Engineering topics

Amin R Nehrir

Publications and source records attributed to Amin R Nehrir.

9 records

NASA NTRS2020

Assessment of Lidar XCH4 Measurements During ACT-America 2019 for Boundary Layer Methane and Emission Model Constraint

Atmospheric methane is a potent and abundant greenhouse gas (GHG) with increasing importance due to rising emissions and their subsequent impact on radiative forcing, indicating an important driving role in climate change. The recent Earth Science Decadal Survey called for further understanding of the sources and sinks of atmospheric methane, the processes that will affect their future concentrations, and identified the need for advanced observing capabilities to elucidate these issues. HALO, a combined methane Differential Absorption Lidar and Aerosol/Cloud High Spectral Resolution Lidar participated in the ACT-America summer 2019 campaign to help constrain the emissions and transport of GHGs in the atmosphere to address some of the aforementioned science goals. In this presentation we will discuss the utility of using an in-situ point measurement versus lidar column measurement for inventory and source identification applications. Comparisons between column measurements of methane from in-situ and lidar will be made to assess whether point measurements are useful when the assumption of a well-mixed boundary layer, a practice commonly used in the community, is valid. We will also show that a lidar column measurement can apportion boundary layer methane from the column and has the ability to identify boundary layer methane enhancements in fair weather conditions using atmospheric backscatter signals, a measurement capability that has the potential to transform the observational strategies of future airborne carbon cycle science investigations. We will also present on the differences and utility of constraining emission models using an in-situ versus lidar column and asses the resulting uncertainties of each.

Rory A Barton-Grimley↗

NASA NTRS2020

Understanding and Improving Model Representation of Aerosol Optical Properties for a Chinese Haze Event Measured During KORUS-AQ

KORUS-AQ was an international cooperative air quality field study in South Korea that measured local and remote sources of air pollution affecting the Korean Peninsula during May–June 2016. Some of the largest aerosol mass concentrations were measured during a Chinese haze transport event (24 May). Air quality forecasts using the WRF-Chem model with aerosol optical depth (AOD) data assimilation captured AOD during this pollution episode but overpredicted surface particulate matter concentrations in South Korea, especially PM2.5, often by a factor of 2 or larger. Analysis revealed multiple sources of model deficiency related to the calculation of optical properties from aerosol mass that explain these discrepancies. Using in situ observations of aerosol size and composition as inputs to the optical properties calculations showed that using a low-resolution size bin representation (four bins) underestimates the efficiency with which aerosols scatter and absorb light (mass extinction efficiency). Besides using finer-resolution size bins (8–16 bins), it was also necessary to increase the refractive indices and hygroscopicity of select aerosol species within the range of values reported in the literature to achieve better consistency with measured values of the mass extinction efficiency (6.7 m2 g−1 observed average) and light-scattering enhancement factor (f(RH)) due to aerosol hygroscopic growth (2.2 observed average). Furthermore, an evaluation of the optical properties obtained using modeled aerosol properties revealed the inability of sectional and modal aerosol representations in WRF-Chem to properly reproduce the observed size distribution, with the models displaying a much wider accumulation mode. Other model deficiencies included an underestimate of organic aerosol density (1.0 g cm−3 in the model vs. observed average of 1.5 g cm−3) and an overprediction of the fractional contribution of submicron inorganic aerosols other than sulfate, ammonium, nitrate, chloride, and sodium corresponding to mostly dust (17 %–28 % modeled vs. 12 % estimated from observations). These results illustrate the complexity of achieving an accurate model representation of optical properties and provide potential solutions that are relevant to multiple disciplines and applications such as air quality forecasts, health impact assessments, climate projections, solar power forecasts, and aerosol data assimilation.

Pablo E. Saide↗

NASA NTRSDate not supplied

TPSAS-NF1676L-11308-DND

The Lidar Atmospheric Sensing Experiment (LASE) system has operated on the NASA ER-2, P-3, and DC-8 aircraft during the past 16 years and participated in 13 field experiments. This system provides vertical profiles of water vapor mixing ratio and aerosol and cloud backscattering. A brief overview is presented on the upgrade of LASE that included refurbishment of laser diode seeding, control and data system, and zenith receiver system. These developments have enhanced the operational reliability of the system and improved its performance. The LASE system was deployed on the NASA DC-8 aircraft recently for the NASA GRIP (Genesis and Rapid Intensification Processes) field experiment, which was conducted during August and September 2010 from operational bases in Fort Lauderdale, FL and St. Croix, VI. Initial measurements from this field experiment are presented including distributions of water vapor, aerosol, and clouds. Temperature profiles from DC-8 dropsondes and nearby radiosondes are used to derive relative humidity profiles from the LASE water vapor mixing ratio profiles. Comparisons of LASE water vapor mixing ratio profiles with those measured by the new AVAPS-II dropsondes from the NASA DC-8 are also presented.

Susan A Kooi↗

NASA NTRSDate not supplied