M. McKay

A climatology of stratospheric aerosol

A global climatology of stratospheric aerosol is created by combining nearly a decade (1979–1981 and 1984–1990) of contemporaneous observations from the Stratospheric Aerosol and Gas Experiment (SAGE I and II) and Stratospheric Aerosol Measurement (SAM II) instruments. One goal of this work is to provide a representative distribution of the aerosol layer for use in radiative and chemical modeling. A table of decadal average 1μm extinction values is included, extending from the tropopause to 35 km and 80°S to 85°N, which allows estimation of surface area density. We find that the aerosol layer is distinctly volcanic in nature and suggest that the decadal average is a more useful estimate of future aerosol loading than a “background” loading, which is never clearly achieved during the data record. This climatology lends insight into the general circulation of the stratosphere. Latitude - altitude sections of extinction ratio at 1 μm are shown, averaged by decade, season, and phase of the quasi-biennial oscillation (QBO). A tropical reservoir region is diagnosed, with an “upper” and a “lower” transport regime. In the tropics above 22 km (upper regime), enhanced lofting occurs in the summer, with suppressed lofting or eddy dilution in the winter. In the extratropics within two scale heights of the tropopause (lower regime), poleward and downward transport is most robust during winter, especially in the northern hemisphere. The transport patterns persist into the subsequent equinoctial season. Ascent associated with QBO easterly shear favors detrainment in the upper regime, while relative descent and poleward spreading during QBO westerly shear favors detrainment in the lower regime. Extinction ratio differences between the winter-spring and summer-fall hemispheres, and differences between the two phases of the QBO, are typically 20–50%. Dynamical implications of the aerosol distributions are explored, with focus on interhemispheric differences, strong subtropical gradients, and the pronounced annual cycle.

Volatile organic compound measurements at Trinidad Head, California, during ITCT 2K2: Analysis of sources, atmospheric composition, and aerosol residence times

[1] We report hourly in-situ observations of C(1)-C(8) speciated volatile organic compounds (VOCs) obtained at Trinidad Head CA in April and May 2002 as part of the NOAA Intercontinental Transport and Chemical Transformation study. Factor analysis of the VOC data set was used to define the dominant processes driving atmospheric chemical composition at the site, and to characterize the sources for measured species. Strong decreases in background concentration were observed for several of the VOCs during the experiment due to seasonal changes in OH concentration. CO was the most important contributor to the total measured OH reactivity at the site at all times. Oxygenated VOCs were the primary component of both the total VOC burden and of the VOC OH reactivity, and their relative importance was enhanced under conditions when local source contributions were minimal. VOC variability exhibited a strong dependence on residence time (s(lnX) = 1.55(tau)(-0.44), r(2) = 0.98; where s(lnX) is the standard deviation of the natural logarithm of the mixing ratio), and this relationship was used, in conjunction with measurements of (222)Rn, to estimate the average OH concentration during the study period (6.1 x 10(5) molec/cm(3)). We also employed the variability-lifetime relationship defined by the VOC data set to estimate submicron aerosol residence times as a function of chemical composition. Two independent measures of aerosol chemical composition yielded consistent residence time estimates. Lifetimes calculated in this manner were between 3 - 7 days for aerosol nitrate, organics, sulfate, and ammonium. The lifetime estimate for methane sulfonic acid ( similar to 12 days) was slightly outside of this range. The lifetime of the total aerosol number density was estimated at 9.8 days.

Partitioning of water flux in a Sierra Nevada ponderosa pine plantation

The weather patterns of the west side of the Sierra Nevada Mountains (cold, wet winters and hot, dry summers) strongly influence how water is partitioned between transpiration and evaporation and result in a specific strategy of water use by ponderosa pine trees (Pinus ponderosa) in this region. To investigate how year-round water fluxes were partitioned in a young ponderosa pine ecosystem in the Sierra Nevada Mountains, water fluxes were continually measured from June 2000 to May 2001 using a combination of sap flow and eddy covariance techniques (above- and below-canopy). Water fluxes were modeled at our study site using a biophysical model, FORFLUX. During summer and fall water fluxes were equally partitioned between transpiration and soil evaporation while transpiration dominated the water fluxes in winter and spring. The trees had high rates of canopy conductance and transpiration in the early morning and mid-late afternoon and a mid-day depression during the dry season. We used a diurnal centroid analysis to show that the timing of high canopy conductance and transpiration relative to high vapor pressure deficit ( D) shifted with soil moisture: during periods of low soil moisture canopy conductance and transpiration peaked early in the day when D was low. Conversely, during periods of high soil moisture canopy conductance and transpiration peaked at the same time or later in the day than D. Our observations suggest a general strategy by the pine trees in which they maximize stomatal conductance, and therefore carbon fixation, throughout the day on warm sunny days with high soil moisture (i.e. warm periods in winter and late spring) and maximize stomatal conductance and carbon fixation in the morning through the dry periods. FORFLUX model estimates of evaporation and transpiration were close to measured/calculated values during the dry period, including the drought, but underestimated transpiration and overestimated evaporation during the wet period.

Annual ozone deposition to a Sierra Nevada ponderosa pine plantation

Ozone concentration and ecosystem scale fluxes were measured continuously from June 1999 to June 2000 above a ponderosa pine plantation at Blodgett Forest, an Ameriflux site located B75 km northeast of Sacramento, CA (1300 m). The ponderosa pine trees were most active during the summer but maintained a low level of activity during the fall, winter, and spring. Cumulative ozone flux for the year was 127 mmol m � 2 with the contribution for each season being 37% for summer, 18% for fall, 15% for winter, and 30% for spring. The high levels of cumulative ozone deposition over non-summer seasons indicate that significant ozone damage may occur during times when ozone concentrations are not at their maximum. Ozone flux is dependent upon both ozone deposition velocity (O3 Vd; how effective the ecosystem is at taking up ozone) and ambient ozone concentration but was found to be more closely related to O3 Vd than to ozone concentration. The relationships between O3 Vd (and therefore ozone flux) and the controlling climatic variables were dynamic over the year, changing mainly with water status and phenology. Understanding how the relationship between ozone deposition and its driving variables interact and change over the year is therefore critical to understanding potential ozone damage to vegetated ecosystems. Additionally, we found that commonly used ozone exposure metrics such as SUM0 (sum of all ozone exposure during the day) were poor predictors of ozone uptake (flux) unless periods of ecosystem stress, such as drought, were excluded. r 2002 Elsevier Science Ltd. All rights reserved.

Measurement of atmospheric nitrous acid at Bodgett Forest during BEARPEX2007

Nitrous acid (HONO) is an important precursor of the hydroxyl radical (OH) in the lower troposphere. Understanding HONO chemistry, particularly its sources and contribution to HOx (=OH+HO2) production, is very important for understanding atmospheric oxidation processes. A highly sensitive instrument for detecting atmospheric HONO based on wet chemistry followed by liquid waveguide long path absorption photometry was deployed in the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) at Blodgett Forest, California in late summer 2007. The median diurnal variation shows minimum HONO levels of about 20– 30 pptv during the day and maximum levels of about 60– 70 pptv at night, a diurnal pattern quite different from the results at various other forested sites. Measured HONO/NO 2 ratios for a 24-h period ranged from 0.05 to 0.13 with a mean ratio of 0.07. Speciation of reactive nitrogen compounds (NOy) indicates that HONO accounted for only ∼3% of total NOy. However, due to the fast HONO loss through phoCorrespondence to: X. Ren (xren@rsmas.miami.edu) tolysis, a strong HONO source (1.59 ppbv day −1) existed in this environment in order to sustain the observed HONO levels, indicating the significant role of HONO in NO y cycling. The wet chemistry HONO measurements were compared to the HONO measurements made with a Chemical Ionization Mass Spectrometer (CIMS) over a three-day period. Good agreement was obtained between the measurements from the two different techniques. Using the expansive suite of photochemical and meteorological measurements, the contribution of HONO photolysis to HOx budget was calculated to be relatively small (6%) compared to results from other forested sites. The lower HONO mixing ratio and thus its smaller contribution to HOx production are attributed to the unique meteorological conditions and low acid precipitation at Blodgett Forest. Further studies of HONO in this kind of environment are needed to test this hypothesis and to improve our understanding of atmospheric oxidation and nitrogen budget. Published by Copernicus Publications on behalf of the European Geosciences Union. 6284 X. Ren et al.: Measurement of atmospheric nitrous acid at Bodgett Forest

Characteristics of fine particle growth events observed above a forested ecosystem in the sierra nevada mountains of california

Atmospheric aerosols from natural and anthropogenic processes have both primary and secondary origins, and can influence human health, visibility, and climate. One key process affecting atmospheric concentrations of aerosols is the formation of new particles and their subsequent growth to larger particle sizes. A field study was conducted at the Blodgett Forest Research Station in the Sierra Nevada Mountains of California from May through September of 2002 to examine the effect of biogenic volatile organic compounds on aerosol formation and processing. The study included in-situ measurements of concentration and biosphere-atmosphere flux of VOCs, ozone, aerosol size distribution, aerosol physical and optical properties, and meteorological variables. Fine particle growth events were observed on approximately 30 percent of the 107 days with complete size distribution data. Average particle growth rates measured during these events were 3.8 ± 1.9 nm hr−1. Correlations between aerosol properties, trace gas concentrations, and meteorological measurements were analyzed to determine conditions conducive to fine particle growth events. Growth events were typically observed on days with a lesser degree of anthropogenic influence, as indicated by lower concentrations of black carbon, carbon monoxide, and total aerosol volume. Days with growth events also had lower temperatures, increased wind speeds, and larger momentum flux. Measurements of ozone concentrations and ozone flux indicate that gas phase oxidation of biogenic volatile organic compounds occur in the canopy, strongly suggesting that a significant portion of the material responsible for the observed particle growth are oxidation products of naturally emitted very reactive organic compounds.

Circulation Deduced from Aerosol Data Averaged by Season and Phase of the Quasibiennial Oscillation

Aerosol measurements obtained from the Stratospheric Aerosol and Gas Experiments (SAGE I and II), for the periods 1979–81 and 1984–91, are averaged by season and by the phase of the quasibiennial oscillation (QBO). Largest values of aerosol extinction ratio are found in the tropical lower stratosphere. Latitude-altitude distributions suggest the existence of an upper and a lower transport regime out of this tropical reservoir. Above 25 km upward detrainment occurs more readily in the subtropics during summer and fall. Detrainment pole-ward and downward occurs within ~5 kilometers above the tropopause most readily during the austral winter and spring and during the boreal fall, winter and spring. Ascent associated with QBO easterly shear favors detrainment in the upper regime, while relative descent and poleward spreading during QBO westerly shear favors detrainment in the lower regime.