Differences in fine particle chemical composition on clear and cloudy days

Area of Science:

• Atmospheric chemistry
• Aerosol science
• Remote sensing

Background:

• Clouds influence particulate matter (PM2.5) mass and chemical makeup.
• Satellite data often exclude cloud-affected areas, limiting air quality assessments.
• Understanding aerosol-cloud interactions is crucial for climate and air quality modeling.

Purpose of the Study:

• To investigate differences in PM2.5 chemical composition between cloudy and clear sky conditions.
• To analyze particle hygroscopicity and aerosol liquid water (ALW) variations.
• To assess the impact of cloud cover on air quality data and model biases.

Main Methods:

• Utilized data from the Interagency Monitoring of PROtected Visual Environments (IMPROVE) network (2010-2014).
• Defined cloudy and clear sky periods using Moderate Resolution Imaging Spectroradiometer (MODIS) cloud flags.
• Compared PM2.5 chemical constituents, hygroscopicity, and ALW between sky conditions.

Main Results:

• Significant regional and seasonal differences in PM2.5 and its composition were observed between cloudy and clear skies.
• In the eastern US, relative humidity alone did not explain ALW variations, indicating the role of emissions and in situ chemistry.
• Cloudy periods showed distinct PM2.5 characteristics compared to clear periods.

Conclusions:

• Cloudy conditions significantly impact PM2.5 measurements and composition.
• Excluding cloudy data introduces a clear-sky bias, potentially hindering accurate air quality modeling.
• Further research is needed to incorporate cloud effects into aerosol-cloud interaction models.