Characteristics and Sources of Nitrous Acid in the Urban Atmosphere of Ji'nan, North China: Results from 1-yr Continuous Observations

Nitrous acid (HONO) is a key reservoir of the hydroxyl radical (OH) and hence plays a central role in atmospheric chemistry. To understand the sources and impact of HONO in the polluted atmosphere of northern China, continuous measurements of HONO and related parameters have been made since September 2015 at an urban site in Ji'nan, the capital city of Shandong province. The year-round data were analyzed preliminarily to examine the temporal variations, vehicle emission factors, heterogeneous conversion, unknown daytime source(s), and impact on OH production. HONO shows well-defined seasonal and diurnal patterns with clear wintertime and nighttime concentration peaks. Elevated HONO levels (e.g., over 5 ppbv) were frequently observed with a maximum value of 8.4 ppbv. The derived HONO/NO2 ratio from direct vehicle emissions varies in the range of 0.8%-3.4%, which is higher than the commonly-adopted value of 0.8% from a tunnel study. Aerosol surface was found to play an important role in facilitating the heterogeneous conversion of NO2 to HONO. A non-PSS HONO budget analysis suggests the existence of significant unknown HONO source(s) at daytime in Ji'nan. The contributions of HONO photolysis and other radical sources to the atmospheric oxidative capacity are also estimated. The present study presents the first continuous year-round record of HONO measurements in China, and more detailed analyses are under way to explore the potential unknown daytime sources.