Quantifying HONO Production from Nitrate Photolysis in a Polluted Atmosphere

Area of Science:

• Atmospheric Chemistry
• Environmental Science
• Photochemistry

Background:

• Particulate nitrate (pNO3-) photolysis is a potential source of nitrous acid (HONO) in the troposphere.
• Previous measurements of the photolysis rate constant (jpNO3-) are uncertain due to the 'shadow effect,' where light extinction within aerosol layers skews results.

Purpose of the Study:

• To develop and apply a novel method for correcting the shadow effect in pNO3- photolysis rate measurements.
• To accurately quantify jpNO3- and its influence on HONO production over the North China Plain during winter haze.

Main Methods:

• A new technique was employed using aerosol filters with identical chemical compositions but varying aerosol loadings to correct for the shadow effect.
• The corrected jpNO3- was quantified for particulate nitrate over the North China Plain.

Main Results:

• The shadow effect correction significantly increased the normalized average jpNO3- at 5 °C from 5.89 × 10^-6 s^-1 to 1.72 × 10^-5 s^-1.
• Corrected jpNO3- decreased with increasing pH and nitrate fraction in PM2.5, showing no correlation with nitrate concentration.
• A parametrization for jpNO3- was developed for atmospheric modeling.

Conclusions:

• The study provides a more accurate method for determining pNO3- photolysis rates, crucial for understanding atmospheric HONO production.
• The findings are vital for improving air quality models, particularly in regions like the North China Plain experiencing significant haze events.