compared to the ground-based measurements, S5P data show, on average, (i) a negative bias for the tropospheric column data, of typically −23 % to −37 % in clean to slightly polluted conditions but reaching values as high as −51 % over highly polluted areas; (ii) a slight negative median difference for the stratospheric column data, of about −0.2 Pmolec cm−2, i.e. approx. −2 % in summer to −15 % in winter; and (iii) a bias ranging from zero to −50 % for the total column data, found to depend on the amplitude of the total NO2 column, with small to slightly positive bias values for columns below 6 Pmolec cm−2 and negative values above. The dispersion between S5P and correlative measurements contains mostly random components, which remain within mission requirements for the stratospheric column data (0.5 Pmolec cm−2) but exceed those for the tropospheric column data (0.7 Pmolec cm−2). While a part of the biases and dispersion may be due to representativeness differences such as different area averaging and measurement times, it is known that errors in the S5P tropospheric columns exist due to shortcomings in the (horizontally coarse) a priori profile representation in the TM5-MP chemical transport model used in the S5P retrieval and, to a lesser extent, to the treatment of cloud effects and aerosols. Although considerable differences (up to 2 Pmolec cm−2 and more) are observed at single ground-pixel level, the near-real-time (NRTI) and offline (OFFL) versions of the S5P NO2 operational data processor provide similar NO2 column values and validation results when globally averaged, with the NRTI values being on average 0.79 % larger than the OFFL values

Ground-based validation of the Copernicus Sentinel-5p TROPOMI NO₂ measurements with the NDACC ZSL-DOAS, MAX-DOAS and Pandonia global networks

International audienceThis paper reports on consolidated ground-based validation results of the atmospheric NO2 data produced operationally since April 2018 by the TROPOMI instrument on board of the ESA/EU Copernicus Sentinel-5 Precursor (S5p) satellite. Tropospheric, stratospheric, and total NO2 column data from S5p are compared to correlative measurements collected from, respectively, 19 Multi-Axis DOAS (MAX-DOAS), 26 NDACC Zenith-Scattered-Light DOAS (ZSL-DOAS), and 25 PGN/Pandora instruments distributed globally. The validation methodology gives special care to minimizing mismatch errors due to imperfect spatio-temporal co-location of the satellite and correlative data, e.g., by using tailored observation operators to account for differences in smoothing and in sampling of atmospheric structures and variability, and photochemical modelling to reduce diurnal cycle effects. Compared to the ground-based measurements, S5p data show, on an average: (i) a negative bias for the tropospheric column data, of typically −23 to −37 % in clean to slightly polluted conditions, but reaching values as high as −51 % over highly polluted areas; (ii) a slight negative bias for the stratospheric column data, of about −0.2 Pmolec/cm2, i.e. approx. −2 % in summer to −15 % in winter; and (iii) a bias ranging from zero to −50 % for the total column data, found to depend on the amplitude of the total NO2 column, with small to slightly positive bias values for columns below 6 Pmolec/cm2 and negative values above. The dispersion between S5p and correlative measurements contains mostly random components, which remain within mission requirements for the stratospheric column data (0.5 Pmolec/cm2), but exceed those for the tropospheric column data (0.7 Pmolec/cm2). While a part of the biases and dispersion may be due to representativeness differences, it is known that errors in the S5p tropospheric columns exist due to shortcomings in the (horizontally coarse) a-priori profile representation in the TM5-MP chemistry transport model used in the S5p retrieval, and to a lesser extent, to the treatment of cloud effects. Although considerable differences (up to 2 Pmolec/cm2 and more) are observed at single ground-pixel level, the near-real-time (NRTI) and off-line (OFFL) versions of the S5p NO2 operational data processor provide similar NO2 column values and validation results when globally averaged, with the NRTI values being on average 0.79 % larger than the OFFL values