Tropospheric HONO distribution and chemistry in the southeastern US

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 southeastern US during the NOMADSS 2013 summer field study. The daytime HONO concentration ranged from low parts per trillion by volume (pptv) in the free troposphere (FT) to mostly within 5-15 pptv in the background planetary boundary layer (PBL). There was no discernible vertical HONO gradient above the lower flight altitude of 300 m in the PBL, and the transport of ground surface HONO was not found to be a significant contributor to the tropospheric HONO budget. The total in situ HONO source mean (±1 SD) was calculated as 53 (±21) pptv h^-1 during the day. The upper-limit contribution from NO-related reactions was 10 (±5) pptv h^-1, and the contribution from photolysis of particulate nitrate (pNO₃) was 38 (±23) pptv h^-1, based on the measured pNO₃ concentrations and the median pNO₃ photolysis rate constant of 2.0 × 10^-4 s^-1 determined in the laboratory using ambient aerosol samples. The photolysis of HONO contributed to less than 10 % of the primary OH source. However, a recycling NOx source via pNO₃ photolysis was equivalent to ~ 2.3 × 10^-6 mol m^-2 h^-1 in the air column within the PBL, a considerable supplementary NOx source in the low-NOx background area. Up to several tens of parts per trillion by volume of HONO were observed in power plant and urban plumes during the day, mostly produced in situ from precursors including NOx and pNO₃. Finally, there was no observable accumulation of HONO in the nocturnal residual layer and the nocturnal FT in the background southeastern US, with an increase in the HONO / NOx ratio of ≤ 3 × 10^-4 h^-1 after sunset.