NITROGENOUS AIR-POLLUTANTS AT A SOUTHERN CALIFORNIA MOUNTAIN FOREST SMOG RECEPTOR-SITE

Ambient levels of the nitrogenous pollutants NO, NO2, nitric acid, nitrous acid, ammonia, particulate nitrate, particulate ammonium, peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) have been measured at a southern California mountain forest location severely impacted by urban photochemical smog. Air quality at the mountain forest location was characterized by high levels of nitric acid (up to 18 ppb) and the phytotoxic peroxyacyl nitrates PAN (up to 22 ppb) and PPN (up to 5 ppb). Alkyl nitrates were below our detection limits of 0.05-0.5 ppb. The (PAN + PPN)/NO2 ratios varied substantially (range 0.03-2.27) and were generally large, with typical 24-h averages of 0.19-0.50. Diurnal variations of the (PAN/PPN)/NO2 ratio exhibited both nighttime and daytime maxima reflecting diurnal variations in PAN (and PPN) thermal stability and photochemical production rates, respectively. Organic nitrogen-containing oxidation products (PAN + PPN) were more abundant than inorganic nitrate (HNO3 + NO3-), with in average organic/inorganic concentration ratio of 2.5 (daytime ratio 1.0; nighttime ratio 3.7). The four oxidation products PAN, PPN, HNO3 and NO3- together accounted for 0.26 of the total reactive nitrogen. The results are discussed with respect to diurnal and seasonal variations and in terms of NO2 atmospheric oxidation pathways. Deposition fluxes and velocities to ponderosa pine have been measured for inorganic nitrate and for ammonium and have been compared with those obtained at other mountain forest locations.