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Mass concentration (mascon) solutions have become a prominent medium for investigating time‐variable gravity recovered by GRACE and GRACE Follow‐On. While GRACE Level‐2 spherical harmonic products require various postprocessing techniques to eliminate correlated noise, mascon formulations employ spatial regularization strategies during the estimation step to improve signal recovery. However, mascon estimation has traditionally required large computing resources and GRACE Level‐1B processing capabilities. In this study, we show that a typical mascon estimation system can be reformulated to allow for the estimation of regularized mascons from Level‐2 spherical harmonics. Provided that spherical harmonic solution covariances are available, the computed mascons will be mathematically equivalent to similar mascons estimated from Level‐1B observations. This method is computationally efficient, better leverages GRACE spherical harmonics than past methods, and matches the performance of typical mascon solutions without locking scientists into predetermined regularization designs. We develop a proof‐of‐concept solution using ITSG‐Grace2018 and compare results with traditional mascons from the Jet Propulsion Lab (JPL) and NASA Goddard Space Flight Center (GSFC). We then assess the effects of spherical harmonic truncation and use of regularization correlations on basin signal recovery. We find that spherical expansions to degree and order 60 provide the minimum expansion necessary to study most basins, while larger expansions help further localize signals. We also find that diagonal regularizations (i.e., regularizations that do not contain inter‐mascon correlations) are adversely affected by leakage, especially across boundaries such as coastlines where signals are not highly correlated, whereas including inter‐mascon correlations and regional boundaries in the regularization greatly improves signal recovery. Plain Language Summary: This study derives a new method for producing surface mass change maps in the form of so‐called "mascons" using GRACE and GRACE Follow‐On data that is computationally simpler but equally as rigorous as typically used methods. Using this method, scientists no longer need specialized orbit determination and estimation software to produce these maps, and therefore this type of work is now accessible to scientists outside of the few major centers that currently produce these maps. At the same time, centers with these specialized software packages can incorporate the computational efficiencies presented here to improve their own processes. As a result, this new method can be used to study individual problems of interest outside the scope of the globally focused solutions available today (for example, to specifically study targeted river basins), and these new studies can additionally incorporate other known information about those problems. We provide a proof‐of‐concept solution showcasing the new method and apply the new method to highlight key design decisions that must be made when producing mascon solutions. Key Points: Computationally efficient mascons computed from GRACE Level‐2 spherical harmonics are equivalent to those from Level‐1B ranging dataThese mascons can be tailored to specific problems of interest and do not require special orbit determination softwareA presented trade study shows the effects of spherical expansion truncation and compares diagonal regularization to that with correlations [ABSTRACT FROM AUTHOR]

Mascon products derived from Gravity Recovery and Climate Experiment satellite gravimetry data are widely used to study the Greenland ice sheet mass balance. However, the products released by different research groups—JPL, CSR, and GSFC—show noticeable discrepancies. To understand them, we compare those mascon products with mascon solutions computed in‐house using a varying regularization parameter. We show that the observed discrepancies are likely dominated by differences in the applied regularization. Furthermore, we present a numerical study aimed at an in‐depth analysis of regularization‐driven biases in the solutions. We demonstrate the ability of our simulations to reproduce 60%–80% of biases observed in real data, which proves that our simulations are sufficiently realistic. After that, we demonstrate that the quality of mascon‐based estimates can be increased by a proper modification of the applied regularization: no correlation between mascons is assumed when they belong to different drainage systems. Using both simulations and real data analysis, we show that the improved regularization mitigates signal leakage between drainage systems by 11%–56%. Finally, we validate various mascon solutions over the SW drainage system, using trends from (i) the GOCO‐06S model and (ii) the Input‐Output Method as control data. In general, the in‐house computed trend estimates are consistent with the trends from CSR and JPL solutions and the trends from the control data. Plain Language Summary: The mass variations of Greenland ice sheet (GrIS) have been widely monitored by the satellite mission named Gravity Recovery and Climate Experiment (GRACE). Currently, there are different research groups (i.e., JPL, CSR, and GSFC), which produce various mascon products from the GRACE data. Noticeable discrepancies, however, are shown in these mascon products. In this study, we try to analyze the reasons that cause the discrepancies, by varying with different strengths of regularization when estimating the mascon products using our variant of the mascon approach. Using both real data and simulation, we show that the observed discrepancies are likely dominated by differences in the applied regularization. Thereafter, we find that by utilizing a proper modification of the spatial constraint, the quality of the mascon estimates in GrIS can be obviously improved by 11%–56%. Finally, the trend estimates of the SW drainage systems from the GOCO‐06S model and the Input‐Output Method are utilized as independent data, to validate the mascon‐based solutions. It is found that the trends from CSR, JPL, and the estimates by the modified spatial constraints are in good agreement with the independent data. Key Points: State‐of‐the‐art GRACE mascon solutions show obvious discrepancies in Greenland mass anomalies over the period 2003–2014 (e.g., reaching the level of 100–200 Gt)The major cause of the large discrepancies is likely caused by different spatial constraintsWe recommend that no smoothing constraints are applied to neighboring mascons across drainage system boundaries [ABSTRACT FROM AUTHOR]

The lunar mascon basins are characterized by high gravity anomalies and thin crust. Study of the density structure beneath the mascon basins can help to understand the origin of these gravity anomalies and infer their formation and evolution. We propose an efficient forward gravity method based on the 3‐D Gauss‐Legendre quadrature (GLQ) and Fast Fourier Transforms combined with the adaptive discretization strategy to ensure high accuracy. The numerical example demonstrates that computational efficiency is increased by about three orders of magnitude compared to the traditional 3D GLQ method. We employ this forward method in a 3‐D inversion of the gravity data derived from the lunar gravity model GL1500E. The inverted results show prominent high‐density structures (namely, mascons) representing significant mantle uplift and thinned crust beneath most impact basins. Marked low‐density rings surround the mascons. Most of the corresponding low‐density anomalies extend from the near‐surface to the Moho, indicating a thick, low‐density crust. Our density model is consistent with the formation processes of mascons that an impact causes collapse of the transient crater and then the shocked mantle drives mantle flow. The low‐density rings surrounding the mascons may stem from the crust thickening following an impact and extensive fracturing of the crustal column. Plain Language Summary: Mascons (i.e., mass concentration) have been found beneath most of the lunar impact basins. Knowledge of their density distribution is vital for understanding of their structure and evolution. The mass excesses in some impact basins extend from the base of the lunar crust to about 60 km depth and correspond to a density anomaly of up to 400 kg/m3. These mass concentrations are usually surrounded by a ring of low‐density crust. These features are consistent with the hypothesis that the mascons were formed after an asteroid impact and the upwelling mantle material filling the impact crater. This high‐density material produces the mass excess under the impact basins, while the broken crustal material forms the thick low‐density crust around the mascons. Key Points: A highly efficient forward modeling method for gravity calculations is proposed and used in inversionProminent high‐density structures represent significant mantle uplift and thinned crust beneath most of the impact basinsNegative density anomalies around the mascons form outer rings which may be associated with crustal thickening [ABSTRACT FROM AUTHOR]

The Long Island Sound Tropospheric Ozone Study (LISTOS) was organized to investigate ozone formation and transport in the New York City metropolitan area and locations downwind. During LISTOS, the University at Albany Atmospheric Sciences Research Center (ASRC) mobile laboratory was used for measuring surface O3, NO2, and aerosol number and mass concentration. Sharp O3 concentration gradients, with ΔO3 Δy−1 over 15 ppb km−1, were measured both at and near the land‐water interface and on the highway on days characterized by high regional O3 concentrations. These large O3 gradients at or near the land‐water interface, and in air masses relatively low in NO2, are shown to be influenced in part by the transport of highly oxidized air masses via sea breeze circulation and convergence with gradient flow. On the highway under regionally high O3 concentrations, strong anticorrelation (R2 = 0.78, p < 0.05) between O3 and NO2 and an absolute slope less than 1 suggested that Ox concentrations (O3 + NO2) increased with increasing NO2. Overall, the on‐road measurements made during LISTOS help to better characterize the interaction between the emitted pollution and the meteorological conditions on Long Island, thereby having potential policy implications. Plain Language Summary: Ozone pollution is a significant problem for the New York City (NYC) Metropolitan Area, which has been classified by the U.S. Environmental Protection Agency as being in noncompliance with the National Ambient Air Quality Standard for ozone. The southern coast of Connecticut is the most severely affected by ozone pollution, which is produced in the atmosphere as it travels downwind from emission sources of precursor gases like oxides of nitrogen and volatile organic compounds. The impact of ozone on a given location involves a complex interplay of emissions, meteorology, and chemistry. Our study explored aspects of this interplay on several polluted days in summer 2018 using a Dodge Sprinter van mobile laboratory equipped with instrumentation to measure ozone, one of the critical oxides of nitrogen, and other parameters. Some of the analysis days yielded significant and rapid changes in ozone concentration near the south or north shore of Long Island. Our results suggest that sea breeze winds can greatly influence ozone concentrations near the coast in some cases. Understanding when and how the sea breeze affects air quality in regions downwind of NYC and other coastal metropolitan areas will help efforts to improve air quality in these locations. Key Points: A mobile laboratory was used for measuring surface O3, NO2, and aerosol during the Long Island Sound Tropospheric Ozone Study (LISTOS)Sharp O3 concentration gradients were measured in low NOx environments at the shoreline on days with high regional O3 concentrationsOne factor influencing some of the sharp ozone gradients was the convergence of a sea breeze containing highly oxidized air and a land/sound breeze [ABSTRACT FROM AUTHOR]

The Gravity Recovery and Climate Experiment (GRACE) and GRACE‐Follow On missions have provided a global history of terrestrial water storage changes since 2002. Traditional GRACE products resolve monthly time‐variable gravity at spatial resolutions of 300–500 km, with many recent efforts focusing on regularized mass concentration (mascon) solutions to better resolve signals spatially. However, monthly resolution inhibits the applicability of GRACE to investigations of sub‐monthly signals. This study presents a new daily mass change solution estimated as deviations from the NASA Goddard Space Flight Center (GSFC) converged monthly mascon product and quantifies the fundamental trade‐off between temporal and spatial resolution in GRACE‐only solutions. As an iteration of the GSFC product, this daily solution represents a mixing of monthly information at higher relative spatial resolution with daily information at lower relative spatial resolution, resolving each temporal scale at the best resolution achievable without requiring hydrological model dependencies in the estimation. The resolution of the daily mascons is 300–400 km at high latitudes and 600–1,000 km lower, depending on each daily orbital track and the proximity of each mascon to constraint region boundaries (e.g., coastlines). Through simulations and model comparisons, we demonstrate that daily signals over areas larger than 400,000 km 2 are recoverable, with basins larger than 800,000 km 2 exhibiting strong signal recovery relative to leakage. This analysis establishes baselines for daily signal recovery from GRACE in the context of longer time scale solutions, characterizes leakage inherent in daily GRACE information, and creates new opportunities for applying GRACE to investigations of sub‐monthly signals. Key Points: Daily solutions are independently estimated as an iteration of the converged GSFC monthly masconsLarge spatial signals are recovered at daily and longer time scalesNoise and leakage are quantified, and significant hydrologic signals are recovered in large basins [ABSTRACT FROM AUTHOR]

An efficient forward modeling algorithm for calculation of gravitational fields in spherical coordinates is developed for 3‐D large‐scale gravity inversion problems. The 3‐D Gauss‐Legendre quadrature (GLQ) is used to calculate the gravitational fields of mass distributions discretized into tesseroids. Equivalence relations in the kernel matrix of the forward modeling are exploited to decrease storage and computation time. The numerical tests demonstrate that the computation time of the proposed algorithm is reduced by approximately 2 orders of magnitude, and the memory requirement is reduced by N′λ times compared with the traditional GLQ method, where N′λ is the number of the model elements in the longitudinal direction. These significant improvements in computational efficiency and storage make it possible to calculate and store the dense Jacobian matrix in 3‐D large‐scale gravity inversions. The equivalence relations can be applied to the Taylor series method or combined with the adaptive discretization to ensure high accuracy. To further illustrate the capability of the algorithm, we present a regional synthetic example. The inverted results show density distributions consistent with the actual model. The computation took about 6.3 hr and 0.88 GB of memory compared with about a dozen days and 245.86 GB for the traditional 3‐D GLQ method. Finally, the proposed algorithm is applied to the gravity field derived from the latest lunar gravity model GL1500E. Three‐dimensional density distributions of the Imbrium and Serenitatis basins are obtained, and high‐density bodies are found at the depths 10–60 km, likely indicating a significant uplift of the high‐density mantle beneath the two mascon basins. Key Points: An efficient forward modeling algorithm with kernel matrix equivalence is developedComputation time and memory requirement of the algorithm are significantly reducedThree‐dimensional density distributions of the Imbrium and Serenitatis basins are obtained [ABSTRACT FROM AUTHOR]

We examine long‐term trends in the near‐surface black carbon mass concentration, using multiyear primary data obtained from a dense network (ARFINET) of observatories over the Indian region. We report for the first time the statistically significant decreasing trend in black carbon mass concentration, based on primary data from this region, at an average rate of ~242 ± 53 ng · m−3 · year−1 during the period 2007–2016. This finding contrasts with the generally increasing trend in the columnar aerosol optical depth, reported earlier, and the steadily increasing trend in anthropogenic activities over this region. The roles of different possible mechanisms, including possible changes in the vertical redistribution of aerosols, are discussed. Over the period 2007–2015, a significant though weak, increasing trend is seen in the contribution from aerosols above 1 km to the columnar aerosol optical depth. These observations imply possible long‐term climate consequences. Key Points: We report the first time regional synthesis of black carbon aerosols over the Indian region using primary dataThe study revealed surprising observation of decreasing trend in the near surface black carbon aerosol concentrationWe found an increasing trend in contribution from free tropospheric aerosols to the columnar aerosol optical depth [ABSTRACT FROM AUTHOR]

Recent developments in mascon (mass concentration) solutions for GRACE (Gravity Recovery and Climate Experiment) satellite data have significantly increased the spatial localization and amplitude of recovered terrestrial Total Water Storage anomalies (TWSA); however, land hydrology applications have been limited. Here we compare TWSA from April 2002 through March 2015 from (1) newly released GRACE mascons from the Center for Space Research (CSR-M) with (2) NASA JPL mascons (JPL-M), and with (3) CSR Tellus gridded spherical harmonics rescaled (sf) (CSRT-GSH.sf) in 176 river basins, ∼60% of the global land area. Time series in TWSA mascons (CSR-M and JPL-M) and spherical harmonics are highly correlated (rank correlation coefficients mostly >0.9). The signal from long-term trends (up to ±20 mm/yr) is much less than that from seasonal amplitudes (up to 250 mm). Net long-term trends, summed over all 176 basins, are similar for CSR and JPL mascons (66-69 km3/yr) but are lower for spherical harmonics (∼14 km3/yr). Long-term TWSA declines are found mostly in irrigated basins (−41 to −69 km3/yr). Seasonal amplitudes agree among GRACE solutions, increasing confidence in GRACE-based seasonal fluctuations. Rescaling spherical harmonics significantly increases agreement with mascons for seasonal fluctuations, but less for long-term trends. Mascons provide advantages relative to spherical harmonics, including (1) reduced leakage from land to ocean increasing signal amplitude, and (2) application of geophysical data constraints during processing with little empirical postprocessing requirements, making it easier for nongeodetic users. Results of this product intercomparison should allow hydrologists to better select suitable GRACE solutions for hydrologic applications. [ABSTRACT FROM AUTHOR]

Recent advances in processing data from the Gravity Recovery and Climate Experiment (GRACE) have led to a new generation of gravity solutions constrained within a Bayesian framework to remove correlated errors rather than relying on empirical filters. The JPL RL05M mascon solution is one such solution, solving for mass variations using spherical cap mass concentration elements (mascons), while relying on external information provided by near-global geophysical models to constrain the solution. This new gravity solution is fundamentally different than the traditional spherical harmonic gravity solution, and as such, requires different care when postprocessing. Here we discuss two classes of postprocessing considerations for the JPL RL05M GRACE mascon solution: (1) reducing leakage errors across land/ocean boundaries, and (2) scaling the solutions to account for leakage errors introduced through parameterizing the gravity solution in terms of mascons. A Coastline Resolution Improvement (CRI) filter is developed to reduce leakage errors across coastlines. Synthetic simulations reveal a reduction in leakage errors of ∼50%, such that residual leakage errors are ∼1 cm equivalent water height (EWH) averaged globally. A set of gain factors is derived to reduce leakage errors for continental hydrology applications. The combined effect of the CRI filter coupled with application of the gain factors, is shown to reduce leakage errors when determining the mass balance of large (>160,000 km2) hydrological basins from 11% to 30% (0.6-1.5 mm EWH) averaged globally, with local improvements up to 38%-81% (9-19 mm EWH). [ABSTRACT FROM AUTHOR]

A sequence of equilibrium figures with the strongest possible mass concentration toward the centre is constructed. The bicubic equation is solved and the explicit equation is derived for the shape of the surface of any equilibrium figure consid-ered. The new sequence of equilibrium figures is constructed proceeding from the assumption that angular rotation velocity of these figures varies as a function of normalized average density. A relation is derived between the normalized angular velocity $ \tilde \Omega ^2$ and the oblateness ε of the equilibrium figure. Oblateness values of figures lie in the range 0 < ε < 1/3. In the extreme case of the weakest possible potential (the largest ε) the figure has the shape of the limiting Roche lens. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]