Los Angeles Carbon Dioxide Concentrations

Introduction

Fig. 1: Map showing the locations of the CO2 measurement sites listed in Table 1. [2] (Source: A. Foster)

Carbon dioxide is the most abundantly emitted and long-lived anthropogenic gas. While CO₂ is generally well-mixed in the atmosphere, its mixing ratios - the number of molecules of a chemical per molecule of dry air - are higher in urban air due to local emissions, resulting in urban CO₂ domes. [1] Gases can become well-mixed in the atmosphere if they are long-lived and are emitted uniformly over time. Although carbon dioxide is a long-lived gas, it is not well-mixed in the atmosphere because its emission rate is not uniform - human emissions of carbon dioxide both change over short time periods and are concentrated in urban areas. [1] This leads to higher CO₂ mixing ratios in urban areas than in surrounding rural areas, referred to as carbon dioxide domes. This report investigates CO₂ concentration measurements over Los Angeles.

Carbon Dioxide Concentrations

Verhulst et al. report observations of carbon dioxide concentrations from the Los Angeles (LA) Megacity Carbon Project in 2015. [2] The Megacities Carbon Project was established to develop carbon monitoring in major cities and address gaps in the knowledge of greenhouse gas emissions. [2] They draw observations from twelve sites, each equipped with a spectrometer to measure CO₂ levels. Their results are represented in Table 1, in which the highest mean CO₂ concentration for 2015 was at the University of Southern California (downtown LA) site, with 434.8 parts per million (ppm), closely followed by Ontario and Compton, with 434.0 and 430.5 ppm respectively. According the NOAA's Mauna Loa Observatory, the average CO₂ concentration in Earth's atmosphere was roughly 400 ppm in 2015, when the Megacity Carbon Project took place. [3] Thus, the CO₂ concentration over downtown LA was 8.7% higher than that of the global atmosphere in 2015.

Locations of these measuring sites are mapped in Fig. 1, including Compton and University of Southern California in the West Coast Basin, California State University Fullerton in the Orange County Coastal Plan, Victorville in the desert, and La Jolla on the coast. Based on their location, the sites differ geophysically and are influenced by different atmospheric factors, causing a difference in CO₂ levels. For instance, the Ontario location receives marine winds coming in from the Pacific, particularly during daytime hours. This marine air picks up emissions from the basin making Ontario a good receptor site for CO₂ concentrations. [2]

Los Angeles is located in a basin surrounded by mountains, which serves to trap air pollution. Additionally, marine inversions cause Los Angeles' polluted air to become trapped. These marine inversions are caused when cold air from the ocean becomes capped by warmer, inland air. [4] The warm air layer prevents the cooler air below from rising and dispersing, so the emissions and pollutants accumulate, contributing to elevated air pollution levels.

As the mountains block emissions from moving inland, the desert experiences lower CO₂ concentrations: the Victorville site observed less CO₂ than those in the basin: 404.7 ppm vs 434.8 ppm. [2] Similarly, stations near the ocean receive marine air with lower CO₂ concentrations, such as the La Jolla site, which has an average CO₂ concentration of 412.9 ppm.

There are also winds moving from the hot desert to the cold ocean. Most notably, the Santa Ana winds originate in the desert and blow down towards the coast. [5] Thus, the Santa Ana winds ventilate the LA Basin and push urban emissions over the Pacific Ocean, leading to decreased CO₂ concentrations observed by in situ measurements.

CO₂ concentrations also fluctuate with respect to the time of day. The San Gabriel Mountains north of the Basin act as a barrier, trapping greenhouse gasses and air pollutants in the Basin. However, around noon, upslope flow and the rising temperature inversion layer allows wind and air to move over the mountains. [6] This enables CO₂ concentrations to build up in the mornings and decrease in the afternoons. The annual average mid-afternoon concentrations are represented in Table 1 and are notably much smaller than the general annual averages: that difference between the all-hours mean and mid-afternoon mean is as great as 18.6 ppm in Ontario and 13.2 ppm in downtown LA. Verhulst et al. report that the in-situ observations show that CO₂ concentrations increase at night and remain high until sunrise, and then drop over the course of the day.

Discussion

The carbon dioxide concentration in a given location is heavily influenced by weather patterns, climate events, and the geophysical attributes of the site.

Accurate observation of CO₂ concentrations enables more precise identification and understanding of CO₂ domes. Jacobson reports how local CO₂ emissions are correlated to increases in local air pollution. [7] Carbon dioxide domes form when high levels of local CO₂ emissions warm the air locally, warmer air evaporates more water, and both higher water vapor concentrations and higher temperatures increase the rate of air pollution formation. [7]

© Anissa Foster. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] M. Z. Jacobson, "Enhancement of Local Air Pollution by Urban CO₂ Domes," Environ. Sci.Technol. 44, 2497 (2010).

[2] K. R. Verhulst et al., "Carbon Dioxide and Methane Measurements From the Los Angeles Megacity Carbon Project," Atmos. Chem. Phys. 17, 8313 (2017).

[3] C. Le Quéré et al., "Global Carbon Budget 2015," Earth Sys. Sci. Data 7, 349 (2015).

[4] A. Miller and D. Ahrens, "Ozone Within and Below the West Coast Temperature Inversion," Tellus 22, 328 (2016).

[5] M. N. Raphael, "The Santa Ana Winds of California," Earth Interact. 7, 1-13 (2003).

[6] R. Lu and R. P. Turco, "Air Pollutant Transport in a Coastal Environment - II. Three-Dimensional Simulations over Los Angeles Basin," Atmos. Chem. Phys. 29, 1499 (1995).

[7] M. Z. Jacobson, "On the Causal Link Between Carbon Dioxide and Air Pollution Mortality," Geophys. Res. Lett. 35, L03809 (2008).