Publication Search Results

ID: 158179
Type: article
Authors: Choi, Haklim; Liu, Xiong; Gonzalez Abad, Gonzalo; Seo, Jongjin; Lee, Kwang-Mog; Kim, Jhoon
Abstract: Clouds act as a major reflector that changes the amount of sunlight reflected to space. Change in radiance intensity due to the presence of clouds interrupts the retrieval of trace gas or aerosol properties from satellite data. In this paper, we developed a fast and robust algorithm, named the fast cloud retrieval algorithm, using a triplet of wavelengths (469, 477, and 485 nm) of the O-2-O-2 absorption band around 477 nm (CLDTO4) to derive the cloud information such as cloud top pressure (CTP) and cloud fraction (CF) for the Geostationary Environment Monitoring Spectrometer (GEMS). The novel algorithm is based on the fact that the difference in the optical path through which light passes with regard to the altitude of clouds causes a change in radiance due to the absorption of O-2-O-2 at the three selected wavelengths. To reduce the time required for algorithm calculations, the look-up table (LUT) method was applied. The LUT was pre-constructed for various conditions of geometry using Vectorized Linearized Discrete Ordinate Radiative Transfer (VLIDORT) to consider the polarization of the scattered light. The GEMS was launched in February 2020, but the observed data of GEMS have not yet been widely released. To evaluate the performance of the algorithm, the retrieved CTP and CF using observational data from the Global Ozone Monitoring Experiment-2 (GOME-2), which cover the spectral range of GEMS, were compared with the results of the Fast Retrieval Scheme for Clouds from the Oxygen A band (FRESCO) algorithm, which is based on the O-2 A-band. There was good agreement between the results, despite small discrepancies for low clouds.

ID: 157568
Type: article
Authors: Bak, Juseon; Liu, Xiong; Birk, Manfred; Wagner, Georg; Gordon, Iouli E.; Chance, Kelly
Abstract: We evaluate different sets of high-resolution ozone absorption cross-section data for use in atmospheric ozone profile measurements in the Hartley and Huggins bands with a particular focus on BDM 1995 (Daumont et al. 1992; Brion et al., 1993; Malicet et al., 1995), currently used in our retrievals, and a new laboratory dataset by Birk and Wagner (2018) (BW). The BDM cross-section data have been recommended to use for retrieval of ozone profiles using spaceborne nadir-viewing backscattered ultraviolet (BUV) measurements since its improved performance was demonstrated against other cross-sections including Bass and Paur (1985) (BP) and those of Serdyuchenko et al. (2014) and Gorshelev et al. (2014) (SER) by the "Absorption Cross-Sections of Ozone" (ACSO) activity. The BW laboratory data were recently measured within the framework of the European Space Agency (ESA) project SEOM-IAS (Scientific Exploitation of Operational Missions - Improved Atmospheric Spectroscopy Databases) to provide an advanced absorption cross-section database. The BW cross-sections are made from measurements at more temperatures and in a wider temperature range than BDM, especially for low temperatures. Relative differences of cross-sections between BW and BDM range from similar to 2% at shorter UV wavelengths to similar to 5% at longer UV wavelengths at warm temperatures. Furthermore, these differences dynamically increase by up to +/- 40% at cold temperatures due to no BDM measurements having been made below 218 K. We evaluate the impact of using different cross-sections on ozone profile retrievals from Ozone Monitoring Instrument (OMI) measurements. Correspondingly, this impact leads to significant differences in individual ozone retrievals by up to 50% in the tropopause where the coldest atmospheric temperatures are observed. Bottom atmospheric layers illustrate the significant change of the retrieved ozone values, with differences of 20% in low latitudes, which is not the case in high latitudes because the ozone retrievals are mainly controlled by a priori ozone information in high latitudes due to less photon penetration down to the lower troposphere. Validation with ozonesonde observations demonstrates that BW and BDM retrievals show altitude-dependent bias oscillations of similar magnitude relative to ozonesonde measurements, much smaller than those of both BP and SER retrievals. However, compared to BDM, BW retrievals show significant reduction in standard deviation, by up to 15 %, especially at the coldest atmospheric temperatures. Such improvement is achieved mainly by the better characterization of the temperature dependence of ozone absorption.

ID: 158888
Type: article
Authors: Kang, Mina; Ahn, Myoung-Hwan; Liu, Xiong; Jeong, Ukkyo; Kim, Jhoon

ID: 155672
Type: article
Authors: Kim, Jhoon; Jeong, Ukkyo; Ahn, Myoung-Hwan; Kim, Jae H.; Park, Rokjin J.; Lee, Hanlim; Song, Chul Han; Choi, Yong-Sang; Lee, Kwon-Ho; Yoo, Jung-Moon; Jeong, Myeong-Jae; Park, Seon Ki; Lee, Kwang-Mog; Song, Chang-Keun; Kim, Sang-Woo; Kim, Young Joon; Kim, Si-Wan; Kim, Mijin; Go, Sujung; Liu, Xiong; Chance, Kelly; Chan Miller, Christopher; Al-Saadi, Jay; Veihelmann, Ben; Bhartia, Pawan K.; Torres, Omar; González-Abad, Gonzalo; Haffner, David P.; Ko, Dai Ho; Lee, Seung Hoon; Woo, Jung-Hun; Chong, Heesung; Park, Sang Seo; Nicks, Dennis; Choi, Won Jun; Moon, Kyung-Jung; Cho, Ara; Yoon, Jongmin; Kim, Sang-kyun; Hong, Hyunkee; Lee, Kyunghwa; Lee, Hana; Lee, Seoyoung; Choi, Myungje; Veefkind, Pepijn; Levelt, Pieternel F.; Edwards, David P.; Kang, Mina; Eo, Mijin; Bak, Juseon; Baek, Kanghyun; Kwon, Hyeong-Ahn; Yang, Jiwon; Park, Junsung; Han, Kyung Man; Kim, Bo-Ram; Shin, Hee-Woo; Choi, Haklim; Lee, Ebony; Chong, Jihyo; Cha, Yesol; Koo, Ja-Ho; Irie, Hitoshi; Hayashida, Sachiko; Kasai, Yasko; Kanaya, Yugo; Liu, Cheng; Lin, Jintai; Crawford, James H.; Carmichael, Gregory R.; Newchurch, Michael J.; Lefer, Barry L.; Herman, Jay R.; Swap, Robert J.; Lau, Alexis K. H.; Kurosu, Thomas P.; Jaross, Glen; Ahlers, Berit; Dobber, Marcel; McElroy, C. Thomas; Choi, Yunsoo
Abstract: The Geostationary Environment Monitoring Spectrometer (GEMS) is scheduled for launch in February 2020 to monitor air quality (AQ) at an unprecedented spatial and temporal resolution from a geostationary Earth orbit (GEO) for the first time. With the development of UV-visible spectrometers at sub-nm spectral resolution and sophisticated retrieval algorithms, estimates of the column amounts of atmospheric pollutants (O₃, NO₂, SO₂, HCHO, CHOCHO, and aerosols) can be obtained. To date, all the UV-visible satellite missions monitoring air quality have been in low Earth orbit (LEO), allowing one to two observations per day. With UV-visible instruments on GEO platforms, the diurnal variations of these pollutants can now be determined. Details of the GEMS mission are presented, including instrumentation, scientific algorithms, predicted performance, and applications for air quality forecasts through data assimilation. GEMS will be on board the Geostationary Korea Multi-Purpose Satellite 2 (GEO- KOMPSAT-2) satellite series, which also hosts the Advanced Meteorological Imager (AMI) and Geostationary Ocean Color Imager 2 (GOCI-2). These three instruments will provide synergistic science products to better understand air quality, meteorology, the long-range transport of air pollutants, emission source distributions, and chemical processes. Faster sampling rates at higher spatial resolution will increase the probability of finding cloud-free pixels, leading to more observations of aerosols and trace gases than is possible from LEO. GEMS will be joined by NASA's Tropospheric Emissions: Monitoring of Pollution (TEMPO) and ESA's Sentinel-4 to form a GEO AQ satellite constellation in early 2020s, coordinated by the Committee on Earth Observation Satellites (CEOS).

ID: 157462
Type: article
Authors: Lu, Xiao; Zhang, Lin; Wu, Tongwen; Long, Michael S.; Wang, Jun; Jacob, Daniel J.; Zhang, Fang; Zhang, Jie; Eastham, Sebastian D.; Hu, Lu; Zhu, Lei; Liu, Xiong; Wei, Min
Abstract: Chemistry plays an indispensable role in investigations of the atmosphere; however, many climate models either ignore or greatly simplify atmospheric chemistry, limiting both their accuracy and their scope. We present the development and evaluation of the online global atmospheric chemical model BCC-GEOS-Chem v1.0, coupling the GEOS-Chem chemical transport model (CTM) as an atmospheric chemistry component in the Beijing Climate Center atmospheric general circulation model (BCC-AGCM). The GEOS-Chem atmospheric chemistry component includes detailed tropospheric HO_x-NO_x-volatile organic compounds-ozone-bromine-aerosol chemistry and online dry and wet deposition schemes. We then demonstrate the new capabilities of BCC-GEOS-Chem v1.0 relative to the base BCC-AGCM model through a 3-year (2012-2014) simulation with anthropogenic emissions from the Community Emissions Data System (CEDS) used in the Coupled Model Intercomparison Project Phase 6 (CMIP6). The model captures well the spatial distributions and seasonal variations in tropospheric ozone, with seasonal mean biases of 0.4-2.2 ppbv at 700-400 hPa compared to satellite observations and within 10 ppbv at the surface to 500 hPa compared to global ozonesonde observations. The model has larger high-ozone biases over the tropics which we attribute to an overestimate of ozone chemical production. It underestimates ozone in the upper troposphere which is likely due either to the use of a simplified stratospheric ozone scheme or to biases in estimated stratosphere-troposphere exchange dynamics. The model diagnoses the global tropospheric ozone burden, OH concentration, and methane chemical lifetime to be 336 Tg, 1.16×10⁶ molecule cm^-3, and 8.3 years, respectively, which is consistent with recent multimodel assessments. The spatiotemporal distributions of NO₂, CO, SO₂, CH₂O, and aerosol optical depth are generally in agreement with satellite observations. The development of BCC-GEOS-Chem v1.0 represents an important step for the development of fully coupled earth system models (ESMs) in China.

ID: 158080
Type: article
Authors: Souri, Amir H.; Nowlan, Caroline R.; González Abad, Gonzalo; Zhu, Lei; Blake, Donald R.; Fried, Alan; Weinheimer, Andrew J.; Wisthaler, Armin; Woo, Jung-Hun; Zhang, Qiang; Miller, Christopher E. Chan; Liu, Xiong; Chance, Kelly
Abstract: The absence of up-to-date emissions has been a major impediment to accurately simulating aspects of atmospheric chemistry and to precisely quantifying the impact of changes in emissions on air pollution. Hence, a nonlinear joint analytical inversion (Gauss-Newton method) of both volatile organic compounds (VOCs) and nitrogen oxide (NO_x) emissions is made by exploiting the Smithsonian Astrophysical Observatory (SAO) Ozone Mapping and Profiler Suite Nadir Mapper (OMPS-NM) formaldehyde (HCHO) and the National Aeronautics and Space Administration (NASA) Ozone Monitoring Instrument (OMI) tropospheric nitrogen dioxide (NO₂) columns during the Korea-United States Air Quality (KORUS-AQ) campaign over East Asia in May-June 2016. Effects of the chemical feedback of NO_x and VOCs on both NO₂ and HCHO are implicitly included by iteratively optimizing the inversion. Emission uncertainties are greatly narrowed (averaging kernels > 0.8, which is the mathematical presentation of the partition of information gained from the satellite observations with respect to the prior knowledge) over medium- to high-emitting areas such as cities and dense vegetation. The prior amount of total NO_x emissions is mainly dictated by values reported in the MIX-Asia 2010 inventory. After the inversion we conclude that there is a decline in emissions (before, after, change) for China (87.94±44.09 Gg d^-1, 68.00±15.94 Gg d^-1, -23 %), North China Plain (NCP) (27.96±13.49 Gg d^-1, 19.05±2.50 Gg d^-1, -32 %), Pearl River Delta (PRD) (4.23±1.78 Gg d^-1, 2.70±0.32 Gg d^-1, -36 %), Yangtze River Delta (YRD) (9.84±4.68 Gg d^-1, 5.77±0.51 Gg d^-1, -41 %), Taiwan (1.26±0.57 Gg d^-1, 0.97±0.33 Gg d^-1, -23 %), and Malaysia (2.89±2.77 Gg d^-1, 2.25±1.34 Gg d^-1, -22 %), all of which have effectively implemented various stringent regulations. In contrast, South Korea (2.71±1.34 Gg d^-1, 2.95±0.58 Gg d^-1, +9 %) and Japan (3.53±1.71 Gg d^-1, 3.96±1.04 Gg d^-1, +12 %) are experiencing an increase in NO_x emissions, potentially due to an increased number of diesel vehicles and new thermal power plants. We revisit the well-documented positive bias (by a factor of 2 to 3) of MEGAN v2.1 (Model of Emissions of Gases and Aerosols from Nature) in terms of biogenic VOC emissions in the tropics. The inversion, however, suggests a larger growth of VOCs (mainly anthropogenic) over NCP (25 %) than previously reported (6 %) relative to 2010. The spatial variation in both the magnitude and sign of NO_x and VOC emissions results in nonlinear responses of ozone production and loss. Due to a simultaneous decrease and increase in NO_x/VOC over NCP and YRD, we observe a ∼53 % reduction in the ratio of the chemical loss of NO_x (LNO_x) to the chemical loss of RO_x (RO₂+HO₂) over the surface transitioning toward NO_x-sensitive regimes, which in turn reduces and increases the afternoon chemical loss and production of ozone through NO₂+OH (-0.42 ppbv h^-1)/HO₂ (and RO₂)+NO (+0.31 ppbv h^-1). Conversely, a combined decrease in NO_x and VOC emissions in Taiwan, Malaysia, and southern China suppresses the formation of ozone. Simulations using the updated emissions indicate increases in maximum daily 8 h average (MDA8) surface ozone over China (0.62 ppbv), NCP (4.56 ppbv), and YRD (5.25 ppbv), suggesting that emission control strategies on VOCs should be prioritized to curb ozone production rates in these regions. Taiwan, Malaysia, and PRD stand out as regions undergoing lower MDA8 ozone levels resulting from the NO_x reductions occurring predominantly in NO_x-sensitive regimes.

ID: 157460
Type: article
Authors: Souri, Amir H.; Nowlan, Caroline R.; Wolfe, Glenn M.; Lamsal, Lok N.; Chan Miller, Christopher E.; González Abad, Gonzalo; Janz, Scott J.; Fried, Alan; Blake, Donald R.; Weinheimer, Andrew J.; Diskin, Glenn S.; Liu, Xiong; Chance, Kelly
Abstract: The authors regret both Fig. S2 and Fig. S3 were inadvertently used a wrong day (06/10 instead of 06/09 due to a difference in the UTC vs local times) for plotting the NASA's LaRC box model output. The new figure shows that both F0AM and LaRC models are in a strong degree of agreement (<10%) suggesting that the non-linear ozone chemistry over Seoul can be reasonably represented by both models. This correction does not impact the conclusion drawn from this study. We thank James H. Crawford for pointing us at the bug.

ID: 157311
Type: article
Authors: Souri, Amir H.; Wang, Huiqun; González Abad, Gonzalo; Liu, Xiong; Chance, Kelly
Abstract: The primary focus of this study is to understand the contribution from excess moisture from crop transpiration to the severity of a heat wave episode that hit the Midwestern U.S. from 16 to 20 July 2011. To elucidate this, we first provide an optimal estimate of the transpiration water vapor flux using satellite total column water vapor retrievals whose accuracy and precision are characterized using independent observations. The posterior transpiration flux is estimated using a local ensemble transform Kalman filter that employs a mesoscale weather model as the forward model. The new estimation suggests that the prior values of transpiration flux from crops are biased high by 15%. We further use the constrained flux to examine the sensitivity of meteorology to the contributions from crops. Over the agricultural areas during daytime, elevated moisture (up to 40%) from crops not only increases humidity (thus the heat index) but also provides a positive radiative forcing by increasing downward longwave radiation (13 ± 4 W m^-2) that results in even higher surface air temperature (+0.4 °C). Consequently, we find that the elevated moisture generally provides positive feedback to aggravate the heat wave, with daytime enhancements of heat index by as large as 3.3 ± 0.8 °C. Due to a strong diurnal cycle in the transpiration, the feedback tends to be stronger in the afternoon (up to 5 °C) and weaker at night. Results offer a potential basis for designing mitigation strategies for the effect of transpiration from agriculture in the future, in addition to improving the estimation of canopy transpiration.

ID: 156335
Type: article
Authors: Souri, Amir Hossein; Nowlan, Caroline R.; Wolfe, Glenn M.; Lamsal, Lok N.; Chan Miller, Christopher E.; Abad, Gonzalo González; Janz, Scott J.; Fried, Alan; Blake, Donald R.; Weinheimer, Andrew J.; Diskin, Glenn S.; Liu, Xiong; Chance, Kelly
Abstract: The nonlinear chemical processes involved in ozone production (P(O₃)) have necessitated using proxy indicators to convey information about the primary dependence of P(O₃) on volatile organic compounds (VOCs) or nitrogen oxides (NO_x). In particular, the ratio of remotely sensed columns of formaldehyde (HCHO) to nitrogen dioxide (NO₂) has been widely used for studying O₃ sensitivity. Previous studies found that the errors in retrievals and the incoherent relationship between the column and the near-surface concentrations are a barrier in applying the ratio in a robust way. In addition to these obstacles, we provide calculational- observational evidence, using an ensemble of 0-D photochemical box models constrained by DC-8 aircraft measurements on an ozone event during the Korea-United States Air Quality (KORUS-AQ) campaign over Seoul, to demonstrate the chemical feedback of NO₂ on the formation of HCHO is a controlling factor for the transition line between NO_x-sensitive and NO_x-saturated regimes. A fixed value (~2.7) of the ratio of the chemical loss of NO_x (LNO_x) to the chemical loss of HO₂+RO₂ (LRO_x) perceptibly differentiates the regimes. Following this value, data points with a ratio of HCHO/NO₂ less than 1 can be safely classified as NO_x-saturated regime, whereas points with ratios between 1 and 4 fall into one or the other regime. We attribute this mainly to the HCHO-NO₂ chemical relationship causing the transition line to occur at larger (smaller) HCHO/NO₂ ratios in VOC-rich (VOC-poor) environments. We then redefine the transition line to LNO_x/LRO_x~2.7 that accounts for the HCHO-NO₂ chemical relationship leading to HCHO = 3.7 × (NO₂ - 1.14 × 10¹⁶ molec.cm^-2). Although the revised formula is locally calibrated (i.e., requires for readjustment for other regions), its mathematical format removes the need for having a wide range of thresholds used in HCHO/NO₂ ratios that is a result of the chemical feedback. Therefore, to be able to properly take the chemical feedback into consideration, the use of HCHO = a × (NO₂ - b) formula should be preferred to the ratio in future works. We then use the Geostationary Trace gas and Aerosol Sensor Optimization (GeoTASO) airborne instrument to study O₃ sensitivity in Seoul. The unprecedented spatial (250 × 250 m²) and temporal (~every 2 h) resolutions of HCHO and NO₂ observations form the sensor enhance our understanding of P(O₃) in Seoul; rather than providing a crude label for the entire city, more in-depth variabilities in chemical regimes are observed that should be able to inform mitigation strategies correspondingly.

ID: 154401
Type: article
Authors: Bak, Juseon; Baek, Kang-Hyeon; Kim, Jae-Hwan; Liu, Xiong; Kim, Jhoon; Chance, Kelly
Abstract: The Geostationary Environment Monitoring Spectrometer (GEMS) is scheduled to be launched in 2019-2020 on board the GEO-KOMPSAT (GEOstationary KOrea Multi-Purpose SATellite)-2B, contributing as the Asian partner of the global geostationary constellation of air quality monitoring. To support this air quality satellite mission, we perform a cross-evaluation of simulated GEMS ozone profile retrievals from OMI (Ozone Monitoring Instrument) data based on the optimal estimation and ozonesonde measurements within the GEMS domain, covering from 5^∘ S (Indonesia) to 45^∘ N (south of the Russian border) and from 75 to 145^∘ E. The comparison between ozonesonde and GEMS shows a significant dependence on ozonesonde types. Ozonesonde data measured by modified Brewer-Mast (MBM) at Trivandrum and New Delhi show inconsistent seasonal variabilities in tropospheric ozone compared to carbon-iodine (CI) and electrochemical condensation cell (ECC) ozonesondes at other stations in a similar latitude regime. CI ozonesonde measurements are negatively biased relative to ECC measurements by 2-4 DU; better agreement is achieved when simulated GEMS ozone retrievals are compared to ECC measurements. ECC ozone data at Hanoi, Kuala Lumpur, and Singapore show abnormally worse agreements with simulated GEMS retrievals than other ECC measurements. Therefore, ECC ozonesonde measurements at Hong Kong, Pohang, Naha, Sapporo, and Tsukuba are finally identified as an optimal reference dataset. The accuracy of simulated GEMS retrievals is estimated to be ̃5.0 % for both tropospheric and stratospheric column ozone with the precision of 15 %and 5 %, which meets the GEMS ozone requirements.

ID: 154165
Type: article
Authors: Bak, Juseon; Liu, Xiong; Sun, Kang; Chance, Kelly; Kim, Jae-Hwan
Abstract: We introduce a method that accounts for errors caused by the slit function in an optimal-estimation-based spectral fitting process to improve ozone profile retrievals from the Ozone Monitoring Instrument (OMI) ultraviolet measurements (270-330 nm). Previously, a slit function was parameterized as a standard Gaussian by fitting the full width at half maximum (FWHM) of the slit function from climatological OMI solar irradiances. This cannot account for the temporal variation in slit function in irradiance, the intra-orbit changes due to thermally induced change and scene inhomogeneity, and potential differences in the slit functions of irradiance and radiance measurements. As a result, radiance simulation errors may be induced due to convolving reference spectra with incorrect slit functions. To better represent the shape of the slit functions, we implement a more generic super Gaussian slit function with two free parameters (slit width and shape factor); it becomes standard Gaussian when the shape factor is fixed to be 2. The effects of errors in slit function parameters on radiance spectra, referred to as pseudo absorbers (PAs), are linearized by convolving high-resolution cross sections or simulated radiances with the partial derivatives of the slit function with respect to the slit parameters. The PAs are included in the spectral fitting scaled by fitting coefficients that are iteratively adjusted as elements of the state vector along with ozone and other fitting parameters. The fitting coefficients vary with cross-track and along-track pixels and show sensitivity to heterogeneous scenes. The PA spectrum is quite similar in the Hartley band below 310 nm for both standard and super Gaussians, but is more distinctly structured in the Huggins band above 310 nm with the use of super Gaussian slit functions. Finally, we demonstrate that some spikes of fitting residuals are slightly smoothed by accounting for the slit function errors. Comparisons with ozonesondes demonstrate noticeable improvements when using PAs for both standard and super Gaussians, especially for reducing the systematic biases in the tropics and midlatitudes (mean biases of tropospheric column ozone reduced from -1.4̃0.7 to 0.0̃0.4 DU) and reducing the standard deviations of tropospheric ozone column differences at high latitudes (by 1 DU for the super Gaussian). Including PAs also makes the retrievals consistent between standard and super Gaussians. This study corroborates the slit function differences between radiance and irradiance, demonstrating that it is important to account for such differences in the ozone profile retrievals.

ID: 154635
Type: article
Authors: Cusworth, Daniel H.; Jacob, Daniel J.; Varon, Daniel J.; Chan Miller, Christopher; Liu, Xiong; Chance, Kelly; Thorpe, Andrew K.; Duren, Riley M.; Miller, Charles E.; Thompson, David R.; Frankenberg, Christian; Guanter, Luis; Randles, Cynthia A.
Abstract: We examine the potential for global detection of methane plumes from individual point sources with the new generation of spaceborne imaging spectrometers (EnMAP, PRISMA, EMIT, SBG, CHIME) scheduled for launch in 2019-2025. These instruments are designed to map the Earth's surface at high spatial resolution (30 m×30 m) and have a spectral resolution of 7-10 nm in the 2200-2400 nm band that should also allow useful detection of atmospheric methane. We simulate scenes viewed by EnMAP (10 nm spectral resolution, 180 signal-to-noise ratio) using the EnMAP end-to- end simulation tool with superimposed methane plumes generated by large- eddy simulations. We retrieve atmospheric methane and surface reflectivity for these scenes using the IMAP-DOAS optimal estimation algorithm. We find an EnMAP precision of 3 %-7 % for atmospheric methane depending on surface type. This allows effective single-pass detection of methane point sources as small as 100 kg h^-1 depending on surface brightness, surface homogeneity, and wind speed. Successful retrievals over very heterogeneous surfaces such as an urban mosaic require finer spectral resolution. We tested the EnMAP capability with actual plume observations over oil/gas fields in California from the Airborne Visible/Infrared Imaging Spectrometer - Next Generation (AVIRIS-NG) sensor (3 m×3 m pixel resolution, 5 nm spectral resolution, SNR 200-400), by spectrally and spatially downsampling the AVIRIS-NG data to match EnMAP instrument specifications. Results confirm that EnMAP can successfully detect point sources of ̃100 kg h^-1 over bright surfaces. Source rates inferred with a generic integrated mass enhancement (IME) algorithm were lower for EnMAP than for AVIRIS- NG. Better agreement may be achieved with a more customized IME algorithm. Our results suggest that imaging spectrometers in space could play an important role in the future for quantifying methane emissions from point sources worldwide.

ID: 154602
Type: article
Authors: Gonzalez Abad, Gonzalo; Souri, Amir Hossein; Bak, Juseon; Chance, Kelly; Flynn, Lawrence E.; Krotkov, Nickolay A.; Lamsal, Lok; Li, Can; Liu, Xiong; Chan Miller, Christopher; Nowlan, Caroline R.; Suleiman, Raid; Wang, Huiqun
Abstract: Over the last five decades, Earth's atmosphere has been extensively monitored from space using different spectral ranges. Early efforts were directed at improving weather forecasts with the first meteorological satellites launched in the 1960s. Soon thereafter, the intersection between weather, climate and atmospheric chemistry led to the observation of atmospheric composition from space. During the 1970s the Nimbus satellite program started regular monitoring of ozone integrated columns and water vapor profiles using the Backscatter Ultraviolet Spectrometer, the Infrared Interferometer Spectrometer and the Satellite Infrared Spectrometer instruments. Five decades after these pioneer efforts, continuous progress in instrument design, and retrieval techniques allow researchers to monitor tropospheric concentrations of a wide range of species with implications for air quality, climate and weather. The time line of historic, present and future space-borne instruments measuring ultraviolet and visible backscattered solar radiation designed to quantify atmospheric trace gases is presented. We describe the instruments technological evolution and the basic concepts of retrieval theory. We include a review of algorithms developed for ozone, nitrogen dioxide, sulfur dioxide, formaldehyde, bromine monoxide, water vapor and glyoxal, a selection of studies using these algorithms, the challenges they face and how these challenges can be addressed. The paper ends by providing insights on the opportunities that new instruments will bring to the atmospheric chemistry, weather and air quality communities and how to address the pressing need for long-term, inter-calibrated data records necessary to monitor the response of the atmosphere to rapidly changing ecosystems.

ID: 155115
Type: article
Authors: Jung, Yeonjin; González Abad, Gonzalo; Nowlan, Caroline R.; Chance, Kelly; Liu, Xiong; Torres, Omar; Ahn, Changwoo
Abstract: Atmospheric aerosols are significant sources of uncertainty in air mass factor (AMF) calculations for trace gas retrievals using ultraviolet measurements from space. Current trace gas retrievals typically do not consider aerosols explicitly as cloud products partially account for aerosol effects. Here, we propose a new measurement-based approach to correct for aerosols explicitly in the AMF calculation, apply it to Ozone Monitoring Instrument (OMI) formaldehyde (HCHO) retrievals and quantify the aerosol-induced HCHO vertical column density difference for three aerosol types (smoke, dust, and sulfate) during 2006-2007. We use OMI aerosol retrievals for aerosol optical properties and vertical profiles to construct lookup tables of scattering weights as functions of geometry, surface pressure, surface albedo, and aerosol information. The average difference between the National Aeronautics and Space Administration operational OMI HCHO product (not considering aerosols) and the results obtained in this study on a global scale are 27%, 6%, and -0.3% for smoke, dust, and sulfate aerosols, respectively. The region with the largest aerosol effects is East China, where the explicit smoke aerosol correction enhances mean HCHO vertical column densities by 35%, with corrections to individual observations sometimes larger than 100%. The quantified aerosol effects are applicable under clear-sky conditions. This study highlights the need to implement aerosol corrections in the AMF calculation for HCHO retrievals. This is particularly relevant in regions with high levels of pollution where aerosols interfere the most with formaldehyde satellite observations.

ID: 155235
Type: article
Authors: Suleiman, Raid M.; Chance, Kelly; Liu, Xiong; González Abad, Gonzalo; Kurosu, Thomas P.; Hendrick, Francois; Theys, Nicolas
Abstract: This paper presents the retrieval algorithm for the operational Ozone Monitoring Instrument (OMI) total bromine monoxide (BrO) data product (OMBRO) developed at the Smithsonian Astrophysical Observatory (SAO) and shows comparisons with correlative measurements and retrieval results. The algorithm is based on direct nonlinear least squares fitting of radiances from the spectral range 319.0-347.5 nm. Radiances are modeled from the solar irradiance, attenuated by contributions from BrO and interfering gases, and including rotational Raman scattering, additive and multiplicative closure polynomials, correction for Nyquist undersampling and the average fitting residual spectrum. The retrieval uses albedo- and wavelength-dependent air mass factors (AMFs), which have been pre-computed using a single mostly stratospheric BrO profile. The BrO cross sections are multiplied by the wavelength-dependent AMFs before fitting so that the vertical column densities (VCDs) are retrieved directly. The fitting uncertainties of BrO VCDs typically vary between 4 and 7×10¹² molecules cm^-2 (̃10 %-20 % of the measured BrO VCDs). Additional fitting uncertainties can be caused by the interferences from O₂-O₂ and H₂CO and their correlation with BrO. AMF uncertainties are estimated to be around 10 % when the single stratospheric-only BrO profile is used. However, under conditions of high tropospheric concentrations, AMF errors due to this assumption of profile can be as high as 50 %. The retrievals agree well with GOME-2 observations at simultaneous nadir overpasses and with ground-based zenith-sky measurements at Harestua, Norway, with mean biases less than -0.22±1.13×¹⁰¹³ and 0.12±0.76×¹⁰¹³ molecules cm^-2, respectively. Global distribution and seasonal variation of OMI BrO are generally consistent with previous satellite observations. Finally, we confirm the capacity of OMBRO retrievals to observe enhancements of BrO over the US Great Salt Lake despite the current retrieval setup considering a stratospheric profile in the AMF calculations. OMBRO retrievals also show significant BrO enhancements from the eruption of the Eyjafjallajökull volcano, although the BrO retrievals are affected under high SO₂ loading conditions by the sub-optimum choice of SO₂ cross sections.

ID: 147898
Type: article
Authors: Hayashida, Sachiko; Kajino, Mizuo; Deushi, Makoto; Sekiyama, Tsuyoshi Thomas; Liu, Xiong
Abstract: We analyzed the ozone (O₃) profile product (PROFOZ) derived from Ozone Monitoring Instrument(OMI) ultraviolet (UV) spectra to reveal spatial and temporal variations in O₃ distributions over China. Although discriminating vertical O₃ in the lower troposphere is a challenge for satellite-borne measurements, previous research has confirmed the reliability of the lowermost layer O₃ of the PROFOZ product, corresponding to 0-3 km, under the high O₃ condition over China by comparison with the airborne measurements. In the present study, we focus on the seasonal variation of O₃ obtained in the lowermost layer over China. We track the O₃ enhancement under polluted conditions by using the ozone anomaly (DeltaO₃), defined as the difference between the retrieval values and a priori values, because our focus is the temporally high O₃ level compared with the background level. We divide the 25-40° N and 100-135° E region into clusters according to the similarity of the seasonal variation in DeltaO₃ at the lowermost layer corresponding to approximately 0-3 km in altitude. Using this cluster analysis, we distinguish the areas in which DeltaO₃ has outstanding seasonality with high values in summer, particularly in June, and low values in winter over the North China Plain and Sichuan basin. The areas with these anomalous DeltaO₃ values correspond to high NO₂ emission areas. We compare the results with model simulations from the Meteorological Research Institute-Chemistry Climate Model (MRI-CCM2) and meteorological data. The areas showing outstanding seasonality also correspond to those of high chemical production rates in June. Along the coastal area, DeltaO₃ values tend to drop to negative in August, which can be attributed to the inflow of clean oceanic air to inland regions. The results presented here again demonstrate the reliability of the lower tropospheric O₃ data from the OMI PROFOZ product and suggest good capability of UV satellite sensors for monitoring O₃ pollution that will be launched in the near future.

ID: 145771
Type: article
Authors: Huang, Guanyu; Liu, Xiong; Chance, Kelly; Yang, Kai; Cai, Zhaonan
Abstract: We validate the Ozone Monitoring Instrument (OMI) ozone profile (PROFOZ v0.9.3) product including ozone profiles between 0.22 and 261 hPa and stratospheric ozone columns (SOCs) down to 100, 215, and 261 hPa from October 2004 through December 2014 retrieved by the Smithsonian Astrophysical Observatory (SAO) algorithm against the latest Microwave Limb Sound (MLS) v4.2x data. We also evaluate the effects of OMI row anomaly (RA) on the retrieval by dividing the data set into before and after the occurrence of serious RA, i.e., pre-RA (2004-2008) and post-RA (2009-2014). During the pre-RA period, OMI ozone profiles agree very well with MLS data. After applying OMI averaging kernels to MLS data, the global mean biases (MBs) are within 3 % between 0.22 and 100 hPa, negative biases are within 3-9 % for lower layers, and the standard deviations (SDs) are 3.5-5 % from 1 to 40 hPa, 6-10 % for upper layers, and 5-20 % for lower layers. OMI shows biases dependent on latitude and solar zenith angle (SZA), but MBs and SDs are mostly within 10 % except for low and high altitudes of high latitudes and SZAs. Compared to the retrievals during the pre-RA period, OMI retrievals during the post-RA period degrade slightly between 5 and 261 hPa with MBs and SDs typically larger by 2-5 %, and degrade much more for pressure less than &tilde; 5 hPa, with larger MBs by up to 8 % and SDs by up to 15 %, where the MBs are larger by 10-15 % south of 40° N due to the blockage effect of

ID: 143826
Type: article
Authors: Huang, Guanyu; Liu, Xiong; Chance, Kelly; Yang, Kai; Bhartia, Pawan K.; Cai, Zhaonan; Allaart, Marc; Ancellet, Gérard; Calpini, Bertrand; Coetzee, Gerrie J. R.; Cuevas-Agulló, Emilio; Cupeiro, Manuel; De Backer, Hugo; Dubey, Manvendra K.; Fuelberg, Henry E.; Fujiwara, Masatomo; Godin-Beekmann, Sophie; Hall, Tristan J.; Johnson, Bryan; Joseph, Everette; Kivi, Rigel; Kois, Bogumil; Komala, Ninong; König-Langlo, Gert; Laneve, Giovanni; Leblanc, Thierry; Marchand, Marion; Minschwaner, Kenneth R.; Morris, Gary; Newchurch, Michael J.; Ogino, Shin-Ya; Ohkawara, Nozomu; Piters, Ankie J. M.; Posny, Françoise; Querel, Richard; Scheele, Rinus; Schmidlin, Frank J.; Schnell, Russell C.; Schrems, Otto; Selkirk, Henry; Shiotani, Masato; Skrivánková, Pavla; Stübi, René; Taha, Ghassan; Tarasick, David W.; Thompson, Anne M.; Thouret, Valérie; Tully, Matthew B.; Van Malderen, Roeland; Vömel, Holger; von der Gathen, Peter; Witte, Jacquelyn C.; Yela, Margarita
Abstract: We validate the Ozone Monitoring Instrument (OMI) Ozone Profile (PROFOZ) product from October 2004 through December 2014 retrieved by the Smithsonian Astrophysical Observatory (SAO) algorithm against ozonesonde observations. We also evaluate the effects of OMI row anomaly (RA) on the retrieval by dividing the dataset into before and after the occurrence of serious OMI RA, i.e., pre-RA (2004-2008) and post-RA (2009-2014). The retrieval shows good agreement with ozonesondes in the tropics and midlatitudes and for pressure ~ 50 hPa after applying OMI averaging kernels to ozonesonde data. The MBs of the stratospheric ozone column (SOC, the ozone column from the tropopause pressure to the ozonesonde burst pressure) are within 2 % with SDs of ~ 50 hPa after applying OMI averaging kernels to ozonesonde data. The MBs of the stratospheric ozone column (SOC, the ozone column from the tropopause pressure to the ozonesonde burst pressure) are within 2 % with SDs of ~ 50 hPa. The SOC MBs increase up to 3 % with SDs as great as 6 % and the TOC SDs increase up to 30 %. The comparison generally degrades at larger solar zenith angles (SZA) due to weaker signals and additional sources of error, leading to worse performance at high latitudes and during the midlatitude winter. Agreement also degrades with increasing cloudiness for pressure > ~ 100 hPa and varies with cross-track position, especially with large MBs and SDs at extreme off-nadir positions. In the tropics and midlatitudes, the post-RA comparison is considerably worse with larger SDs reaching 2 % in the stratosphere and 8 % in the troposphere and up to 6 % in TOC. There are systematic differences that vary with latitude compared to the pre-RA comparison. The retrieval comparison demonstrates good long-term stability during the pre-RA period but exhibits a statistically significant trend of 0.14-0.7 % year^-1 for pressure for pressure ^-1 in SOC, and -0. 33 DU year^-1 in TOC during the post-RA period. The spatiotemporal variation of retrieval performance suggests the need to improve OMI's radiometric calibration especially during the post-RA period to maintain the long-term stability and reduce the latitude/season/SZA and cross-track dependency of retrieval quality.

ID: 144778
Type: article
Authors: Sun, Kang; Liu, Xiong; Huang, Guanyu; González Abad, Gonzalo; Cai, Zhaonan; Chance, Kelly; Yang, Kai
Abstract: The Ozone Monitoring Instrument (OMI) has been successfully measuring the Earth's atmospheric composition since 2004, but the on-orbit behavior of its slit functions has not been thoroughly characterized. Preflight measurements of slit functions have been used as a static input in many OMI retrieval algorithms. This study derives on-orbit slit functions from the OMI irradiance spectra assuming various function forms, including standard and super-Gaussian functions and a stretch to the preflight slit functions. The on-orbit slit functions in the UV bands show U-shaped cross-track dependences that cannot be fully represented by the preflight ones. The full widths at half maximum (FWHM) of the stretched preflight slit functions for detector pixels at large viewing angles are up to 30 % larger than the nadir pixels for the UV1 band, 5 % larger for the UV2 band, and practically flat in the VIS band. Nonetheless, the on-orbit changes of OMI slit functions are found to be insignificant over time after accounting for the solar activity, despite of the decaying of detectors and the occurrence of OMI row anomaly. Applying the derived on-orbit slit functions to ozone-profile retrieval shows substantial improvements over the preflight slit functions based on comparisons with ozonesonde validations.

ID: 142859
Type: article
Authors: Sun, Kang; Liu, Xiong; Nowlan, Caroline R.; Cai, Zhaonan; Chance, Kelly V.; Frankenberg, Christian; Lee, Richard A. M.; Pollock, Randy; Rosenberg, Robert; Crisp, David
Abstract: Accurately characterizing the instrument line shape (ILS) of the Orbiting Carbon Observatory-2 (OCO-2) is challenging and highly important due to its high spectral resolution and requirement for retrieval accuracy (0. 25 %) compared to previous spaceborne grating spectrometers. On-orbit ILS functions for all three bands of the OCO-2 instrument have been derived using its frequent solar measurements and high-resolution solar reference spectra. The solar reference spectrum generated from the 2016 version of the Total Carbon Column Observing Network (TCCON) solar line list shows significant improvements in the fitting residual compared to the solar reference spectrum currently used in the version 7 Level 2 algorithm in the O₂ A band. The analytical functions used to represent the ILS of previous grating spectrometers are found to be inadequate for the OCO-2 ILS. Particularly, the hybrid Gaussian and super-Gaussian functions may introduce spurious variations, up to 5 % of the ILS width, depending on the spectral sampling position, when there is a spectral undersampling. Fitting a homogeneous stretch of the preflight ILS together with the relative widening of the wings of the ILS is insensitive to the sampling grid position and accurately captures the variation of ILS in the O₂ A band between decontamination events. These temporal changes of ILS may explain the spurious signals observed in the solar-induced fluorescence retrieval in barren areas.