Long-term measurements of NO₃ radical at a semiarid urban site: 2. Seasonal trends and loss mechanisms

This study is the first to present long-term measurements of the nitrate radical in an urban location. Extensive nitrate radical measurements were conducted together with ancillary parameters during a continuous two year campaign (2005-2007) in the semiarid location of Jerusalem. The average nighttime NO₃ concentration was 27.3 ± 43.5 ppt, the highest ever reported, with a seasonal average peak during summer (33.3 ± 55.8 pptv) with maximum levels exceeding 800 pptv. Significant diurnal changes in NO₃ concentrations were observed, caused by an unusual nighttime increase in ozone concentrations. The NO₃ loss processes exhibited strong seasonal variability. Homogeneous gas-phase losses were the main removal processes during summer and spring. The heterogeneous losses of N ₂O₅, averaged over the entire campaign, contributed to less than half of the direct losses even though they dominated the winter seasons and part of the autumn months. Statistical regression analysis showed that NO₃ was inversely correlated with relative humidity and positively correlated with temperature and to a lesser extent with NO ₂ and O₃, indicating that the heterogeneous removal processes were also important. The diurnal behavior of NO₃ was examined using a one-dimensional chemical transport model. The simulations showed that NO₃ trends and concentrations were influenced mainly by changes in ozone and nitrogen oxide levels and that the very high levels of NO₃ can be explained by the entrainment of fresh ozone from the upper atmospheric levels. After sunset and in the early morning, the homogeneous processes are the major loss pathways, while the heterogeneous N ₂O₅ removal pathway dominates the intermediate times.