Assessment of Annual and Seasonal Fluxes of Particulate Matter from Atmospheric Deposition to Lake Tahoe

Abstract

Lake Tahoe, an oligotrophic lake appreciated for its fresh water and geographic setting, has been reported of its declining water clarity over the past few decades due to nutrient and sediment particle inputs. Contributions from atmospheric deposition of particulate matter (PM) have been suggested to be substantial, yet inadequately quantified. This study established three long-term monitoring sites (July 2013 – August 2014) to measure 24-hr, size-resolved dry and wet PM deposition in near-shore, offshore-background, and upper watershed conditions in the Lake Tahoe Basin. The objectives are to: 1) investigate spatiotemporal variations of PM deposition flux, 2) obtain dry deposition velocity using mass deposition flux and PM concentration measurements, and 3) provide estimated annual number deposition flux (NDF) via dry and wet processes. Dry deposition was quantified on passive substrates using a scanning electron microscope, while wet deposition was based on particles suspended in rain and/or snow water analyzed by laser diffraction spectroscopy. The results show higher NDFdry and greater monthly variability at the near-shore than the offshore and upper-watershed sites, suggesting substantial impacts of nearby beach, traffic, and construction activities. Seasonal NDFdry and NDFwet (1–20 µm) were consistent across all three monitoring sites, characterized by higher NDFdry in fall, winter and summer but higher NDFwet in spring when precipitation occurred more frequently. The annual NDFdry+wet, estimated between 4.31 x 10^19 and 8.61 x 10^19 particles per year, is in the same order of magnitude with a previous estimate of 7.4 x 10^19 (0.5–16 µm) particles per year by the Lake Tahoe Total Maximum Daily Load (TMDL) using an independent approach. Comparison of the NDFdry with conventional eddy correlation measurements was examined through short-term collocated monitoring.