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Ozone concentrations in the Po Valley of northern Italy often exceedinternational regulations. As both a source of radicals and an intermediatein the oxidation of most volatile organic compounds (VOCs), formaldehyde(HCHO) is a useful tracer for the oxidative processing of hydrocarbons thatleads to ozone production. We investigate the sources of HCHO in the PoValley using vertical profile measurements acquired from the airship ZeppelinNT over an agricultural region during the PEGASOS 2012 campaign. Using a 1-Dmodel, the total VOC oxidation rate is examined and discussed in the contextof formaldehyde and ozone production in the early morning. While model andmeasurement discrepancies in OH reactivity are small (on average3.4 ± 13%), HCHO concentrations are underestimated by as much as1.5 ppb (45%) in the convective mixed layer. A similar underestimate inHCHO was seen in the 2002–2003 FORMAT Po Valley measurements, though theadditional source of HCHO was not identified. Oxidation of unmeasured VOCprecursors cannot explain the missing HCHO source, as measured OH reactivityis explained by measured VOCs and their calculated oxidation products. Weconclude that local direct emissions from agricultural land are the mostlikely source of missing HCHO. Model calculations demonstrate that radicalsfrom degradation of this non-photochemical HCHO source increase model ozoneproduction rates by as much as 0.6 ppb h<sup>−1</sup> (12%) before noon

Evidence for an unidentified non-photochemical ground-level source of formaldehyde in the Po Valley with potential implications for ozone production