Seasonal Patterns of Dry Deposition at a High‐Elevation Site in the Colorado Rocky Mountains

Abstract

In the Colorado Rocky Mountains, high‐elevation barren soils are deficient in carbon (C) and phosphorus (P) and enriched in nitrogen (N). The seasonal variability of dry deposition and its contributions to alpine elemental budgets is critical to understanding how dry deposition influences biogeochemical cycling in high‐elevation environments. In this 2 year study, we evaluated dry and wet deposition inputs to the Niwot Ridge Long Term Ecological Research (NWT LTER) site in the Colorado Rocky Mountains. The total organic C flux in wet + dry (including soluble and particulate C) deposition was >30 kg C ha−1 yr−1 and represents a substantial input for this C‐limited environment. Our side‐by‐side comparison of dry deposition collectors with and without marble insert indicated that the insert improved retention of dry deposition by ~28%. Annual average dry deposition fluxes of water‐soluble organic carbon (4.25 kg C ha−1 yr−1) and other water‐soluble constituents, including ammonium (0.16 kg NH4+ha−1 yr−1), nitrate (1.99 kg NO3− ha−1 yr−1), phosphate (0.08 kg PO43− ha−1 yr−1), and sulfate (1.20 kg SO42− ha−1 yr−1), were comparable to those in wet deposition, with highest values measured in the summer. Backward trajectory analyses implicate air masses passing through the arid west and Four Corners, USA, as dominant source areas for dry deposition, especially in spring months. Synchronous temporal patterns of deposition observed at the NWT LTER site and a distant Rocky Mountain National Park Clean Air Status and Trends Network site indicate that seasonal dry deposition patterns are regional phenomena with important implications for the larger Rocky Mountain region.

Water soluble organic aerosols in the Colorado Rocky Mountains, USA: composition, sources and optical properties

“…Similar seasonal trends of total dissolved nitrogen and dissolved phosphorus in wet deposition have been observed by Mladenov et al 1. and Oldani10 at the same sampling site. The sample collected during 16–30 November 2010 has the highest concentration of TSP (Supplementary Table S1) and much lower mean and median OC and WSOC concentrations than the other samples, suggesting a contribution from dust enriched with inorganics.…”

“…Inputs of organic carbon, in particular, can influence redox reactions in the water column and sediments and can fuel microbial activity resulting in unanticipated consequences for otherwise isolated waters that we regard as our pristine reference sites. For the Rocky Mountains, studies have shown that the organic carbon load in atmospheric deposition is substantial110, but direct evidence of the sources of organic aerosols is thus far lacking.…”

“…1) in the Colorado Rocky Mountains. Based on significant correlations between DOC concentrations and sulfate and nitrate in summer wet deposition1, air mass trajectories that pass through the Front Range of Colorado in the summer10, and summer upslope conditions that carry air masses above the atmospheric boundary layer, the authors hypothesized that the summertime organic aerosol load in high elevation site in the Colorado Rocky Mountains is substantially contributed by Colorado Front Range derived anthropogenic primary and secondary organic aerosol (SOA) emissions.…”

Atmospheric Dust Deposition Varies by Season and Elevation in the Colorado Front Range, USA

“…When discounting the site influenced by construction (Skywatch) and the spring season influenced by pollen deposition, our dust fluxes align with the most recent global compilation of direct dust flux observations, which reports the expected range for sites 10-1,000 km from secondary dust source at 1.0-20 g m −2 yr −1 (Lawrence & Neff, 2009). In addition, the fluxes measured at our alpine, subalpine, and montane sites are within the range reported by other atmospheric deposition studies in the Colorado Front Range (Liptzin & Seastedt, 2009;Oldani et al, 2017) and elsewhere in the Rocky Mountain West (Brahney et al, 2014;Dahms & Rawlins, 1996;Lawrence et al, 2010;Munroe et al, 2019). However, the dust fluxes we measured at lower elevations are some of the highest fluxes reported for the Rocky Mountains, even discounting fluxes measured at Skywatch and during the spring season ( Figure 6).…”

“…If this is the case, the summertime elevation trend in mass fluxes described above may not extend to spring or annual mass fluxes. On the other hand, high-elevation sites may experience peak dust deposition during summer months, as was the case for a subalpine site on Niwot Ridge during 2013 (Oldani et al, 2017). The springtime delivery of dust to the Front Range may vary greatly from year to year, due to variability in dust storm transport path (Reynolds et al, 2016), whereas the summertime increase in dust deposition due to local anthropogenic sources may have lower interannual variability.…”