# Research

Here is a list of specific research topics:

Feel free to contact me (dienwu@caltech.edu) if you’re interested in my work.

# Anthropogenic CO2 emissions for cities

### The model development of the Column version of the Stochastic Time-Inverted Lagrangian Transport

X-STILT (Wu et al., 2018, GMD) is modified based upon the STILT model version 2 (Lin et al., 2003, Fasoli et al., 2018); Model code archived on GitHub; with a Cesium animation of X-STILT on YouTube.

• Initially designed for OCO-2 with dependences from its sounding-specific vertical sensivities (e.g., AK and PWF), but has now been modified to adapt to other sensors including OCO-3 and TROPOMI (i.e., CO, NO2, CH4).

• Provide the atmospheric transport and calculate the imprints of potential upwind sources and sinks onto downwind satellite column measurements (so-called column footprint)

• Provide comprehensive horizontal and vertical transport uncertainties of column CO2 (XCO2)

### The application of X-STILT in calculating per capita CO2 emissions for cities from space

Scientific significance
We provide the first independent observational constraint on per capita emissions and their relationship with population density and per capita GDP, by using spaceborne CO2 measurements from OCO-2.

• Combined with population density data, we were able to derive independent emission estimates of per capita CO2 emissions for 20 cities around the world and explore their relationship with population density using satellite data (Wu et al., 2020, ERL).

• Previous urban-scaling studies relied on bottom-up inventory that has implicit assumption between emissions and population density. However, this method provides (1) an objective definition of urban extent and (2) observational constraints from satellite retrieved mixing ratios, which were lacking from previous work.

• This work was featured by NASA.

# Biogenic CO2 fluxes for cities

A new approach to separate biogenic fluxes from anthropogenic emissions: A Model for Urban Biogenic CO2 Fluxes: Solar-Induced Fluorescence for Modeling Urban biogenic Fluxes (SMUrF, Wu et al., 2021, GMD).

• Evaluate the biogenic fluxes against available flux tower observations
• Reveal differences between biogenic vs. anthropogenic fluxes over 40 cities
• Reveal urban-rural contrast in NEE fluxes and their diurnal cycles
• Estimate the resulting imprints on atmospheric column CO2 (using X-STILT).

Feel free to check out the model scripts and data release from here.

# Intra-city combustion efficiency and sectoral contributions

Research questions (Wu et al. ACPD)

1. Is it possible to quantify accurate spatially-resolved emission ratio between CO and CO2 from asynchronous satellite measurements?
2. Can the combustion efficiency for a given sector be determined without using prior emission inventories?
• Examine several KEY factors that can influence the interpretatin of ERCO using TROPOMI and OCO-3.

• Identify distinct combustion efficiencies between cities and within city area (i.e., areas strongly affected by heavy industry in a city).

Ongoing work