Welcome to another edition of our “Molecule of the Month” series, where we dive into the fascinating world of atmospheric compounds that shape our environment and influence our lives. This month, we explore nitric oxide (NO), a reactive nitrogen compound that plays a pivotal role in atmospheric processes, air quality, and environmental chemistry.

 

The Importance of Nitric Oxide:

Role in Atmospheric Chemistry: Nitric oxide is a critical precursor in the formation of nitrogen dioxide (NO₂), which, together with NO, constitutes the group known as nitrogen oxides (NOₓ). NOₓ compounds drive key atmospheric reactions, including the formation of ozone (O₃) in the troposphere and its depletion in the stratosphere.

Air Quality and Health: Nitric oxide significantly impacts urban air quality. While not directly harmful at typical ambient concentrations, its rapid conversion to NO₂ and subsequent reactions can lead to the formation of ground-level ozone and particulate matter (PM). These secondary pollutants are linked to respiratory and cardiovascular diseases.

Climate Implications: NO contributes indirectly to climate forcing. Through its role in the production of ozone—a potent greenhouse gas—it affects the radiative balance of the atmosphere. Additionally, NOₓ emissions influence the lifetime of methane, another key greenhouse gas, by altering the abundance of hydroxyl radicals (OH).

 

One specific example highlighting the importance of measuring NO directly is in urban air quality management. In cities with heavy traffic, vehicles are a major source of NO emissions. Directly measuring NO allows for precise identification of pollution hotspots and the effectiveness of traffic management strategies. Additionally, NO has a very short lifetime in the atmosphere, typically lasting only a few seconds before it reacts with other compounds to form nitrogen dioxide (NO₂) or other nitrogen oxides. This short lifetime means that NO concentrations can vary significantly over short distances and time periods. Therefore, direct measurement is essential to capture these rapid changes and provide accurate data for air quality management. NO eddy covariance is crucial for accurately measuring the exchange rates of nitrogen monoxide between the surface and the atmosphere, providing high-resolution data essential for understanding urban pollution dynamics. The Breathe London project exemplifies the importance of direct NO measurement, as it enabled precise identification of pollution hotspots and demonstrated the effectiveness of low-emission zones in reducing vehicle emissions. Together, these approaches highlight the value of advanced monitoring techniques in improving urban air quality and informing policy decisions.

There are different sources of Nitric Oxide:

Natural Sources: (I) Biogenic Emissions: Soil microbes emit NO during the natural processes of nitrification and denitrification, particularly in agricultural regions. (II) Lightning: Lightning strikes produce NO through the high-temperature oxidation of atmospheric nitrogen (N₂) and oxygen (O₂). This process is a major natural source of NO in the upper troposphere.

Anthropogenic Sources: (I) Combustion Processes: The combustion of fossil fuels in vehicles, power plants, and industrial facilities is the dominant human source of NO emissions. (II) Biomass Burning: Forest fires and agricultural burning release significant amounts of NO.

To understand and mitigate the impacts of NO on air quality and climate, precise measurement techniques are essential:

Chemiluminescence Detection (CLD): This standard technique measures NO by reacting it with ozone, producing a chemiluminescent light signal proportional to the NO concentration. CLD offers high sensitivity and specificity for NO detection, real-time measurement capabilities, and its well-established use in air quality monitoring. However, it requires regular maintenance and calibration, can be expensive due to the need for specialized equipment, and may experience interference from other reactive gases.

Laser-Induced Fluorescence (LIF): LIF involves exciting NO molecules with a laser and detecting the resulting fluorescence. This method provides high spatial and temporal resolution, exceptional sensitivity for detecting low concentrations of NO, and minimal background interference, which leads to accurate measurements. On the downside, LIF requires complex and expensive equipment, needs careful calibration and maintenance, and is generally limited to laboratory or controlled field environments due to the complexity of the equipment.

Electrochemical Sensors: Compact and cost-effective, electrochemical sensors detect NO by its reaction with specific electrodes. Although less sensitive than advanced techniques, they are easily portable, making them suitable for use in the field, cost effective compared to other advanced techniques, and low power consumption with ease of use. However, they have lower sensitivity and specificity than, for example, LAS, have a limited lifetime requiring frequent replacement, and are susceptible to interference from other gases.

Differential Optical Absorption Spectroscopy (DOAS): This technique measures the absorption of ultraviolet and visible light by NO in the atmosphere. It is useful for long-path measurements, can cover large areas, measures multiple gases simultaneously, and is suitable for remote sensing and urban air quality studies. The cons include the need for a clear line of sight for accurate measurements, potential interference from atmospheric conditions such as fog and rain, and the complexity of initial setup and calibration.

Here, at MIRO Analytical, we use direct laser absorption spectroscopy (LAS) in the mid-infrared region to measure water vapor together with up to 9 other gases. This technique offers highest sensitivity, selectivity, and real-time monitoring capabilities, making it ideal for trace-level, real-time continuous monitoring.

 

Conclusion:

Nitric oxide is more than just a trace gas; it is a dynamic participant in atmospheric processes that influence air quality, human health, and the global climate. By monitoring and understanding NO emissions, we can better address the challenges posed by air pollution and its broader environmental impacts.

Stay tuned for the next edition of “Molecule of the Month,” where we continue to unveil the atmospheric compounds shaping our world!

 

 

 

 

About the author: Dr Jonas Bruckhuisen studied chemistry at the RWTH University in Aachen, Germany, before obtaining his PhD in gas phase spectroscopy and atmospheric science from the Université du Littoral Côte d’Opale in Dunkirk, France. He has been application scientist at MIRO Analytical since 2023.