Seasonal and diurnal variations of particulate nitrate and organic matter

Atmospheric Chemistry and Physics Discussions This discussion paper is/has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP if available. Abstract Nitrate and several organic compounds such as dicarboxylic acids (e.g. succinic acid, glutaric acid), Polycyclic Aromatic Hydrocarbons (PAHs) or n-alkanes form the group of the most volatile compounds in atmospheric aerosol particles. The transition of these compounds between gas and particulate phase may significantly change the aerosol 5 particles radiative properties, the heterogeneous chemical properties, and, naturally, the total particulate mass concentration. To better assess these time-dependent effects , three intensive field experiments were conducted in 2008–2009 at the Central European EMEP research station Melpitz (Germany) using an Aerodyne Aerosol Mass Spectrometer (AMS). Data coverage from all seasons highlighted organic matter as be-10 ing the most important particulate fraction during summertime, while the nitrate fraction was more prevalent in winter. The variation in particulate nitrate was inherently linked to the gas-to-particle-phase equilibrium of ammonium nitrate, which depends on ambient temperature and relative humidity. During short episodes immediately after dawn, the particulate nitrate seems to disobey this dependency so that additional local nitrate 15 formation, such as from HONO photolysis is needed as an explanation. During the summer 2008's experiment, a remarkable diurnal evolution in the oxidation state of the organic matter became evident, which could be correlated to hydroxyl radical (OH) and ozone concentrations indicating photochemical transformation process. In summer, the organic particulate matter seems to be heavily influenced by regional secondary 20 formation and transformation processes, facilitated by photochemical production processes as well as a diurnal cycling of the substances between the gas and particulate phase. In winter, these processes were obviously much weaker, so that organic matter apparently originated mainly from aged particles and long range transport.