Browsing by Author "Abuhassan, Nader"
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Item Analysis of the performance characteristics of the five-channel Microtops II Sun photometer for measuring aerosol optical thickness and precipitable water vapor(AGU, 2002-07-12) Ichoku, Charles; Levy, Robert; Kaufman, Yoram J.; Remer, Lorraine; Li, Rong-Rong; Martins, Vanderlei J.; Holben, Brent N.; Abuhassan, Nader; Slutsker, Ilya; Eck, Thomas; Pietras, ChristopheFive Microtops II Sun photometers were studied in detail at the NASA Goddard Space Flight Center (GSFC) to determine their performance in measuring aerosol optical thickness (AOT or τaλ) and precipitable column water vapor (W). Each derives τaλ from measured signals at four wavelengths λ (340, 440, 675, and 870 nm), and W from the 936 nm signal measurements. Accuracy of τaλ and W determination depends on the reliability of the relevant channel calibration coefficient (V0). Relative calibration by transfer of parameters from a more accurate Sun photometer (such as the Mauna-Loa-calibrated AERONET master Sun photometer at GSFC) is more reliable than Langley calibration performed at GSFC. It was found that the factory-determined value of the instrument constant for the 936 nm filter (k = 0.7847) used in the Microtops' internal algorithm is unrealistic, causing large errors in V₀(₉₃₆), τₐ₉₃₆, and W. Thus, when applied for transfer calibration at GSFC, whereas the random variation of V0 at 340 to 870 nm is quite small, with coefficients of variation (CV) in the range of 0 to 2.4%, at 936 nm the CV goes up to 19%. Also, the systematic temporal variation of V0 at 340 to 870 nm is very slow, while at 936 nm it is large and exhibits a very high dependence on W. The algorithm also computes τa936 as 0.91 τa870, which is highly simplistic. Therefore, it is recommended to determine τa936 by logarithmic extrapolation from τₐ₆₇₅ and τₐ₈₇₀. From the operational standpoint of the Microtops, apart from errors that may result from unperceived cloud contamination, the main sources of error include inaccurate pointing to the Sun, neglecting to clean the front quartz window, and neglecting to calibrate correctly. If these three issues are adequately taken care of, the Microtops can be quite accurate and stable, with root-mean-square (rms) differences between corresponding retrievals from clean calibrated Microtops and the AERONET Sun photometer being about ±0.02 at 340 nm, decreasing down to about ±0.01 at 870 nm.Item Atmospheric NO₂ dynamics and impact on ocean color retrievals in urban nearshore regions(AGU, 2014-06-03) Tzortziou, Maria; Herman, Jay; Ahmad, Ziauddin; Loughner, Christopher P.; Abuhassan, Nader; Cede, AlexanderUrban nearshore regions are characterized by strong variability in atmospheric composition, associated with anthropogenic emissions and meteorological processes that influence the circulation and accumulation of atmospheric pollutants at the land-water interface. If not adequately corrected in satellite retrievals of ocean color, this atmospheric variability can impose a false impression of diurnal and seasonal changes in nearshore water quality and biogeochemical processes. Consideration of these errors is important for measurements from polar orbiting ocean color sensors but becomes critical for geostationary satellite missions having the capability for higher frequency and higher spatial resolution observations of coastal ocean dynamics. We examined variability in atmospheric NO₂ over urban nearshore environments in the Eastern US, Europe, and Korea, using a new network of ground-based Pandora spectrometers and Aura-OMI satellite observations. Our measurements in the US and in Europe revealed clear diurnal and day-of-the-week patterns in total column NO₂ (TCNO₂), temporal changes as large as 0.8 DU within 4 h, and spatial variability as large as 0.7 DU within an area often covered by just a single OMI pixel. TCNO₂ gradients were considerably stronger over the coastal cities of Korea. With a coarse resolution and an overpass at around 13:30 local time, OMI cannot detect this strong variability in NO₂, missing pollution peaks from industrial and rush hour activities. Observations were combined with air quality model simulations and radiative transfer calculations to estimate the impact of atmospheric NO₂ variability on satellite retrievals of coastal ocean remote sensing reflectance and biogeochemical variables (i.e., chlorophyll and CDOM).Item Atmospheric Trace Gas (NO₂ and O₃) Variability in South Korean Coastal Waters, and Implications for Remote Sensing of Coastal Ocean Color Dynamics(MDPI, 2018-10-03) Tzortziou, Maria; Parker, Owen; Lamb, Brian; Herman, Jay; Lamsal, Lok; Stauffer, Ryan; Abuhassan, NaderCoastal environments are highly dynamic, and are characterized by short-term, local-scale variability in atmospheric and oceanic processes. Yet, high-frequency measurements of atmospheric composition, and particularly nitrogen dioxide (NO₂) and ozone (O₃) dynamics, are scarce over the ocean, introducing uncertainties in satellite retrievals of coastal ocean biogeochemistry and ecology. Combining measurements from different platforms, the Korea-US Ocean Color and Air Quality field campaign provided a unique opportunity to capture, for the first time, the strong spatial dynamics and diurnal variability in total column (TC) NO₂ and O₃ over the coastal waters of South Korea. Measurements were conducted using a shipboard Pandora Spectrometer Instrument specifically designed to collect accurate, high-frequency observations from a research vessel, and were combined with ground-based observations at coastal land sites, synoptic satellite imagery, and air-mass trajectory simulations to assess source contributions to atmospheric pollution over the coastal ocean. TCO₃ showed only small (<20%) variability that was driven primarily by larger-scale meteorological processes captured successfully in the relatively coarse satellite imagery from Aura-OMI. In contrast, TCNO₂ over the ocean varied by more than an order of magnitude (0.07–0.92 DU), mostly affected by urban emissions and highly dynamic air mass transport pathways. Diurnal patterns varied widely across the ocean domain, with TCNO₂ in the coastal area of Geoje and offshore Seoul varying by more than 0.6 DU and 0.4 DU, respectively, over a period of less than 3 h. On a polar orbit, Aura-OMI is not capable of detecting these short-term changes in TCNO₂. If unaccounted for in atmospheric correction retrievals of ocean color, the observed variability in TCNO₂ would be misinterpreted as a change in ocean remote sensing reflectance, Rrs, by more than 80% and 40% at 412 and 443 nm, respectively, introducing a significant false variability in retrievals of coastal ocean ecological processes from space.Item The Cabauw Intercomparison campaign for Nitrogen Dioxide measuring Instruments (CINDI): design, execution, and early results(EGU, 2012-02-27) Piters, A. J. M.; Boersma, K. F.; Kroon, M.; Hains, J. C.; Abuhassan, Nader; Cede, A.; Herman, Jay; et alFrom 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 within 25% of one another, and MAX-DOAS aerosol optical thickness agrees within 20–30% with AERONET data. For the in-situ NO₂ instrument using a molybdenum converter, a bias was found as large as 5 ppbv during day time, when compared to the other in-situ instruments using photolytic converters.Item Changes in the surface broadband shortwave radiation budget during the 2017 eclipse(EGU, 2020-09-09) Wen, Guoyong; Marshak, Alexander; Tsay, Si-Chee; Herman, Jay; Jeong, Ukkyo; Abuhassan, Nader; Swap, Robert; Wu, DongWhile solar eclipses are known to greatly diminish the visible radiation reaching the surface of the Earth, less is known about the magnitude of the impact. We explore both the observed and modeled levels of change in surface radiation during the eclipse of 2017. We deployed a pyranometer and Pandora spectrometer instrument to Casper, Wyoming, and Columbia, Missouri, to measure surface broadband shortwave (SW) flux and atmospheric properties during the 21 August 2017 solar eclipse event. We performed detailed radiative transfer simulations to understand the role of clouds in spectral and broadband solar radiation transfer in the Earth's atmosphere for the normal (non-eclipse) spectrum and red-shift solar spectra for eclipse conditions. The theoretical calculations showed that the non-eclipse-to-eclipse surface flux ratio depends strongly on the obscuration of the solar disk and slightly on the cloud optical depth. These findings allowed us to estimate what the surface broadband SW flux would be for hypothetical non-eclipse conditions from observations during the eclipse and further to quantify the impact of the eclipse on the surface broadband SW radiation budget. We found that the eclipse caused local reductions of time-averaged surface flux of about 379 W m⁻² (50 %) and 329 W m⁻² (46 %) during the ∼3 h course of the eclipse at the Casper and Columbia sites, respectively. We estimated that the Moon's shadow caused a reduction of approximately 7 %–8 % in global average surface broadband SW radiation. The eclipse has a smaller impact on the absolute value of surface flux reduction for cloudy conditions than a clear atmosphere; the impact decreases with the increase in cloud optical depth. However, the relative time-averaged reduction of local surface SW flux during a solar eclipse is approximately 45 %, and it is not sensitive to cloud optical depth. The reduction of global average SW flux relative to climatology is proportional to the non-eclipse and eclipse flux difference in the penumbra area and depends on cloud optical depth in the Moon's shadow and geolocation due to the change in solar zenith angle. We also discuss the influence of cloud inhomogeneity on the observed SW flux. Our results not only quantify the reduction of the surface solar radiation budget, but also advance the understanding of broadband SW radiative transfer under solar eclipse conditions.Item Comparison of Near-Surface NO₂ Pollution With Pandora Total Column NO₂ During the Korea-United States Ocean Color (KORUS OC) Campaign(AGU, 2019-11-12) Thompson, Anne M.; Stauffer, Ryan M.; Boyle, Tyler P.; Kollonige, Debra E.; Miyazaki, Kazuyuki; Tzortziou, Maria; Herman, Jay; Abuhassan, Nader; Jordan, Carolyn E.; Lamb, Brian T.Near‐surface air quality (AQ) observations over coastal waters are scarce, a situation that limitsour capacity to monitor pollution events at land‐water interfaces. Satellite measurements of total column(TC) nitrogen dioxide (NO₂) observations are a useful proxy for combustion sources, but the once dailysnapshots available from most sensors are insufficient for tracking the diurnal evolution and transport ofpollution. Ground‐based remote sensors like the Pandora Spectrometer Instrument (PSI) that have beendeveloped to verify space‐based TC NO₂and other trace gases are being tested for routine use as certifiedAQ monitors. The KORUS‐OC (Korea‐United States Ocean Color) cruise aboard the R/VOnnuriinMay–June 2016 represented an opportunity to study AQ near the South Korean coast, a region affected byboth local/regional and long‐distance pollution sources. Using PSI data in direct‐Sun mode and in situsensors for shipboard ozone, CO, and NO₂, we explore, for thefirst time, relationships between TC NO₂andsurface AQ in this coastal region. Three case studies illustrate the value of the PSI and complexities in thesurface‐column NO₂relationship caused by varying meteorological conditions. Case Study 1(25–26 May 2016) exhibited a high correlation of surface NO₂to TC NO₂measured by both PSI and Aura'sOzone Monitoring Instrument, but two other cases displayed poor relationships between in situ andTC NO₂due to decoupling of pollution layers from the surface. With suitable interpretation the PSI TC NO₂measurement demonstrates good potential for working with upcoming geostationary satellites toadvance diurnal tracking of pollution.Item Comparison of ozone retrievals from the Pandora spectrometer system and Dobson spectrophotometer in Boulder, Colorado(EGU, 2015-08-24) Herman, Jay; Evans, R.; Cede, A.; Abuhassan, Nader; Petropavlovskikh, I.; McConville, G.A comparison of retrieved total column ozone (TCO) amounts between the Pandora #34 spectrometer system and the Dobson #061 spectrophotometer from directsun observations was performed on the roof of the Boulder, Colorado, NOAA building. This paper, part of an ongoing study, covers a 1-year period starting on 17 December 2013. Both the standard Dobson and Pandora TCO retrievals required a correction, TCOcorr = TCO (1 + C(T )), using a monthly varying effective ozone temperature, TE, derived from a temperature and ozone profile climatology. The correction is used to remove a seasonal difference caused by using a fixed temperature in each retrieval algorithm. The respective corrections C(TE) are CPandora = 0.00333(TE−225) and CDobson = −0.0013(TE − 226.7) per degree K. After the applied corrections removed most of the seasonal retrieval dependence on ozone temperature, TCO agreement between the instruments was within 1 % for clear-sky conditions. For clear-sky observations, both co-located instruments tracked the day-to-day variation in total column ozone amounts with a correlation of r² = 0.97 and an average offset of 1.1 ± 5.8 DU. In addition, the Pandora TCO data showed 0.3 % annual average agreement with satellite overpass data from AURA/OMI (Ozone Monitoring Instrument) and 1 % annual average offset with Suomi-NPP/OMPS (Suomi National Polar-orbiting Partnership, the nadir viewing portion of the Ozone Mapper Profiler Suite).Item Evaluating the impact of spatial resolution on tropospheric NO2 column comparisons within urban areas using high-resolution airborne data(EGU, 2019-11-22) Judd, Laura M.; Al-Saadi, Jassim A.; Janz, Scott J.; Kowalewski, Matthew G.; Pierce, R. Bradley; Szykman, James J.; Valin, Lukas C.; Swap, Robert; Cede, Alexander; Mueller, Moritz; Tiefengraber, Martin; Abuhassan, Nader; Williams, DavidNASA deployed the GeoTASO airborne UV–visible spectrometer in May–June 2017 to produce high-resolution (approximately 250 m×250 m) gapless NO₂ datasets over the western shore of Lake Michigan and over the Los Angeles Basin. The results collected show that the airborne tropospheric vertical column retrievals compare well with ground-based Pandora spectrometer column NO₂ observations (r²=0.91 and slope of 1.03). Apparent disagreements between the two measurements can be sensitive to the coincidence criteria and are often associated with large local variability, including rapid temporal changes and spatial heterogeneity that may be observed differently by the sunward-viewing Pandora observations. The gapless mapping strategy executed during the 2017 GeoTASO flights provides data suitable for averaging to coarser areal resolutions to simulate satellite retrievals. As simulated satellite pixel area increases to values typical of TEMPO (Tropospheric Emissions: Monitoring Pollution), TROPOMI (TROPOspheric Monitoring Instrument), and OMI (Ozone Monitoring Instrument), the agreement with Pandora measurements degraded, particularly for the most polluted columns as localized large pollution enhancements observed by Pandora and GeoTASO are spatially averaged with nearby less-polluted locations within the larger area representative of the satellite spatial resolutions (aircraft-to-Pandora slope: TEMPO scale =0.88; TROPOMI scale =0.77; OMI scale =0.57). In these two regions, Pandora and TEMPO or TROPOMI have the potential to compare well at least up to pollution scales of 30×10¹⁵ molecules cm⁻². Two publicly available OMI tropospheric NO₂ retrievals are found to be biased low with respect to these Pandora observations. However, the agreement improves when higher-resolution a priori inputs are used for the tropospheric air mass factor calculation (NASA V3 standard product slope =0.18 and Berkeley High Resolution product slope =0.30). Overall, this work explores best practices for satellite validation strategies with Pandora direct-sun observations by showing the sensitivity to product spatial resolution and demonstrating how the high-spatial-resolution NO₂ data retrieved from airborne spectrometers, such as GeoTASO, can be used with high-temporal-resolution ground-based column observations to evaluate the influence of spatial heterogeneity on validation results.Item The first evaluation of formaldehyde column observations by improved Pandora spectrometers during the KORUS-AQ field study(EGU, 2018-08-31) Spinei, Elena; Whitehill, Andrew; Fried, Alan; Tiefengraber, Martin; Herman, Jay; Abuhassan, Nader; et alThe Korea–United States Air Quality Study (KORUS-AQ) conducted during May–June 2016 offered the first opportunity to evaluate direct-sun observations of formaldehyde (HCHO) total column densities with improved Pandora spectrometer instruments. The measurements highlighted in this work were conducted both in the Seoul megacity area at the Olympic Park site (37.5232◦ N, 27.1260◦ E; 26 m a.s.l.) and at a nearby rural site downwind of the city at the Mount Taehwa research forest site (37.3123◦ N, 127.3106◦ E; 160 m a.s.l.). Evaluation of these measurements was made possible by concurrent ground-based in situ observations of HCHO at both sites as well as overflight by the NASA DC-8 research aircraft. The flights provided in situ measurements of HCHO to characterize its vertical distribution in the lower troposphere (0–5 km). Diurnal variation in HCHO total column densities followed the same pattern at both sites, with the minimum daily values typically observed between 6:00 and 7:00 local time, gradually increasing to a maximum between 13:00 and 17:00 before decreasing into the evening. Pandora vertical column densities were compared with those derived from the DC-8 HCHO in situ measured profiles augmented with in situ surface concentrations below the lowest altitude of the DC-8 in proximity to the ground sites. A comparison between 49 column densities measured by Pandora vs. aircraft-integrated in situ data showed that Pandora values were larger by 16 % with a constant offset of 0.22 DU (Dobson units; R 2 = 0.68). Pandora HCHO columns were also compared with columns calculated from the surface in situ measurements over Olympic Park by assuming a wellmixed lower atmosphere up to a ceilometer-measured mixedlayer height (MLH) and various assumptions about the small residual HCHO amounts in the free troposphere up to the tropopause. The best comparison (slope = 1.03±0.03; intercept = 0.29±0.02 DU; and R 2 = 0.78±0.02) was achieved assuming equal mixing within ceilometer-measured MLH combined with an exponential profile shape. These results suggest that diurnal changes in HCHO surface concentrations can be reasonably estimated from the Pandora total column and information on the mixed-layer height. More work is needed to understand the bias in the intercept and the slope relative to columns derived from the in situ aircraft and surface measurements.Item High precision, absolute total column ozone measurements from the Pandora spectrometer system: Comparisons with data from a Brewer double monochromator and Aura OMI(AGU, 2012-08-22) Tzortziou, Maria; Herman, Jay; Cede, Alexander; Abuhassan, NaderWe present new, high precision, high temporal resolution measurements of total column ozone (TCO) amounts derived from ground-based direct-sun irradiance measurements using our recently deployed Pandora single-grating spectrometers. Pandora's small size and portability allow deployment at multiple sites within an urban air-shed and development of a ground-based monitoring network for studying small-scale atmospheric dynamics, spatial heterogeneities in trace gas distribution, local pollution conditions, photochemical processes and interdependencies of ozone and its major precursors. Results are shown for four mid- to high-latitude sites where different Pandora instruments were used. Comparisons with a well calibrated double-grating Brewer spectrometer over a period of more than a year in Greenbelt MD showed excellent agreement and a small bias of approximately 2 DU (or, 0.6%). This was constant with slant column ozone amount over the full range of observed solar zenith angles (15–80°), indicating adequate Pandora stray light correction. A small (1–2%) seasonal difference was found, consistent with sensitivity studies showing that the Pandora spectral fitting TCO retrieval has a temperature dependence of 1% per 3°K, with an underestimation in temperature (e.g., during summer) resulting in an underestimation of TCO. Pandora agreed well with Aura-OMI (Ozone Measuring Instrument) satellite data, with average residuals of <1% at the different sites when the OMI view was within 50 km from the Pandora location and OMI-measured cloud fraction was <0.2. The frequent and continuous measurements by Pandora revealed significant short-term (hourly) temporal changes in TCO, not possible to capture by sun-synchronous satellites, such as OMI, alone.Item Intercomparison of slant column measurements of NO₂ and O₄ by MAX-DOAS and zenith-sky UV and visible spectrometers(EGU, 2010-11-23) Roscoe, H. K.; Roozendael, M. Van; Fayt, C.; Piesanie, A. du; Abuhassan, Nader; Cede, A.; Herman, Jay; et alIn 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.Item Langley Calibration Analysis of Solar Spectroradiometric Measurements: Spectral Aerosol Optical Thickness Retrievals(AGU, 2018-04-06) Jeong, Ukkyo; Tsay, Si-Chee; Pantina, Peter; Butler, James J.; Loftus, Adrian M.; Abuhassan, Nader; Herman, Jay; Dimov, Alexander; Holben, Brent N.; Swap, Robert J.Aerosol optical thickness (τaer) is a fundamental parameter for analyzing aerosol loading and associated radiative effects. The τaer can constrain many inversion algorithms using passive/active sensor measurements to retrieve other aerosol properties and/or the abundance of trace gases. In the next wave of spectroradiometric observations from geostationary platforms, we envision that a strategically distributed network of robust, well-calibrated ground-based spectroradiometers will comprehensively complement spaceborne measurements in spectral and temporal domains. Spectral τaer can be accurately obtained from direct-Sun measurements based on the Langley calibration method, which allows for the analysis of distinct spectral features of the calibration results. In this study, we present a spectral τaer retrieval algorithm for an in-house developed, field deployable spectroradiometer instrument covering wavelengths from ultraviolet to near-infrared (UV-Vis-NIR). The spectral total optical thickness obtained from the Langley calibration method is partitioned into molecular and particulate components by utilizing a least squares method. The resulting high temporal-resolution τaer and Ångström Exponent can be used effectively for cloud screening. The new algorithm was applied to month-long measurements acquired from the rooftop at National Aeronautics and Space Administration Goddard Space Flight Center’s Building 33. The retrieved τaer demonstrated excellent agreement with those from well-calibrated Aerosol Robotic Network Sun photometers at all overlapping wavelengths (correlation coefficients higher than 0.98). In addition, empirical stray light corrections considerably improved τaer retrievals at short wavelengths in the UV. The continuous spectrum of τaer from UV-Vis-NIR spectroradiometers is expected to provide more informative constraints for retrieval of additional aerosol properties such as refractive indices, size, and bulk vertical distribution.Item New Observations of Upper Tropospheric NO₂ from TROPOMI(EGU Publications, 2020-10-08) Marais, Eloise A.; Roberts, John F.; Ryan, Robert G.; Eskes, Henk; Boersma, K. Folkert; Choi, Sungyeon; Joiner, Joanna; Abuhassan, Nader; Redondas, Alberto; Grutter, Michel; Cede, Alexander; Gomez, Laura; Navarro-Comas, MonicaNitrogen oxides (NOₓ ≡ NO + NO₂) in the NOₓ-limited upper troposphere (UT) are long-lived and so have a large influence on the oxidizing capacity of the troposphere and formation of the greenhouse gas ozone. Models misrepresent NOₓ in the UT and observations to address deficiencies in models are sparse. Here we obtain a year of near-global seasonal mean mixing ratios of NO₂ in the UT (450–180 hPa) at 1 ° x 1° by applying cloud-slicing to partial columns of NO₂ from TROPOMI. This follows refinement of the cloud-slicing algorithm with synthetic partial columns from the GEOS-Chem chemical transport model. We find that synthetic cloud-sliced UT NO₂ are spatially consistent (R = 0.64) with UT NO₂ calculated across the same cloud pressure range and scenes as are cloud-sliced (“true” UT NO₂), but the cloud-sliced UT NO₂ is 11–22 % more than the "true" all-sky seasonal mean. The largest contributors to differences between synthetic cloud-sliced and “true” UT NO₂ are target resolution of the cloud-sliced product and uniformity of overlying stratospheric NO₂. TROPOMI, prior to cloud-slicing, is corrected for a 13 % underestimate in stratospheric NO₂ variance and a 50 % overestimate in free tropospheric NO₂ determined by comparison to Pandora total columns at high-altitude sites in Mauna Loa, Izaña and Altzomoni, and MAX-DOAS and Pandora tropospheric columns at Izaña. Two cloud-sliced seasonal mean UT NO₂ products for June 2019 to May 2020 are retrieved from corrected TROPOMI total columns using distinct TROPOMI cloud products that assume clouds are reflective boundaries (FRESCO-S) or water droplet layers (ROCINN-CAL). TROPOMI UT NO₂ typically ranges from 20-30 pptv over remote oceans to > 80 pptv over locations with intense seasonal lightning. Spatial coverage is mostly in the tropics and subtropics with FRESCO-S and extends to the midlatitudes and polar regions with ROCINN-CAL, due to its greater abundance of optically thick clouds and wider cloud top altitude range. TROPOMI UT NO₂ seasonal means are spatially consistent (R = 0.6–0.8) with an existing coarser spatial resolution (5° latitude x 8° longitude) UT NO₂ product from the Ozone Monitoring Instrument (OMI). UT NO₂ from TROPOMI is 12–26 pptv more than that from OMI due to increase in NO₂ with altitude from the OMI pressure ceiling (280 hPa) to that for TROPOMI (180 hPa), but possibly also systematic altitude differences between the TROPOMI and OMI cloud products. The TROPOMI UT NO₂ product offers potential to evaluate and improve representation of UT NOₓ in models and supplement aircraft observations that are sporadic and susceptible to large biases in the UT.Item NO₂ and HCHO measurements in Korea from 2012 to 2016 from Pandora spectrometer instruments compared with OMI retrievals and with aircraft measurements during the KORUS-AQ campaign(EGU, 2018-08-08) Herman, Jay; Spinei, Elena; Fried, Alan; Kim, Jhoon; Kim, Jae; Kim, Woogyung; Cede, Alexander; Abuhassan, Nader; Segal-Rozenhaimer, MichalNine Pandora spectrometer instruments (PSI) were installed at eight sites in South Korea as part of the KORUS-AQ (Korea U.S.-Air Quality) field study integrating information from ground, aircraft, and satellite measurements for validation of remote sensing air-quality studies. The PSI made direct-sun measurements of total vertical column NO₂, C(NO₂), with high precision (0.05 DU, where 1 DU = 2.69 × 1016 molecules cm−2 ) and accuracy (0.1 DU) that were retrieved using spectral fitting techniques. Retrieval of formaldehyde C(HCHO) total column amounts were also obtained at five sites using the recently improved PSI optics. The C(HCHO) retrievals have high precision, but possibly lower accuracy than for NO₂ because of uncertainty about the optimum spectral window for all ground-based and satellite instruments. PSI direct-sun retrieved values for C(NO₂) and C(HCHO) are always significantly larger than OMI (AURA satellite Ozone Monitoring Instrument) retrieved C(NO₂) and C(HCHO) for the OMI overpass local times (KST = 13.5±0.5 h). In urban areas, PSI C(NO₂) 30- day running averages are at least a factor of two larger than OMI averages. Similar differences are seen for C(HCHO) in Seoul and nearby surrounding areas. Late afternoon values of C(HCHO) measured by PSI are even larger, implying that OMI early afternoon measurements underestimate the effect of poor air quality on human health. The primary cause of OMI underestimates is the large OMI field of view (FOV) that includes regions containing low values of pollutants. In relatively clean areas, PSI and OMI are more closely in agreement. C(HCHO) amounts were obtained for five sites, Yonsei University in Seoul, Olympic Park, Taehwa Mountain, Amnyeondo, and Yeoju. Of these, the largest amounts of C(HCHO) were observed at Olympic Park and Taehwa Mountain, surrounded by significant amounts of vegetation. Comparisons of PSI C(HCHO) results were made with the Compact Atmospheric Multispecies Spectrometer CAMS during overflights on the DC-8 aircraft for Taehwa Mountain and Olympic Park. In all cases, PSI measured substantially more C(HCHO) than obtained from integrating the CAMS altitude profiles. PSI C(HCHO) at Yonsei University in Seoul frequently reached 0.6 DU and occasionally exceeded 1.5 DU. The semi-rural site, Taehwa Mountain, frequently reached 0.9 DU and occasionally exceeded 1.5 DU. Even at the cleanest site, Amnyeondo, C(HCHO) occasionally exceeded 1 DU.Item NO₂ column amounts from ground-based Pandora and MFDOAS spectrometers using the direct-sun DOAS technique: Intercomparisons and application to OMI validation(AGU, 2009-07-15) Herman, Jay; Cede, Alexander; Spinei, Elena; Mount, George; Tzortziou, Maria; Abuhassan, NaderVertical column amounts of nitrogen dioxide, C(NO₂), are derived from ground-based direct solar irradiance measurements using two new and independently developed spectrometer systems, Pandora (Goddard Space Flight Center) and MFDOAS (Washington State University). We discuss the advantages of C(NO₂) retrievals based on Direct Sun - Differential Optical Absorption Spectroscopy (DS-DOAS). The C(NO₂) data are presented from field campaigns using Pandora at Aristotle University (AUTH), Thessaloniki, Greece; a second field campaign involving both new instruments at Goddard Space Flight Center (GSFC), Greenbelt, Maryland; a Pandora time series from December 2006 to October 2008 at GSFC; and a MFDOAS time series for spring 2008 at Pacific Northwest National Laboratory (PNNL), Richland, Washington. Pandora and MFDOAS were compared at GFSC and found to closely agree, with both instruments having a clear-sky precision of 0.01 DU (1 DU = 2.67 × 10¹⁶ molecules/cm²) and a nominal accuracy of 0.1 DU. The high precision is obtained from careful laboratory characterization of the spectrometers (temperature sensitivity, slit function, pixel to pixel radiometric calibration, and wavelength calibration), and from sufficient measurement averaging to reduce instrument noise. The accuracy achieved depends on laboratory-measured absorption cross sections and on spectrometer laboratory and field calibration techniques used at each measurement site. The 0.01 DU precision is sufficient to track minute-by-minute changes in C(NO₂) throughout each day with typical daytime values ranging from 0.2 to 2 DU. The MFDOAS instrument has better noise characteristics for a single measurement, which permits MFDOAS to operate at higher time resolution than Pandora for the same precision. Because Pandora and MFDOAS direct-sun measurements can be made in the presence of light to moderate clouds, but with reduced precision (∼0.2 DU for moderate cloud cover), a nearly continuous record can be obtained, which is important when matching OMI overpass times for satellite data validation. Comparisons between Pandora and MFDOAS with OMI are discussed for the moderately polluted GSFC site, between Pandora and OMI at the AUTH site, and between MFDOAS and OMI at the PNNL site. Validation of OMI measured C(NO₂) is essential for the scientific use of the satellite data for air quality, for atmospheric photolysis and chemistry, and for retrieval of other quantities (e.g., accurate atmospheric correction for satellite estimates of ocean reflectance and bio-optical properties). Changes in the diurnal variability of C(NO₂) with season and day of the week are presented based on the 2-year time series at GSFC measured by the Pandora instrument.Item Ozone comparison between Pandora #34, Dobson #061, OMI, and OMPS in Boulder, Colorado, for the period December 2013–December 2016(EGU, 2017-09-27) Herman, Jay; Evans, Robert; Cede, Alexander; Abuhassan, Nader; Petropavlovskikh, Irina; McConville, Glenn; Miyagawa, Koji; Noirot, BrandonA one-time-calibrated (in December 2013) Pandora spectrometer instrument (Pan #034) has been compared to a periodically calibrated Dobson spectroradiometer (Dobson #061) co-located in Boulder, Colorado, and compared with two satellite instruments over a 3-year period (December 2013–December 2016). The results show good agreement between Pan #034 and Dobson #061 within their statistical uncertainties. Both records are corrected for ozone retrieval sensitivity to stratospheric temperature variability obtained from the Global Modeling Initiative (GMI) and Modern-Era Retrospective analysis for Research and Applications (MERRA-2) model calculations. Pandora #034 and Dobson #061 differ by an average of 2.1 ± 3.2 % when both instruments use their standard ozone absorption cross sections in the retrieval algorithms. The results show a relative drift (0.2 ± 0.08 % yr ⁻¹ ) between Pandora observations against NOAA Dobson in Boulder, CO, over a 3-year period of continuous operation. Pandora drifts relative to the satellite Ozone Monitoring Instrument (OMI) and the Ozone Mapping Profiler Suite (OMPS) are +0.18 ± 0.2 % yr ⁻¹ and −0.18 ± 0.2 % yr ⁻¹ , respectively, where the uncertainties are 2 standard deviations. The drift between Dobson #061 and OMPS for a 5.5-year period (January 2012–June 2017) is −0.07 ± 0.06 % yr ⁻¹ .Item Reduction in 317–780 nm radiance reflected from the sunlit Earth during the eclipse of 21 August 2017(EGU, 2018-07-25) Herman, Jay; Wen, Guoyong; Marshak, Alexander; Blank, Karin; Huang, Liang; Cede, Alexander; Abuhassan, Nader; Kowalewski, MatthewTen wavelength channels of calibrated radiance image data from the sunlit Earth are obtained every 65 min during Northern Hemisphere summer from the EPIC (Earth Polychromatic Imaging Camera) instrument on the DSCOVR (Deep Space Climate Observatory) satellite located near the Earth–Sun Lagrange 1 point (L ₁), about 1.5 million km from the Earth. The L ₁ location permitted seven observations of the Moon’s shadow on the Earth for about 3 h during the 21 August 2017 eclipse. Two of the observations were timed to coincide with totality over Casper, Wyoming, and Columbia, Missouri. Since the solar irradiances within five channels (λi = 388, 443, 551, 680, and 780 nm) are not strongly absorbed in the atmosphere, they can be used for characterizing the eclipse reduction in reflected radiances for the Earth’s sunlit face containing the eclipse shadow. Five channels (λi = 317.5, 325, 340, 688, and 764 nm) that are partially absorbed in the atmosphere give consistent reductions compared to the non-absorbed channels. This indicates that cloud reflectivities dominate the 317.5–780 nm radiances reflected back to space from the sunlit Earth’s disk with a significant contribution from Rayleigh scattering for the shorter wavelengths. An estimated reduction of 10 % was obtained for spectrally integrated radiance (387 to 781 nm) reflected from the sunlit Earth towards L ₁ for two sets of observations on 21 August 2017, while the shadow was in the vicinity of Casper, Wyoming (42.8666◦ N, 106.3131◦ W; centered on 17:44:50 UTC), and Columbia, Missouri (38.9517◦ N, 92.3341◦ W; centered on 18:14:50 UTC). In contrast, when non-eclipse days (20 and 23 August) are compared for each wavelength channel, the change in reflected light is much smaller (less than 1 % for 443 nm compared to 9 % (Casper) and 8 % (Columbia) during the eclipse). Also measured was the ratio Rₑₙ (λi) of reflected radiance on adjacent non-eclipse days divided by radiances centered in the eclipse totality region with the same geometry for all 10 wavelength channels. The measured Rₑₙ(443 nm) was smaller for Columbia (169) than for Casper (935), because Columbia had more cloud cover than Casper. Rₑₙ (λi) forms a useful test of a 3-D radiative transfer models for an eclipse in the presence of optically thin clouds. Specific values measured at Casper with thin clouds are Rₑₙ(340 nm) = 475, Rₑₙ(388 nm) = 3500, Rₑₙ(443 nm) = 935, Rₑₙ(551 nm) = 5455, Rₑₙ(680 nm) = 220, and Rₑₙ(780 nm) = 395. Some of the variability is caused by changing cloud amounts within the moving region of totality during the 2.7 min needed to measure all 10 wavelength channels.Item The SMART‐s Trace Gas and Aerosol Inversions: I. Algorithm Theoretical Basis for Column Property Retrievals(AGU, 2020-03-12) Jeong, Ukkyo; Tsay, Si‐Chee; Giles, David M.; Holben, Brent N.; Swap, Robert J.; Abuhassan, Nader; Herman, JayThe SMART-s (Spectral Measurements for Atmospheric Radiative Transfer—spectroradiometer) acquires Sun/sky observations for retrieving optimal information on trace gases and aerosols with minimal assumptions. Overall, the algorithm of SMART-s incorporates a series of retrievals, from fundamental quantities (i.e., column abundance of trace gases and aerosol loading) to higher-order geophysical parameters (e.g., aerosol physicochemical properties and vertical profiles), utilizing Sun/sky spectral radiance measurements. This paper describes the theoretical basis for column retrievals of trace gases and aerosols. Associated profile retrievals will be presented in follow-up papers. The current algorithm retrieves the fine/coarse mode of the particle size distribution and spectral complex index of refraction and, thereby, the spectral aerosol single-scattering albedo ω0. SMART-s retrieval is unique particularly in its high spectral resolution of the complex index of refraction and ω0 from near-ultraviolet to near-infrared wavelengths, which is pivotal information for atmospheric chemistry, climate and other inversions. We theoretically assessed information content and retrieval accuracy of the algorithm and compared different type of measurements including the Aerosol Robotic Network (AERONET) and standard Pandora. For the same levels of radiometric accuracy, SMART-s measurements provide the most informative aerosol retrievals based on theoretical error analyses. Higher spectral resolution measurements are particularly beneficial for particle size distribution and fine-mode refractive index retrievals. We applied this algorithm to the AERONET Sun/sky measurements at Kanpur, India, in 2016 to assess algorithm consistency. Even with different assumptions and numerical methods for the inversion, SMART-s retrieved aerosol parameters agreed well with the AERONET operational products (e.g., absolute mean bias errors less than 0.01 for ω₀).Item Two Air Quality Regimes in Total Column NO₂ over the Gulf of Mexico in May 2019: Shipboard and Satellite Views(AGU, 2023-03-07) Thompson, Anne M.; Kollonige, Debra E.; Stauffer, Ryan M.; Kotsakis, Alexander E.; Abuhassan, Nader; Lamsal, Lok N.; Swap, Robert J.; Blake, Donald R.; Townsend-Small, Amy; Wecht, Holli D.The Satellite Coastal and Oceanic Atmospheric Pollution Experiment (SCOAPE) cruise in the Gulf of Mexico (GOM) was conducted in May 2019 by NASA and the Bureau of Ocean Energy Management to determine the feasibility of using satellite data to measure air quality (AQ) in a region of concentrated oil and natural gas (ONG) operations. SCOAPE featured nitrogen dioxide (NO₂ ) instrumentation (Pandora, Teledyne API analyzer) at Cocodrie, LA (29.26°, -90.66°), and on the Research Vessel Point Sur operating off the Louisiana coast with measurements of ozone, carbon monoxide (CO) and volatile organic compounds (VOC). The findings: (1) both satellite and Pandora NO₂ 1 observations revealed two AQ regimes over the GOM, the first influenced by tropical air in 10-14 May, the second influenced by flow from urban areas on 15-17 May; (2) Comparisons of OMI v4 and TROPOMI v1.3 TC (total column) NO₂ data with all Pandora NO₂ column observations on the Point Sur averaged 13% agreement with the largest difference during 15-17 May (~20%). At Cocodrie, LA, at the same time, the satellite-Pandora agreement was ~5%. (3) Three new-model Pandora instruments displayed a TC NO₂ precision of 0.01 Dobson Units (~5%); (4) Regions of smaller and older operations displayed high methane (CH4 ) readings, presumably from leakage; VOC were also detected at high concentrations. Given an absence of regular AQ data in and near the GOM, SCOAPE data constitute a baseline against which future observations can be compared.Item Underestimation of column NO₂ amounts from the OMI satellite compared to diurnally varying ground-based retrievals from multiple PANDORA spectrometer instruments(EGU, 2019-10-23) Herman, Jay; Abuhassan, Nader; Kim, Jhoon; Kim, Jae; Dubey, Manvendra; Raponi, Marcelo; Tzortziou, MariaRetrievals of total column NO₂ (TCNO₂) are compared for 14 sites from the Ozone Measuring Instrument (OMI using OMNO₂-NASA v3.1) on the AURA satellite and from multiple ground-based PANDORA spectrometer instruments making direct-sun measurements. While OMI accurately provides the daily global distribution of retrieved TCNO₂, OMI almost always underestimates the local amount of TCNO₂ by 50 % to 100 % in polluted areas, while occasionally the daily OMI value exceeds that measured by PANDORA at very clean sites. Compared to local ground-based or aircraft measurements, OMI cannot resolve spatially variable TCNO₂ pollution within a city or urban areas, which makes it less suitable for air quality assessments related to human health. In addition to systematic underestimates in polluted areas, OMI’s selected 13:30 Equator crossing time polar orbit causes it to miss the frequently much higher values of TCNO₂ that occur before or after the OMI overpass time. Six discussed Northern Hemisphere PANDORA sites have multi-year data records (Busan, Seoul, Washington DC, Waterflow, New Mexico, Boulder, Colorado, and Mauna Loa), and one site in the Southern Hemisphere (Buenos Aires, Argentina). The first four of these sites and Buenos Aires frequently have high TCNO₂ (TCNO₂ > 0.5 DU). Eight additional sites have shorter-term data records in the US and South Korea. One of these is a 1-year data record from a highly polluted site at City College in New York City with pollution levels comparable to Seoul, South Korea. OMI-estimated air mass factor, surface reflectivity, and the OMI 24 km × 13 km FOV (field of view) are three factors that can cause OMI to underestimate TCNO₂. Because of the local inhomogeneity of NOx emissions, the large OMI FOV is the most likely factor for consistent underestimates when comparing OMI TCNO₂ to retrievals from the small PANDORA effective FOV (measured in m2 ) calculated from the solar diameter of 0.5◦ .