ALBERT

Description: 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 (NOx = NO + NO2) (R2 = 0.52), the sum of all known NOx-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 NOx 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 NOx and pNO3 as precursors. While HONO was not a significant OH precursor compared to O3 under low NOx conditions in the air column, it was an important intermediate product of a photochemical renoxification process recycling nitric acid and nitrate back to NOx. Finally, the HONO / NOx 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.