Air humidity affects secondary aerosol formation in different pathways

Area of Science:

• Atmospheric Chemistry
• Environmental Science
• Air Pollution Research

Background:

• Haze pollution and particulate matter (PM2.5) chemical composition exhibit distinct patterns during winter in the North China Plain (NCP).
• Understanding the influence of air humidity on aerosol formation is crucial for effective air quality management.

Purpose of the Study:

• To investigate the impact of air humidity on PM2.5 chemical composition during winter haze episodes in Tianjin.
• To differentiate the effects of relative humidity (RH) and absolute humidity on secondary aerosol generation.

Main Methods:

• In situ observation of haze pollution characteristics and PM2.5 chemical composition during winter 2017-2018 in Tianjin.
• Analysis of the relationship between meteorological parameters (humidity, temperature) and aerosol chemical species.

Main Results:

• Nitrate concentrations showed a stronger correlation with absolute humidity, while sulfate concentrations were more sensitive to relative humidity.
• Daytime secondary aerosol formation is influenced by water vapor, ozone, and solar radiation, particularly during transitional seasons.
• Nighttime haze formation is favored by low temperatures and high RH, promoting nitric acid partitioning and N2O5 hydrolysis.
• Sulfate mass fraction increased at lower temperatures (T < 0°C) and higher RH (>80%), driven by enhanced SO2 dissolution and aqueous oxidation rates.

Conclusions:

• Air humidity plays a critical role in modulating PM2.5 chemical composition and secondary aerosol formation during winter haze in the NCP.
• The distinct responses of nitrate and sulfate to different humidity metrics highlight the complexity of aerosol chemistry under varying meteorological conditions.
• Stringent meteorological conditions are likely required for significant sulfate formation via aqueous reactions given current low regional SO2 levels.