Open AccessNitrous acid formation in the urban atmosphere: Gradient measurements of NO 2 and HONO over grass in Milan, Italy
Author(s) -
Stutz Jochen,
Alicke Björn,
Neftel Albrecht
Publication year - 2002
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2001jd000390
Subject(s) - nitrous acid , troposphere , atmosphere (unit) , aerosol , differential optical absorption spectroscopy , deposition (geology) , environmental science , atmospheric sciences , environmental chemistry , absorption (acoustics) , chemistry , meteorology , materials science , physics , geology , inorganic chemistry , paleontology , sediment , composite material
The source of nitrous acid, HONO, in the troposphere remains uncertain, even after two decades of research. It is currently believed that HONO is formed by heterogeneous conversion of NO 2 on either the ground or the aerosol surface. While this conversion has been studied in the laboratory, few atmospheric studies have been reported. Here we present the first simultaneous determination of the vertical gradients and fluxes of HONO, its precursor NO 2 , and SO 2 over a flat grass surface in the polluted atmosphere. The measurements were performed in Milan, Italy, during the Limitation of Oxidant Production/Pianura Padana Produzione di Ozono (LOOP/PIPAPO) study in summer 1998, using differential optical absorption spectroscopy. While deposition of NO 2 onto the ground was frequently observed, heterogeneous HONO formation was much smaller than expected. We can explain our observation by a mechanism that consists of a combination of NO 2 and HONO deposition, and a heterogeneous conversion of NO 2 to HONO on the ground. The compensation point for deposition and formation of HONO is characterized by a HONO/NO 2 ratio of ∼0.03, indicating that only one HONO molecule is released into the gas phase for every 33 NO 2 molecules deposited. Our measurements also show that direct emission of HONO is an important source in strongly polluted areas.