Abstract
Abstract. Visible Multifilter Rotating Shadowband Radiometer (vis-MFRSR) data were collected at Storm Peak Laboratory (SPL), a mountain-top facility in northwest Colorado, from 1999 to 2011 and in 2013. From 2011 to 2014, in situ measurements of aerosol light scattering were also obtained. Using these data sets together, the seasonal impact of dust and biomass burning is considered for the western USA. Analysis indicates that the median contributions to spring and summer aerosol optical depth (AOD) from dust and biomass-burning aerosols across the data set are comparable. The mean AOD is slightly greater in the summer, with significantly more frequent and short-duration high AOD measurements due to biomass-burning episodes than in the spring. The Ångström exponent showed a significant increase in the summer for both the in situ and vis-MFRSR data, suggesting an increase in combustion aerosols. Spring dust events are less distinguishable in the in situ data than the column measurement, suggesting that a significant amount of dust may be found above the elevation of SPL, 3220 m a.s.l. Twenty-two known case studies of intercontinental dust, regional dust, and biomass-burning events were investigated. These events were found to follow a similar pattern, in both aerosol loading and Ångström exponent, as the seasonal mean signal in both the vis-MFRSR and ground-based nephelometer. This data set highlights the wide-scale implications of a warmer, drier climate on visibility in the western USA.
Highlights
The effect of aerosol particles is critical in understanding Earth’s radiation budget, yet significant uncertainties in the radiative properties of aerosols globally and on regional scales prevent the needed accuracy within numerical models to define future climate change
Using Interagency Monitoring for Protected Visual Environments (IMPROVE) data combined with MODerate resolution Imaging Spectroradiometer (MODIS) imagery, Tong et al (2012) found that regional dust in the western USA peaked from March to July
While dust has a large seasonal impact on aerosol loading at Storm Peak Laboratory (SPL), our results indicate that smoke may have a similar impact based on similar median aerosol optical depth (AOD) values for a dust-dominated spring (0.06) and a smokedominated summer (0.07)
Summary
The effect of aerosol particles is critical in understanding Earth’s radiation budget, yet significant uncertainties in the radiative properties of aerosols globally and on regional scales prevent the needed accuracy within numerical models to define future climate change. When considering only the direct effect of aerosols on global climate, the Intergovernmental Panel on Climate Change (IPCC) uncertainty estimate is currently greater than the effect at −0.35 ± 0.5 Wm−2 and in urgent need of further research (Boucher et al, 2013). Understanding the source region of the aerosol is critical for emission control policy both for air quality and visibility. The US Environmental Protection Agency (EPA) Regional Haze Rule (US EPA, 2003) mandated a schedule of increasing emission controls to achieve “natural visibility conditions” in these Class I areas by 2064. Unlike the rest of the USA, visibility has not improved in the Intermountain/southwestern (−116 to −100◦ longitude) regions over the last 2 decades (Hand et al, 2014) and, some aerosol contributors to visibility degradation are increasing
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