Photolysis of Particulate Nitrate as a Source of HONO and NOx

Photolysis of nitric acid on the surface has been found recently to be greatly enhanced from that in the gas phase. Yet, photolysis of particulate nitrate (pNO₃) associated with atmospheric aerosols is still relatively unknown. Here, aerosol filter samples were collected both near the ground surface and throughout the troposphere on board the NSF/NACR C-130 aircraft. The photolysis rate constants of pNO₃ were determined from these samples by directly monitoring the production rates of nitrous acid (HONO) and nitrogen dioxide (NO₂) under UV light (>290 nm) irradiation. Scaled to the tropical noontime condition on the ground level (solar zenith angle = 0°), the normalized photolysis rate constants (jpNO₃N) are in the range from 6.2 × 10⁻⁶ s⁻¹ to 5.0 × 10⁻⁴ s⁻¹ with a median of 8.3 × 10⁻⁵ s⁻¹ and a mean (±1 SD) of (1.3 ± 1.2) × 10⁻⁴ s⁻¹. Chemical compositions, specifically nitrate loading and organic matter, affect the rate of photolysis. Extrapolated to ambient pNO₃ loading conditions, e.g. ≤ 10 nmol m⁻³, the mean jpNO₃N value is over 1.8 × 10⁻⁴ s⁻¹ in the suburban, rural, and remote environments. Photolysis of particulate nitrate is thus a source of HONO and NO₂ in the troposphere.