Systematic variations in the concentration of NOx (NO Plus NO2) at Niwot Ridge, Colorado

Parrish, D. D.; Hahn, Carole J.; Fahey, D. W.; Williams, E. J.; Bollinger, M. J.; Hübler, G.; Buhr, M. P.; Murphy, P. C.; Trainer, M.; Hsie, E.‐Y.; Liu, S. C.; Fehsenfeld, F. C.

doi:10.1029/jd095id02p01817

Cited by 118 publications

(83 citation statements)

References 63 publications

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“…Previous ambient studies at NWT have found typical wintertime (NO ? NO 2 ) values of 200-1,000 pptv (Parrish et al 1990), and it is not unusual that the sum of oxidized nitrogen gases (NO y ) can exceed 1 ppbv (Fahey et al 1986;Ridley et al 1990). Our experiments do not allow for an evaluation of the contribution of these other species to the detected NO 2 signal, nor do we have measurements that allow for an assessment of the NO 2 to NO conversion efficiency of our TEI 42TL instrument.…”

Section: Methodsmentioning

confidence: 99%

Fluxes and chemistry of nitrogen oxides in the Niwot Ridge, Colorado, snowpack

et al. 2009

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The effect of snow cover on surfaceatmosphere exchanges of nitrogen oxides (nitrogen oxide (NO) ? nitrogen dioxide (NO 2 ); note, here 'NO 2 ' is used as surrogate for a series of oxidized nitrogen gases that were detected by the used monitor in this analysis mode) was investigated at the high elevation, subalpine (3,340 m asl) Soddie site, at Niwot Ridge, Colorado. Vertical (NO ? NO 2 ) concentration gradient measurements in interstitial air in the deep (up to *2.5 m) snowpack were conducted with an automated sampling and analysis system that allowed for continuous observations throughout the snow-covered season. These measurements revealed sustained, highly elevated (NO ? NO 2 ) mixing ratios inside the snow. Nitrogen oxide concentrations were highest at the bottom of the snowpack, reaching levels of up to 15 ppbv during mid-winter. Decreasing mixing ratios with increasing distance from the soilsnow interface were indicative of an upwards flux of NO from the soil through the snowpack, and out of the snow into the atmosphere, and imply that biogeochemical processes in the subnival soil are the predominant NO source. Nitrogen dioxide reached maximum levels of *3 ppbv in the upper layers of the snowpack, i.e., *20-40 cm below the surface. This behavior suggests that a significant fraction of NO is converted to NO 2 during its diffusive transport through the snowpack. Ozone showed the opposite behavior, with rapidly declining levels below the snow surface. The mirroring of vertical profiles of ozone and the NO 2 /(NO ? NO 2 ) ratio suggest that titration of ozone by NO in the snowpack contributes to the ozone reaction in the snow and to the ozone surface deposition flux. However, this surface efflux of (NO ? NO 2 ) can only account for a minor fraction of ozone deposition flux over snow that has been reported at other mid-latitude sites. Neither (NO ? NO 2 ) nor ozone levels in the interstitial air showed a clear dependence on incident solar irradiance, much in contrast to observations in polar snow. Comparisons with findings from polar snow studies reveal a much different (NO ? NO 2 ) and ozone snow chemistry in this alpine environment. Snowpack concentration gradients and diffusion theory were applied to estimate an average, wintertime (NO ? NO 2 ) flux of 0.005-0.008 nmol m -2 s -1 , which is of similar magnitude as reported (NO ? NO 2 ) fluxes

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Section: Methodsmentioning

confidence: 99%

Fluxes and chemistry of nitrogen oxides in the Niwot Ridge, Colorado, snowpack

et al. 2009

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“…11d). These conditions can occur in winter on Niwot Ridge after a frontal system passes through the region (Parrish et al 1990). The temperature of the snow surface at NWT was estimated to be around 228C, which was very close to the dewpoint temperature (Fig.…”

Section: Surface Condensation Effects On Snow Temperature Changesmentioning

confidence: 99%

“…Several meteorological events occur just before the snowpack warm-up: 1) the wind speed dropped; 2) the abovecanopy wind direction continued downslope, but the subcanopy wind direction switched to upslope (i.e., decoupling occurred); 3) the water vapor content of the air reached a maximum; and 4) the dewpoint temperature (T d ) approached the snow surface temperature. Upslope winds at the site can be generated by diurnal surface heating, large-scale pressure gradients, or mesoscale phenomena due to the mountainous topography (Parrish et al 1990;Turnipseed et al 2004), with decoupling of the above-canopy and subcanopy air occurring within the forested areas ). In Fig.…”

Section: Surface Condensation Effects On Snow Temperature Changesmentioning

confidence: 99%

Snow Temperature Changes within a Seasonal Snowpack and Their Relationship to Turbulent Fluxes of Sensible and Latent Heat

Burns

Molotch

Williams

et al. 2014

Journal of Hydrometeorology

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Snowpack temperatures from a subalpine forest below Niwot Ridge, Colorado, are examined with respect to atmospheric conditions and the 30-min above-canopy and subcanopy eddy covariance fluxes of sensible Q h and latent Q e heat. In the lower snowpack, daily snow temperature changes greater than 18C day 21 occurred about 1-2 times in late winter and early spring, which resulted in transitions to and from an isothermal snowpack. Though air temperature was a primary control on snowpack temperature, rapid snowpack warm-up events were sometimes preceded by strong downslope winds that kept the nighttime air (and canopy) temperature above freezing, thus increasing sensible heat and longwave radiative transfer from the canopy to the snowpack. There was an indication that water vapor condensation on the snow surface intensified the snowpack warm-up. In late winter, subcanopy Q h was typically between 210 and 10 W m 22 and rarely had a magnitude larger than 20 W m 22 . The direction of subcanopy Q h was closely related to the canopy temperature and only weakly dependent on the time of day. The daytime subcanopy Q h monthly frequency distribution was near normal, whereas the nighttime distribution was more peaked near zero with a large positive skewness. In contrast, above-canopy Q h was larger in magnitude (100-400 W m 22 ) and primarily warmed the forest-surface at night and cooled it during the day. Around midday, decoupling of subcanopy and above-canopy air led to an apparent cooling of the snow surface by sensible heat. Sources of uncertainty in the subcanopy eddy covariance flux measurements are suggested. Implications of the observed snowpack temperature changes for future climates are discussed.

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“…The forest is roughly 60 km northwest of a large urban region on the plains (the Denver, Colorado, metropolitan area). A series of experiments focused on photochemical processes in air established that pollutants from the Denver urban area can be transported to the Niwot Ridge forest by easterly winds in summer (Roberts et al, 1985;Parrish et al, 1990Parrish et al, , 1991. Although CH 4 was not measured in those studies, carbon monoxide at Niwot Ridge tends to be higher during summer easterly winds (Parrish et al, 1991).…”

Section: Environmental Conditions and Observed Ch 4 And Comentioning

confidence: 99%

Soil, plant, and transport influences on methane in a subalpine forest under high ultraviolet irradiance

et al. 2009

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Abstract. Recent studies have demonstrated direct methane emission from plant foliage under aerobic conditions, particularly under high ultraviolet (UV) irradiance. We examined the potential importance of this phenomenon in a high-elevation conifer forest using micrometeorological techniques. Vertical profiles of methane and carbon dioxide in forest air were monitored every 2 h for 6 weeks in summer 2007. Day to day variability in above-canopy CH 4 was high, with observed values in the range 1790 to 1910 nmol mol −1 . High CH 4 was correlated with high carbon monoxide and related to wind direction, consistent with pollutant transport from an urban area by a well-studied mountain-plain wind system. Soils were moderately dry during the study. Vertical gradients of CH 4 were small but detectable day and night, both near the ground and within the vegetation canopy. Gradients near the ground were consistent with the forest soil being a net CH 4 sink. Using scalar similarity with CO 2 , the magnitude of the summer soil CH 4 sink was estimated at ∼1.7 mg CH 4 m −2 h −1 , which is similar to other temperate forest upland soils. The high-elevation forest was naturally exposed to high UV irradiance under clear sky conditions, with observed peak UVB irradiance >2 W m −2 . Gradients and means of CO 2 within the canopy under daytime conditions showed net uptake of CO 2 due to photosynthetic drawdown as expected. No evidence was found for a significant foliar CH 4 source in the vegetation canopy, even under high UV conditions. While the possibility of a weak foliar source Correspondence to: D. R. Bowling (bowling@biology.utah.edu) cannot be excluded given the observed soil sink, overall this subalpine forest was a net sink for atmospheric methane during the growing season.

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Systematic variations in the concentration of NO_x (NO Plus NO₂) at Niwot Ridge, Colorado

Cited by 118 publications

References 63 publications

Fluxes and chemistry of nitrogen oxides in the Niwot Ridge, Colorado, snowpack

Fluxes and chemistry of nitrogen oxides in the Niwot Ridge, Colorado, snowpack

Snow Temperature Changes within a Seasonal Snowpack and Their Relationship to Turbulent Fluxes of Sensible and Latent Heat

Soil, plant, and transport influences on methane in a subalpine forest under high ultraviolet irradiance

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