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. Here we report the measurement results of nitrous acid (HONO) and a suite ofrelevant parameters on the NCAR C-130 research aircraft in the southeastern USduring the NOMADSS 2013 summer field study. The daytime HONO concentrationranged 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 flightaltitude of 300 m in the PBL, and the transport of ground surface HONO wasnot found to be a significant contributor to the tropospheric HONO budget. Thetotal in situ HONO source mean ( ± 1 SD) was calculated as 53( ± 21) pptv h −1 during the day. The upper-limit contribution from NOx -related reactions was 10 ( ± 5) pptv h −1 , and thecontribution from photolysis of particulate nitrate ( pNO3 ) was 38( ± 23) pptv h −1 , based on the measured pNO3 concentrationsand the median pNO3 photolysis rate constant of 2.0  ×  10 −4  s −1 determined in the laboratory using ambient aerosol samples. Thephotolysis of HONO contributed to less than 10 % of the primary OH source.However, a recycling NOx source via pNO3 photolysis was equivalentto ∼  2.3  ×  10 −6  mol m −2  h −1 in the aircolumn within the PBL, a considerable supplementary NOx source in thelow- NOx background area. Up to several tens of parts per trillion by volume of HONO were observedin power plant and urban plumes during the day, mostly produced in situfrom precursors including NOx and pNO3 . Finally, there was noobservable accumulation of HONO in the nocturnal residual layer and thenocturnal FT in the background southeastern US, with an increase in theHONO  ∕ NOx ratio of ≤  3  ×  10 −4  h −1 after sunset.

Tropospheric HONO distribution and chemistry in the southeastern US