The IPAC-NC field campaign: a pollution and oxidization pool in the lower atmosphere over Huabei, China

Abstract. In the past decades, regional air pollution characterized by photochemical smog and grey haze-fog has become a severe environmental problem in China. To investigate this, a field measurement campaign was performed in the Huabei region, located between 32–42° N latitude in eastern China, during the period 2 April–16 May 2006 as part of the project "Influence of Pollution on Aerosols and Cloud Microphysics in North China" (IPAC-NC). It appeared that strong pollution emissions from urban and industrial centers tend to accumulate in the lower atmosphere over the central area of Huabei. We observed widespread, very high SO₂ mixing ratios, about 20–40 ppbv at 0.5–1.5 km altitude and 10–30 ppbv at 1.5–3.0 km altitude. Average CO mixing ratios were 0.65–0.7 ppmv at 0.5–1.5 km altitude, and very high CO around 1 ppmv was observed during some flights, and even higher levels at the surface. We find the high pollution concentrations to be associated with enhanced levels of OH and HO₂ radicals, calculated with a chemical box model constrained by the measurements. In the upper part of the boundary layer and in the lower free troposphere, high CO and SO₂ compete with relatively less NO₂ in reacting with OH, being efficiently recycled through HO₂, preventing a net loss of HO_x radicals. In addition to reactive hydrocarbons and CO, the oxidation of SO₂ causes significant ozone production over Huabei (up to ~13% or 2.0 ppbv h⁻¹ at 0.8 km altitude). Our results indicate that the lower atmosphere over Huabei is not only strongly polluted but also acts as an oxidation pool, with pollutants undergoing very active photochemistry over this part of China.