Preliminary measurements of summer nitric acid and ammonia concentrations in the Lake Tahoe Basin air-shed: implications for dry deposition of atmospheric nitrogen

Over the past 50 years, Lake Tahoe, an alpine lake located in the Sierra Ne vada mountains on the border between California and Nevada, has seen a decl ine in water clarity. With significant urbanization within its borders and major urban areas 130 km upwind of the prevailing synoptic airflow, it is b elieved the Lake Tahoe Basin is receiving substantial nitrogen (N) input vi a atmospheric deposition during summer and fall. We present preliminary inf erential flux estimates to both lake surface and forest canopy based on emp irical measurements of ambient nitric acid (HNO3), ammonia (NH3), and ammon ium nitrate (NH4NO3) concentrations, in an effort to identify the major con tributors to and ranges of atmospheric dry N deposition to the Lake Tahoe B asin. Total flux from dry deposition ranges from 1.2 to 8.6 kg N ha(-1) for the summer and fall dry season and is significantly higher than wet deposi tion, which ranges from 1.7 to 2.9 kg N ha(-1) year(-1). These preliminary results suggest that dry deposition of HNO3 is the major source of atmosphe ric N deposition for the Lake Tahoe Basin, and that overall N deposition is similar in magnitude to deposition reported for sites exposed to moderate N pollution in the southern California mountains. (C) 2001 Elsevier Science Ltd. All rights reserved.