Photolysis of Particulate Nitrate as a Source of HONO and NOx
2017
Ye, Chunxiang | Zhang, Ning | Gao, Honglian | Zhou, Xianliang
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.
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