Browsing by Author "Shah, Viral"
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Item ModelingToolkit: A Composable Graph Transformation System For Equation-Based ModelingMa, Yingbo; Gowda, Shashi; Anantharaman, Ranjan; Laughman, Chris; Shah, Viral; Rackauckas, ChrisGetting good performance out of numerical equation solvers requires that the user has provided stable and efficient functions representing their model. However, users should not be trusted to write good code. In this manuscript we describe ModelingToolkit (MTK), a symbolic equation-based modeling system which allows for composable transformations to generate stable, efficient, and parallelized model implementations. MTK blurs the lines of traditional symbolic computing by acting directly on a user's numerical code. We show the ability to apply graph algorithms for automatically parallelizing and performing index reduction on code written for differential-algebraic equation (DAE) solvers, "fixing" the performance and stability of the model without requiring any changes to on the user's part. We demonstrate how composable model transformations can be combined with automated data-driven surrogate generation techniques, allowing machine learning methods to generate accelerated approximate models within an acausal modeling framework. These reduced models are shown to outperform the Dymola Modelica compiler on an HVAC model by 590x at 3\% error. Together, this demonstrates MTK as a system for bringing the latest research in graph transformations directly to modeling applications.Item Nitrogen oxides in the free troposphere: Implications for tropospheric oxidants and the interpretation of satellite NO2 measurements(EGU, 2022-07-21) Shah, Viral; Jacob, Daniel J.; Dang, Ruijun; Lamsal, Lok N.; Steenrod, Stephen; et alSatellite-based retrievals of tropospheric NO2 columns are used to infer NOx (NO+NO2) emissions at the surface. These retrievals rely on model information for the vertical distribution of NO2. The free tropospheric background above 2 km is particularly important because the sensitivity of the retrievals increases with altitude. Free tropospheric NOx also has a strong effect on tropospheric OH and ozone concentrations. Here we use observations from three aircraft campaigns (SEAC4RS, DC3, and ATom) and four atmospheric chemistry models (GEOS-Chem, GMI, TM5, and CAMS) to evaluate the model capabilities for simulating background NOx and attribute this background to sources. NO2 measurements over the southeast US during SEAC4RS and DC3 show increasing concentrations in the upper troposphere above 10 km, which is not replicated by GEOS-Chem although the model is consistent with the NO measurements. Using concurrent NO, NO2 and ozone observations from a DC3 flight in a thunderstorm outflow, we show that NO2 measurements in the upper troposphere are biased high, plausibly due to interference from thermally labile NO2 reservoirs, such as peroxynitric acid (HNO4) and methyl peroxy nitrate (MPN). We find that NO2 concentrations calculated from the NO measurements and NO-NO2 photochemical steady state (PSS) are more reliable to evaluate the vertical profiles of NO2 in models. GEOS-Chem reproduces the shape of the PSS-inferred NO2 profiles throughout the troposphere for SEAC4RS and DC3 but overestimates NO2 concentrations by about a factor of 2. The model underestimates MPN and alkyl nitrate concentrations, suggesting missing organic NOx chemistry. On the other hand, the standard GEOS-Chem model underestimates NO observations from the ATom campaigns over the Pacific and Atlantic Oceans, indicating a missing NOx source over the oceans. We find that we can account for this missing source by including in the model the photolysis of particulate nitrate on sea salt aerosols at rates inferred from laboratory studies and field observations of nitrous acid (HONO) over the Atlantic. The average NO2 column density for the ATom campaign in the GEOS-Chem simulation is 2.4×1014 molec cm-2 with particulate nitrate photolysis and 1.5×1014 molec cm-2 without, compared to 1.9×1014 molec cm-2 in the observations (using PSS NO2) and 1.4–2.4×1014 molec cm-2 in the GMI, TM5 and CAMS models. We find from GEOS-Chem that lightning is the main primary NOx source in the free troposphere over the tropics and southern midlatitudes, but aircraft emissions dominate at northern midlatitudes in winter and in summer over the oceans. Particulate nitrate photolysis increases ozone concentrations by up to 5 ppbv in the free troposphere in the northern extratropics in the model, which would largely correct the low model bias relative to ozonesonde observations. Global tropospheric OH concentrations increase by 19 %. The contribution of the free tropospheric background to the tropospheric NO2 columns observed by satellites over the contiguous US increases from 25 % in winter to 65 % in summer according to the GEOS-Chem vertical profiles. This needs to be accounted for when deriving NOx emissions from satellite NO2 column measurements.Item Tropospheric NO₂ vertical profiles over South Korea and their relation to oxidant chemistry: Implications for geostationary satellite retrievals and the observation of NO₂ diurnal variation from space(EGU, 2022-12-06) Yang, Laura Hyesung; Jacob, Daniel J.; Colombi, Nadia K.; Zhai, Shixian; Bates, Kelvin H.; Shah, Viral; Beaudry, Ellie; Yantosca, Robert M.; Lin, Haipeng; Brewer, Jared F.; Chong, Heesung; Travis, Katherine R.; Crawford, James H.; Lamsal, Lok; Koo, Ja-Ho; Kim, JhoonTropospheric nitrogen dioxide (NO2) is of central importance for air quality, climate forcing, and nitrogen deposition to ecosystems. The Geostationary Environment Monitoring Spectrometer (GEMS) is now providing high-density NO2 satellite data including diurnal variation over East Asia. The NO2 retrieval requires independent vertical profile information from a chemical transport model (CTM) to compute the air mass factor (AMF) that relates the NO2 column along the line of sight to the NO2 vertical column. Here, we 25 use aircraft observations from the Korea-United States Air Quality (KORUS-AQ) campaign over the Seoul Metropolitan Area (SMA) and around the Korean peninsula to better understand the factors controlling the NO2 vertical profile, its diurnal variation, the implications for the AMF, and the ability of the GEOS-Chem CTM to compute the AMF and its variability. Proper representation of oxidant chemistry is critical for the CTM simulation of NO2 vertical profiles and is achieved in GEOS-Chem through new model developments 30 including aerosol nitrate photolysis, reduced uptake of hydroperoxy (HO2) radicals by aerosols, and accounting for atmospheric oxidation of volatile chemical products (VCPs). We find that the tropospheric NO2 columns measured from space are mainly contributed by the planetary boundary layer (PBL) below 2 km altitude, reflecting the highly polluted conditions. Repeated measurements of NO2 vertical profiles over SMA at different times of day show that diurnal change in mixing depth affecting the NO2 vertical profile 35 induces a diurnal variation in AMF of comparable magnitude to the diurnal variation in the NO2 column. GEOS-Chem captures this diurnal variation in AMF and more generally the variability in the AMF for the KORUS-AQ NO2 vertical profiles (2.7% mean bias, 7.6% precision)Item Wintertime Formaldehyde: Airborne Observations and Source Apportionment Over the Eastern United States(American Geophysical Union, 2021-01-12) Green, Jaime R.; Fiddler, Marc N.; Fibiger, Dorothy L.; McDuffie, Erin E.; Aquino, Janine; Campos, Teresa; Shah, Viral; Jaeglé, Lyatt; Thornton, Joel A.; DiGangi, Joshua P.; Wolfe, Glenn; Bililign, Solomon; Brown, Steven S.Formaldehyde (HCHO) is generated from direct urban emission sources and secondary production from the photochemical reactions of urban smog. HCHO is linked to tropospheric ozone formation, and contributes to the photochemical reactions of other components of urban smog. In this study, pollution plume intercepts during the Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign were used to investigate and characterize the formation of HCHO in relation to several anthropogenic tracers. Analysis of aircraft intercepts combined with detailed chemical box modeling downwind of several cities suggests that the most important contribution to observed HCHO was primary emission. A box model analysis of a single plume suggested that secondary sources contribute to 21 ± 10% of the observed HCHO. Ratios of HCHO/CO observed in the northeast US, from Ohio to New York, ranging from 0.2% to 0.6%, are consistent with direct emissions combined with at most modest photochemical production. Analysis of the nocturnal boundary layer and residual layer from repeated vertical profiling over urban influenced areas indicate a direct HCHO emission flux of 1.3 × 10¹⁴ molecules cm⁻² h⁻¹. In a case study in Atlanta, GA, nighttime HCHO exhibited a ratio to CO (0.6%–1.8%) and was anti-correlated with O₃. Observations were consistent with mixing between direct HCHO emissions in urban air masses with those influenced by more rapid HCHO photochemical production. The HCHO/CO emissions ratios determined from the measured data are 2.3–15 times greater than the NEI 2017 emissions database. The largest observed HCHO/CO was 1.7%–1.8%, located near co-generating power stations.