Daytime formation of nitrous acid at a coastal remote site in Cyprus indicating a common ground source of atmospheric HONO and NO

Abstract. Characterization of daytime sources of nitrous acid (HONO) is crucial to understand atmospheric oxidation and radical cycling in the planetary boundary layer. HONO and numerous other atmospheric trace constituents were measured on the Mediterranean island of Cyprus during the CYPHEX (CYprus PHotochemical EXperiment) campaign in summer 2014. Average volume mixing ratios of HONO were 35 pptv (±25 pptv) with a HONO ∕ NO_x ratio of 0.33, which was considerably higher than reported for most other rural and urban regions. Diel profiles of HONO showed peak values in the late morning (60 ± 28 pptv around 09:00 local time) and persistently high mixing ratios during daytime (45 ± 18 pptv), indicating that the photolytic loss of HONO is compensated by a strong daytime source. Budget analyses revealed unidentified sources producing up to 3.4  ×  10⁶ molecules cm⁻³ s⁻¹ of HONO and up to 2.0  ×  10⁷ molecules cm⁻³ s⁻¹ NO. Under humid conditions (relative humidity  >  70 %), the source strengths of HONO and NO exhibited a close linear correlation (R² = 0.72), suggesting a common source that may be attributable to emissions from microbial communities on soil surfaces.