Tropospheric HONO Distribution and Chemistry in the Southeast U.S.

Here we report the measurement results of nitrous acid (HONO) and a suite of relevant parameters on the NCAR C-130 research aircraft in the Southeast U.S. during NOMADSS 2013 summer field study. Daytime HONO concentrations ranged from low parts per trillion by volume (pptv) in the free troposphere (FT) to mostly within 5–15 pptv in the background terrestrial air masses, and to up to 40 pptv in the industrial and urban plumes in the planetary boundary layer (PBL). There was no discernable vertical HONO distribution trend in the PBL above the lowest flight altitude of 300 m, indicating that the ground surface HONO source was not a significant contributor to the HONO budget in the measurement altitude between 300 m and 4.7 km. While there was a strong correlation between the concentrations of HONO and oxides of nitrogen (NO_x = NO + NO₂) (R² = 0.52), the sum of all known NO_x-related HONO formation mechanisms was found to account for less 20 % of the daytime HONO source in the background terrestrial air masses, due to the low level of NO_x and surface area density of aerosol particles. Photolysis of particulate nitrate (pNO₃) appeared to be the major daytime HONO source in the background terrestrial air masses, based on the measured pNO₃ concentration and the median value of 2.0 × 10⁻⁴ s⁻¹ for pNO₃ photolysis rate constant determined in the laboratory using ambient aerosol samples collected during the field study. Within the power plant and industrial plumes encountered, daytime HONO was predominantly produced by secondary formation processes involving both NO_x and pNO₃ as precursors. While HONO was not a significant OH precursor compared to O₃ under low NO_x conditions in the air column, it was an important intermediate product of a photochemical renoxification process recycling nitric acid and nitrate back to NO_x. Finally, the HONO / NO_x ratio stayed relatively constant for several hours after sunset in the nocturnal residual layer, suggesting no significant night-time volume HONO source existed in the nocturnal residual layer and the nocturnal FT under background conditions. [ABSTRACT FROM AUTHOR]