Metropolitan areas play a central role in the context of global climate change. More than half of the world's population now lives in cities, and these urban areas are responsible for more than 70 % of carbon dioxide (CO2) emissions from fossil fuels, as well as for significant amounts of anthropogenic methane (CH4). For this reason, organized monitoring projects are being established in major metropolises around the world (including European cities such as Paris and London) to track these greenhouse gases.

Emission sources in urban environments

The complexity of metropolitan areas lies in the fact that they mix diverse natural and anthropogenic sources of emissions:

• Carbon dioxide (CO2): Anthropogenic sources include primarily the combustion of fossil fuels (transportation, burning of natural gas for electricity generation and seasonal heating or cooling). Biosphere CO2 fluxes include plant and soil respiration, as well as photosynthesis.
• Methane (CH4): It can be produced by both biogenic and thermogenic processes. Biogenic methane is produced by microbial decomposition of organic matter (landfills, wastewater treatment plants, agriculture). Thermogenic methane is associated with processing and leaks from natural gas infrastructure, but also with incomplete combustion of fossil fuels.

What causes variability in CO2 and CH4 concentrations?

The atmosphere in the city is not static. Greenhouse gas concentrations are subject to constant changes, which are influenced not only by the emissions themselves, but also by meteorological conditions and planetary boundary layer (PBL) dynamics.

The key concept in urban measurement is the so-called. local enhancement. This is the difference between the measured concentration in the city and the "background" value of clean air flowing into the city from the ocean, sea or continent.

The following factors influence the variability of these increases:

1. Partial (daily) variability: The highest concentration spikes of CO2 and CH4 often occur at night and early in the morning. This is because at night the planetary boundary layer is shallower and more stable, preventing local surface emissions from dispersing into the upper atmosphere. Conversely, in the afternoon (usually between 12:00 and 16:00), when the atmosphere is better mixed, concentrations tend to be lower and show less variance.
2. Seasonal variability: Concentrations and increases are generally higher in winter than in summer. This phenomenon is caused by increased anthropogenic emissions from heating in the residential and commercial sectors, as well as by a lower mixing layer height in the winter.
3. Spatial variability: In the most urbanized and industrialized parts of cities, average concentrations are logically highest. To illustrate from large monitoring networks: densely built-up city centers can show mid-afternoon increases of about 15 to 20 ppm for CO2 and 80 to 150 ppb for CH4 compared to clean background air.

Furthermore, CH4 data often exhibit a „long-tail“ distribution, meaning that a smaller number of measurements capture extremely high concentrations. These episodes are typically caused by local unplanned leaks from gas infrastructure (so-called fugitive emissions).

The impact of sudden changes in human activity

Modern measurement networks are sensitive enough to capture changes in emissions in real time. A notable European and global example was lockdowns during the COVID-19 pandemic. These restrictions led to a massive drop in traffic, which was immediately reflected in reduced emissions. Micrometeorological measurements and monitoring stations recorded a drop in CO2 emissions during April 2020 of between 33 % and 45 %, depending on the specific metropolitan hub and its traffic load. Interestingly, the drop in carbon monoxide (CO) in some areas did not exactly correspond to the drop in CO2, which scientists attribute to a possible change in the composition of the vehicle fleet during the lockdowns (for example, continued freight transport versus a decrease in older, more polluting passenger cars).

How is this data obtained and verified?

Two approaches are combined to understand emissions and their variability:

• Bottom-up approach: It uses data on local activity (fuel consumption, traffic density) and emission inventories. These inventories are increasingly being carried out with high spatial resolution down to the level of individual buildings or streets (e.g. the Hestia system).
• Top-down approach: It consists of direct measurement of gas concentrations in the atmosphere. This is done using continuous spectroscopic analyzers located on tall communication towers or on the roofs of buildings (so-called. Atmospheric Integrity Records or AIR), which provides an undistorted chronological record of the air.

For local and metropolitan flows, they are also often used micrometeorological methods, such as "eddy covariance" (EC) or Monin-Obukhov similarity theory (MOST), which can very accurately capture emissions at the level of smaller neighborhoods (with an area of approximately 1 km²).

Continuous monitoring of CO2 and CH4 in metropolitan areas – whether in Europe or elsewhere in the world – reveals that the urban atmosphere is a highly dynamic system. While inventories tell us what cities „should“ produce, modern access networks and atmospheric records show us exactly what is actually happening in the air. This precise data is an essential tool for urban planning, verifying the effectiveness of climate action and reducing the carbon footprint. JRi&CO2AI