Robert Wegener

Missing gas-phase source of hono inferred from zeppelin measurements in the troposphere

On a Zeppelin Nitrous acid (HONO) is an important atmospheric trace gas that acts as a precursor of tropospheric hydroxyl-radicals (OH), which is responsible for the self-cleansing capacity of the atmosphere and which also controls the concentrations of greenhouse gases, such as methane and ozone. How HONO is made is a mystery. Flying onboard a Zeppelin over the Po Valley in Northern Italy, Li et al. (p. 292) discovered HONO in the undisturbed morning troposphere, indicating that HONO must be produced there, rather than mixed from the surface. The high HONO concentrations are likely to have been formed by a light-dependent gas-phase source that probably consumed OH or HO2 radicals, which hints that the impact of HONO on the abundance of OH in the entire troposphere may be substantially overestimated. The tropospheric production of HONO from a light-dependent gas-phase source raises questions about its impact on OH. Gaseous nitrous acid (HONO) is an important precursor of tropospheric hydroxyl radicals (OH). OH is responsible for atmospheric self-cleansing and controls the concentrations of greenhouse gases like methane and ozone. Due to lack of measurements, vertical distributions of HONO and its sources in the troposphere remain unclear. Here, we present a set of observations of HONO and its budget made onboard a Zeppelin airship. In a sunlit layer separated from Earth’s surface processes by temperature inversion, we found high HONO concentrations providing evidence for a strong gas-phase source of HONO consuming nitrogen oxides and potentially hydrogen oxide radicals. The observed properties of this production process suggest that the generally assumed impact of HONO on the abundance of OH in the troposphere is substantially overestimated.

Stable carbon isotope composition of secondary organic aerosol from β‐pinene oxidation

[1] A chamber study was carried out to investigate the stable carbon isotopic composition (δ 13 C) of secondary organic aerosol (SOA) formed from ozonolysis of β-pinene. β-Pinene (600 ppb) with a known δ 13 C value (-30.1%‰) and 500 ppb ozone were injected into the chamber in the absence of light and the resulting SOA was collected on preheated quartz fiber filters. Furthermore, δ 13 C values of the gas-phase β-pinene and one of its oxidation products, nopinone, were measured using a gas chromatograph coupled to an isotope ratio mass spectrometer (GC-IRMS). β-Pinene was progressively enriched with the heavy carbon isotope due to the kinetic isotope effect (KIE). The KIE of the reaction of β-pinene with ozone was measured to be 1.0026 ( O3 e 2.6 ± 1.5%o). The δ 13 C value of total secondary organic aerosol was very similar to that of its precursor (average = -29.6 ± 0.2%o) independent of experiment time. Nopinone, one of the major oxidation products of β-pinene, was found in both the gas and aerosol phases. The gas-phase nopinone was heavier than the initial β-pinene by 1.3%o but lighter than the corresponding aerosol-phase nopinone. On average, the gas-phase nopinone was lighter by 2.3‰ than the corresponding aerosol-phase nopinone. The second product found in the SOA was detected as acetone, but it desorbed from the filter at a higher temperature than nopinone, which indicates that it is a pyrolysis product. The acetone showed a much lower δ 13 C (-36.6‰) compared to the initial β-pinene δ 13 C.

Response to Comment on “Missing gas-phase source of HONO inferred from Zeppelin measurements in the troposphere”

Ye et al. have determined a maximum nitrous acid (HONO) yield of 3% for the reaction HO2·H2O + NO2, which is much lower than the yield used in our work. This finding, however, does not affect our main result that HONO in the investigated Po Valley region is mainly from a gas-phase source that consumes nitrogen oxides.