The contribution of biological particles to observed particulate organic carbon at a remote high altitude site
Introduction
Atmospheric aerosols impact climate through direct and indirect forcing, degrade air quality and visibility, and have detrimental effects on human health, and are therefore important on many scales. Particulate organic carbon (POC) is a significant component of atmospheric aerosols in polluted urban airsheds as well as in more remote regions (e.g., Zhang et al., 2007). Despite the importance of POC in the atmosphere, the sources of POC are often undetermined. Many model simulations of POC in polluted atmospheres underestimate the POC mass by as much as an order of magnitude when compared to observations (e.g., Heald et al., 2005), whereas remote areas show closer agreement, but are nonetheless highly uncertain due to slow rates of POC formation (Tunved et al., 2006). Possible explanations for this model/measurement discrepancy include incorrect emission estimates of primary POC and precursor volatile organic compounds, missing precursors of secondary organic aerosol production, and missing chemical and physical processes that lead to secondary organic aerosol production. Several studies suggest that a main source of measured POC is modern, in other words not derived from fossil fuel sources. For example, Ke et al. (2007) used organic tracer-based chemical mass balance (CMB) modeling and radiocarbon (C-14) measurements in the Tennessee Valley Region of the eastern U.S. and determined that as much as 84% of the observed summertime POC was contemporary (i.e., modern carbon, and not from fossil fuel combustion).
One major source of contemporary POC is secondary aerosol formation via the photo-oxidation of biogenic volatile organic compounds. Although this is estimated to be a significant source of atmospheric POC (e.g., Henze and Seinfeld, 2006), inclusion of secondary aerosols does not necessarily rectify the aforementioned model/measurement discrepancies. For example, Sakulyanontvittaya et al. (2008) show that inclusion of biogenic sesquiterpene emissions and subsequent secondary aerosol formation improves model performance in regional chemical model simulations of the U.S.; however, the model results still underpredict POC when compared to network observations. Another source of POC to the atmosphere that is not currently considered in most model simulations is primary biological particles, which include bacteria, fungal spores, and plant pollen. Recent studies suggest that primary biological particles can contribute significantly to atmospheric POC; however, the results of these studies are limited and vary substantially. Based on measurements in the Amazon, Elbert et al. (2007) attributed an average of 35% of the total aerosol mass to be fungal spores. At a continental background site in the Austrian Alps in March 2000, Bauer et al. (2002b) measured biological components in atmospheric samples and estimated that bacteria comprise to 0.03% and fungal spores 0.9% of the total observed POC mass. However, at a suburban site, Bauer et al. (2008) report that fungal spores comprise 6% and 14% of the POC mass concentration in the spring and summer, respectively. Not only are primary biological particles potentially an important contribution of organic aerosol in the atmosphere, specific microbial species may directly influence climatic conditions by acting as cloud and ice nucleators (e.g., Vali, 1971, Vali, 1996).
In March and April 2008, measurements of POC and biological particles were made at a remote mountain laboratory. Using these measurements, the potential contribution of biological particles to the observed mass of particulate organic carbon with diameters less than 10 μm (POC10) is estimated. The results of the observations and analysis suggest that the biological component of the observed POC10 was substantial and further study is warranted.
Section snippets
Methods
The Storm Peak Aerosol and Cloud Characterization Study (SPACCS08) was conducted from March 24 though April 15, 2008 at the Storm Peak Laboratory (SPL), located on top of Mt. Werner within the Steamboat Springs ski resort in Colorado (40.45°N, 106.73°W; 3200 m ASL). Measurements of meteorological variables, particle number and size distribution, organic and elemental carbon mass concentrations in particles with aerodynamic diameters less than 10 μm, and biological particle concentrations and
Results
Averaged over the study period from March 24 to April 15, 2008, hourly total particle number concentrations measured with the SMPS and the APS averaged 1900 cm−3 and 140 cm−3, respectively (Fig. 1). These concentrations are similar to those reported by Lowenthal et al. (2002) during a study at SPL in February–May, 2001, where they observed particle number concentrations (particles > 10 nm) of 1221 and 3893 cm−3 during cloudy and clear conditions, respectively. These values are typical of
Conclusions
The results of this study suggest that biological particles may significantly contribute to the mass of organic carbon observed in atmospheric aerosols. This confirms some results suggested by C14 measurements. The observations presented here imply that, in order to fully understand the sources of atmospheric POC, biological particles must be considered. This means that measurements of these components should be included in future field studies, and that model studies need to include the
Acknowledgements
The authors thank Ed Dunlea for useful discussions and sharing his data from the 2004 study. The National Center for Atmospheric Research is sponsored by the National Science Foundation. The Desert Research Institute is an equal opportunity service provider and employer. The Desert Research Institute is a permittee of the Medicine-Bow Routt National Forests. We appreciate the assistance from Steamboat Ski and Resort Corporation for continued support of SPL and our research projects. Additional
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