“…Contrary to the detected missing HONO source near the ground, recent airborne measurements (500-1200 m a.g.l., above ground level) observed HONO concentrations, which could be explained by gas-phase reactions only (Li et al, 2014;Neuman et al, 2016). However, vertical gradient studies showed higher HONO concentrations near the ground than in higher altitudes, indicating a ground level source (Harrison and Kitto, 1994;Kleffmann et al, 2003;Ren et al, 2011;Stutz et al, 2002;VandenBoer et al, 2013;Villena et al, 2011;Zhou et al, 2011;Wong et al, 2012Wong et al, , 2013Vogel et al, 2003;Zhang et al, 2009;Young et al, 2012). This is supported by gas exchange studies showing that HONO and NO can be emitted from (natural) soil and biological soil crusts (biocrusts, BSCs), even without applying atmospheric NO 2 (Su et al, 2011;Oswald et al, 2013;Mamtimin et al, 2016;Weber et al, 2015;Meixner and Yang, 2006).…”