Rethinking Nitrate Aerosol Substitution for Sulfate in Winter Haze

Area of Science:

• Atmospheric chemistry
• Environmental science
• Air pollution research

Background:

• Nitrate aerosol formation significantly impacts the global nitrogen cycle and urban air quality.
• Previous research suggested sulfate reduction enhances nitrate formation through thermodynamic equilibrium shifts.
• A distinct winter haze regime in North China challenges this widely accepted model.

Purpose of the Study:

• To investigate the relationship between sulfate abatement and nitrate aerosol formation in North China.
• To identify the factors driving nitrate aerosol behavior under specific environmental conditions.
• To re-evaluate the effectiveness of clean-air measures concerning nitrate pollution.

Main Methods:

• Long-term atmospheric observations in North China during the 2010s.
• Utilized a full thermodynamic equilibrium model for data interpretation.
• Employed a regional air-quality model to simulate atmospheric processes.

Main Results:

• Observed a sustained winter haze regime where nitrate aerosols did not increase despite drastic SO2 reduction (>80%).
• Identified low-temperature and ammonia (NH3)-rich conditions as key drivers, pushing total nitrate proportion to near 100%.
• Demonstrated that increased NH3 did not alter thermodynamic equilibrium to favor more nitrate aerosols.

Conclusions:

• The widely proposed transition from NH3-poor to NH3-rich conditions for nitrate formation may not apply universally.
• Persistent thermodynamic constraints in high-NH3 environments can prevent expected nitrate aerosol increases.
• Misunderstanding these constraints may lead to inaccurate assessments of nitrate aerosol trends and air pollution control effectiveness.