Cambridge, MA, United States
Cambridge, MA, United States

Time filter

Source Type

Liu X.,University of Maryland, Baltimore | Liu X.,Harvard - Smithsonian Center for Astrophysics | Liu X.,NASA | Bhartia P.K.,NASA | And 4 more authors.
Atmospheric Chemistry and Physics | Year: 2010

We validate OMI ozone profiles between 0.22-215 hPa and stratospheric ozone columns down to 215 hPa (SOC215) against v2.2 MLS data from 2006. The validation demonstrates convincingly that SOC can be derived accurately from OMI data alone, with errors comparable to or smaller than those from current MLS retrievals, and it demonstrates implicitly that tropospheric ozone column can be retrieved accurately from OMI or similar nadir-viewing ultraviolet measurements alone. The global mean biases are within 2.5% above 100 hPa and 5-10% below 100 hPa; the standard deviations of the differences (1Ï) are 3.5-5% between 1-50 hPa, 6-9% above 1 hPa and 8-15% below 50 hPa. OMI shows some latitude and solar zenith angle dependent biases, but the mean biases are mostly within 5% and the standard deviations are mostly within 2-5% except for low altitudes and high latitudes. The excellent agreement with MLS data shows that OMI retrievals can be used to augment the validation of MLS and other stratospheric ozone measurements made with even higher vertical resolution than that for OMI. OMI SOC215 shows a small bias of 0.6% with a standard deviation of 2.8%. When compared as a function of latitude and solar zenith angle, the mean biases are within 2% and the standard deviations range from 2.1 to 3.4%. Assuming 2% precision for MLS SOC215, we deduce that the upper limits of random-noise and smoothing errors for OMI SOC215 range from 0.6% in the southern tropics to 2.8% at northern middle latitudes.

Van Donkelaar A.,Dalhousie University | Martin R.V.,Dalhousie University | Martin R.V.,Harvard - Smithsonian Center for Astrophysics | Spurr R.J.D.,RT Solutions Inc. | And 5 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2013

We develop an optimal estimation (OE) algorithm based on top-of-atmosphere reflectances observed by the MODIS satellite instrument to retrieve near-surface fine particulate matter (PM2.5). The GEOS-Chem chemical transport model is used to provide prior information for the Aerosol Optical Depth (AOD) retrieval and to relate total column AOD to PM2.5. We adjust the shape of the GEOS-Chem relative vertical extinction profiles by comparison with lidar retrievals from the CALIOP satellite instrument. Surface reflectance relationships used in the OE algorithm are indexed by land type. Error quantities needed for this OE algorithm are inferred by comparison with AOD observations taken by a worldwide network of sun photometers (AERONET) and extended globally based upon aerosol speciation and cross correlation for simulated values, and upon land type for observational values. Significant agreement in PM2.5 is found over North America for 2005 (slope = 0.89; r = 0.82; 1-σ error = 1 μg/m3 + 27%), with improved coverage and correlation relative to previous work for the same region and time period, although certain subregions, such as the San Joaquin Valley of California are better represented by previous estimates. Independently derived error estimates of the OE PM2.5 values at in situ locations over North America (of ±(2.5 μg/m3 + 31%) and Europe of ±(3.5 μg/m3 + 30%) are corroborated by comparison with in situ observations, although globally (error estimates of ±(3.0 μg/m3 + 35%), may be underestimated. Global population-weighted PM2.5 at 50% relative humidity is estimated as 27.8 μg/m 3 at 0.1°× 0.1°resolution. © 2013. American Geophysical Union. All Rights Reserved.

Wang J.,University of Nebraska - Lincoln | Wang J.,NASA | Xu X.,University of Nebraska - Lincoln | Spurr R.,RT Solutions Inc. | And 2 more authors.
Remote Sensing of Environment | Year: 2010

A new algorithm, using the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite reflectance and aerosol single scattering properties simulated from a chemistry transport model (GEOS-Chem), is developed to retrieve aerosol optical thickness (AOT) over land in China during the spring dust season. The algorithm first uses a "dynamic lower envelope" approach to sample the MODIS dark-pixel reflectance data in low AOT conditions, to derive the local surface visible (0.65μm)/near infrared (NIR, 2.1μm) reflectance ratio. Joint retrievals of AOT at 0.65μm and surface reflectance at 2.1μm are then performed, based on the time, location, and spectral-dependent single scattering properties of the dusty atmosphere as simulated by the GEOS-Chem. A linearized vector radiative transfer model (VLIDORT) that simultaneously computes the top-of-atmosphere reflectance and its Jacobian with respect to AOT, is used in the forward component of the inversion of MODIS reflectance to AOT. Comparison of retrieved AOT results in April and May of 2008 with AERONET observations shows a strong correlation (R=0.83), with small bias (0.01), and small RMSE (0.17); the figures are a substantial improvement over corresponding values obtained with the MODIS Collection 5 AOT algorithm for the same study region and time period. The small bias is partially due to the consideration of dust effect at 2.1 μm channel, Without Which The Bias Is -0.05. The surface PM10 (particulate matter with diameter less than 10μm) concentrations derived using this improved AOT retrieval show better agreement with ground observations than those derived from GEOS-Chem simulations alone, or those inferred from the MODIS Collection 5 AOT. This study underscores the value of using satellite reflectance to improve the air quality modeling and monitoring. © 2010 Elsevier Inc.

Vasilkov A.,Science Systems And Applications Inc. | Joiner J.,NASA | Spurr R.,RT Solutions Inc.
Atmospheric Measurement Techniques | Year: 2013

Quantifying the impact of rotational-Raman scattering (RRS) on the O 2 A- and B-bands is important as these bands can be used for cloud and aerosol characterization for trace-gas retrievals including CO2 and CH4. In this paper, we simulate the spectral effects of RRS for various viewing geometries and instruments with different spectral resolutions. We also examine how aerosols affect the amount of RRS filling-in. We show that the filling-in effects of RRS are relatively small, but not negligible, in these O2absorption bands, particularly for high-spectral-resolution instruments. For comparison, we also compare and contrast the spectral signatures of RRS with those of terrestrial chlorophyll fluorescence. © Author(s) 2013.

Liu X.,University of Maryland, Baltimore | Liu X.,Harvard - Smithsonian Center for Astrophysics | Liu X.,NASA | Bhartia P.K.,NASA | And 3 more authors.
Atmospheric Chemistry and Physics | Year: 2010

Ozone profiles from the surface to about 60 km are retrieved from Ozone Monitoring Instrument (OMI) ultraviolet radiances using the optimal estimation technique. OMI provides daily ozone profiles for the entire sunlit portion of the earth at a horizontal resolution of 13 km×48 km for the nadir position. The retrieved profiles have sufficient accuracy in the troposphere to see ozone perturbations caused by convection, biomass burning and anthropogenic pollution, and to track their spatiotemporal transport. However, to achieve such accuracy it has been necessary to calibrate OMI radiances carefully (using two days of Aura/Microwave Limb Sounder data taken in the tropics). The retrieved profiles contain ∼6-7 degrees of freedom for signal, with 5-7 in the stratosphere and 0-1.5 in the troposphere. Vertical resolution varies from 7-11 km in the stratosphere to 10-14 km in the troposphere. Retrieval precisions range from 1% in the middle stratosphere to 10% in the lower stratosphere and troposphere. Solution errors (i.e., root sum square of precisions and smoothing errors) vary from 1-6% in the middle stratosphere to 6-35% in the troposphere, and are dominated by smoothing errors. Total, stratospheric, and tropospheric ozone columns can be retrieved with solution errors typically in the few Dobson unit range at solar zenith angles less than 80°.

Rault D.F.,NASA | Spurrb R.,RT SOLUTIONS Inc.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

An alternative algorithm is being developed to retrieve ozone vertical distribution information from the OMPS/LP sensor which will be manifested on the upcoming NPOESS Preparatory Project (NPP) platform in late 2011. In contrast to most limb sensors retrieval methods, the proposed algorithm will forgo the spherical symmetry assumption for the atmospheric structure, and will attempt to simultaneously retrieve the ozone distribution in both the vertical and the along-track directions. The paper describes the two-dimensional forward model as well as the methods which have been developed to simultaneously retrieve a whole orbit of data. Sample retrieval results are shown to illustrate the technique.© 2010 SPIE.

Lin J.-T.,Peking University | Martin R.V.,Dalhousie University | Martin R.V.,Harvard - Smithsonian Center for Astrophysics | Boersma K.F.,Royal Netherlands Meteorological Institute | And 8 more authors.
Atmospheric Chemistry and Physics | Year: 2014

Retrievals of tropospheric nitrogen dioxide (NO2) from the Ozone Monitoring Instrument (OMI) are subject to errors in the treatments of aerosols, surface reflectance anisotropy, and vertical profile of NO2. Here we quantify the influences over China via an improved retrieval process. We explicitly account for aerosol optical effects (simulated by nested GEOS-Chem at 0.667 long. × 0.5 lat. and constrained by aerosol measurements), surface reflectance anisotropy, and high-resolution vertical profiles of NO2 (simulated by GEOS-Chem). Prior to the NO2 retrieval, we derive the cloud information using consistent ancillary assumptions.

We compare our retrieval to the widely used DOMINO v2 product, using MAX-DOAS measurements at three urban/suburban sites in East China as reference and focusing the analysis on the 127 OMI pixels (in 30 days) closest to the MAX-DOAS sites. We find that our retrieval reduces the interference of aerosols on the retrieved cloud properties, thus enhancing the number of valid OMI pixels by about 25%. Compared to DOMINO v2, our retrieval better captures the day-to-day variability in MAX-DOAS NO2 data (R2 Combining double low line 0.96 versus 0.72), due to pixel-specific radiative transfer calculations rather than the use of a look-up table, explicit inclusion of aerosols, and consideration of surface reflectance anisotropy. Our retrieved NO2 columns are 54% of the MAX-DOAS data on average, reflecting the inevitable spatial inconsistency between the two types of measurement, errors in MAX-DOAS data, and uncertainties in our OMI retrieval related to aerosols and vertical profile of NO2.

Sensitivity tests show that excluding aerosol optical effects can either increase or decrease the retrieved NO2 for individual OMI pixels with an average increase by 14%. Excluding aerosols also complexly affects the retrievals of cloud fraction and particularly cloud pressure. Employing various surface albedo data sets slightly affects the retrieved NO2 on average (within 10%). The retrieved NO2 columns increase when the NO2 profiles are taken from MAX-DOAS retrievals (by 19% on average) or TM4 simulations (by 13%) instead of GEOS-Chem simulations. Our findings are also relevant to retrievals of other pollutants (e.g., sulfur dioxide, ormaldehyde, glyoxal) from UV-visible backscatter satellite instruments. © 2014 Author(s).

Spurr R.,RT Solutions Inc.
Journal of Quantitative Spectroscopy and Radiative Transfer | Year: 2014

The linearized radiative transfer models VLIDORT and LIDORT will deliver profile weighting functions (Jacobians) with respect to layer optical properties. We derive transformation rules for the conversion of layer Jacobian output to weighting functions defined for level (layer boundary) quantities such as volume mixing ratio, temperature and pressure. In a related development, we discuss the derivation of bulk-property atmospheric Jacobians with respect to quantities such as the temperature shift, the surface pressure and scaling parameters for constituent profiles. We also present some rules for calculating Jacobians for parameters characterizing aerosol loading regimes. An appendix contains linearization (with respect to temperature and pressure) of the trace species cross-sections derived from the HITRAN line spectroscopy data base. © 2014 Elsevier Ltd.

Bak J.,Pusan National University | Kim J.H.,Pusan National University | Spurr R.J.D.,RT Solutions Inc. | Liu X.,Harvard - Smithsonian Center for Astrophysics | Newchurch M.J.,University of Alabama in Huntsville
Remote Sensing of Environment | Year: 2012

This paper addresses several aspects related to ultraviolet (UV) measurements from an upcoming geostationary orbit (GEO) satellite mission. First, using simulated cloud coverage based on the MTSAT (Multi-functional Transport Satellite 1R) 650. nm channel as a proxy, we find that GEO observations can increase clear-sky coverage by ~. 4 times. Secondly, we examine the feasibility of improved ozone detection with GEO observations, using synthetic GEMS spectra with GEO geometries along with the TOMS total ozone algorithm and an ozone profile algorithm based on the optimal estimation method. The sensitivity of ozone retrievals to the lower troposphere is limited at large solar and satellite zenith angles, especially above 70° under clear-sky conditions. Third, we evaluate two new ideas for improving ozone retrieval sensitivity in the troposphere at large angle geometries; (1) a geosynchronous orbit with an inclination-angle of 30°, with smaller values of viewing zenith angles than those from a geostationary orbit; (2) a combination of UV measurements at multiple solar zenith angles to improve the accuracy of ozone profile retrievals. © 2011 Elsevier Inc.

Spurr R.,RT Solutions Inc. | Natraj V.,Jet Propulsion Laboratory
Journal of Quantitative Spectroscopy and Radiative Transfer | Year: 2011

Performance is an issue for radiative transfer simulations in hyper-spectral remote sensing backscatter retrieval algorithms. 2-Stream models are often used to speed up flux and radiance calculations. Here we present a linearized 2-stream multiple-scatter code, with the ability to generate analytic weighting functions with respect to any atmospheric or surface property. We examine 2-stream accuracy for the satellite intensity diffuse field, and the corresponding Jacobians for total ozone column and surface albedo, for an application in the ozone UV Huggins bands. © 2011 Elsevier Ltd.

Loading RT Solutions Inc. collaborators
Loading RT Solutions Inc. collaborators