Accounting for Non-Detects: Application to Satellite Ammonia Observations

Area of Science:

• Atmospheric Science
• Remote Sensing
• Environmental Monitoring

Background:

• Satellite measurements of minor atmospheric species like ammonia (NH3) often contain signals below the sensor's detection limit.
• Failure to account for these
• non-detects

Purpose of the Study:

• To develop and apply a methodology for explicitly identifying and accounting for sub-detection limit satellite measurements.
• To improve the accuracy of averaged satellite data for atmospheric species with weak spectral signals.
• To assess the impact of non-detects on ammonia (NH3) concentration data derived from satellite observations.

Main Methods:

• Utilizing information content from satellite signals to identify non-detects.
• Applying a consistent approach to account for these identified non-detects.
• Implementing the methodology on the CrIS Fast Physical Retrieval (CFPR) ammonia product.

Main Results:

• In high ammonia (NH3) emission regions (> 7.5 ppbv), non-detects are infrequent (<5%) and have a minor impact (<5% decrease) on averaged values.
• In low ammonia (NH3) concentration regions (< 1 ppbv), non-detects can exceed 70% of measurements.
• Accounting for non-detects in low-concentration areas can decrease annual averaged values by over 50%, aligning distributions with surface observations.

Conclusions:

• The developed methodology provides a more realistic averaged dataset for ammonia (NH3) by addressing undetected measurements.
• Accurate handling of non-detects is crucial for reliable satellite-based atmospheric composition studies, especially for species with weak signals.
• The findings highlight the significant impact of undetected data on satellite-derived ammonia (NH3) concentrations in regions with low ambient levels.