Evaluation of background levels of air pollutants over Israel

The present paper characterizes the background concentration of SO₂, NO, NO₂, NO_x^*, O₃, nitrate and sulfate aerosols throughout a three season campaign at a remote isolated site in southern Israel. Upper and lower thresholds were established for background levels based on two sets of criteria. The average background levels were 0.8, <DL, 2.0, 58.0, 76.6 and 86.6 for SO₂, NO, NO_x, O₃, sulfate and nitrate respectively (ppbv for gaseous species and nm m^-3 for the aerosols). Background concentrations of the primary pollutants were lower by a factor of 10-20 as compared to other sites in Israel, clearly showing that the remote background site was not influenced by local anthropogenic pollution. The nitrate and sulfate levels measured at the remote site were similar to those observed at other sites throughout Israel indicating that external sources are the major contributors to these pollution levels in Israel. The limited difference between the aerosol levels measured at a remote site as compared to polluted sites demonstrates that there is only a minor local contribution to nitrate and sulfate levels in Israel. The vast majority of the air masses that were measured showed high ratios of photochemical N-age and S-age, reflecting transport times of 2-3 days. Taken together with air trajectories, the transport time indicates that the majority of the air pollution reaching Israel is a result of long-range transport (LRT) originated in eastern and southern Europe. The present background levels provide an important database in attempts to address important policy questions regarding local air quality and the determination of the contribution of LRT to local air pollution levels. Further, regional meteorology appears to be a key influence on the concentrations in this and other background sites.