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.