Photolysis of Particulate Nitrate as a Source of HONO and NO_x

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 (j_pNO^N) 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 j_pNO^N 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.