Nitrous acid formation on Zea mays leaves by heterogeneous reaction of nitrogen dioxide in the laboratory

Abstract

Nitrous acid (HONO) is of considerable interest because it is an important precursor of hydroxyl radicals (OH), a key species in atmospheric chemistry. HONO sources are still not well understood, and air quality models fail to predict OH as well as HONO mixing ratios. As there is little knowledge about the potential contribution of plant surfaces to HONO emission, this laboratory work investigated HONO formation by heterogeneous reaction of NO₂ on Zea mays. Experiments were carried out in a flow tube reactor; HONO, NO₂ and NO were measured online with a Long Path Absorption Photometer (LOPAP) and a NOx analyzer. Tests were performed on leaves under different conditions of relative humidity (5-58%), NO₂ mixing ratio representing suburban to urban areas (10-80 ppbv), spectral irradiance (0-20 W m^-2) and temperature (288-313 K). Additional tests on plant wax extracts from Zea mays leaves showed that this component can contribute to the observed HONO formation. Temperature and NO₂ mixing ratios were the two environmental parameters that showed substantially increased HONO emissions from Zea mays leaves. The highest HONO emission rates on Zea mays leaves were observed at 313 K for 40 ppbv of NO₂ and 40% RH and reached values of (5.6 ± 0.8) × 10⁹ molecules cm^-2 s^-1. Assuming a mixing layer of 300 m, the HONO flux from Zea mays leaves was estimated to be 171 ± 23 pptv h^-1 during summertime, which is comparable to what has been reported for soil surfaces.

Keywords: Atmospheric chemistry; HONO sources; Hydroxyl radicals; Leaf surface; Solid/gas reactivity.