Most terrestrial carbon sequestration at mid-latitudes in the Northern Hemisphere occurs in seasonal, montane forest ecosystems. Winter respiratory carbon dioxide losses from these ecosystems are high, and over half of the carbon assimilated by photosynthesis in the summer can be lost the following winter. The amount of winter carbon dioxide loss is potentially susceptible to changes in the depth of the snowpack; a shallower snowpack has less insulation potential, causing colder soil temperatures and potentially lower soil respiration rates. Recent climate analyses have shown widespread declines in the winter snowpack of mountain ecosystems in the western USA and Europe that are coupled to positive temperature anomalies. Here we study the effect of changes in snow cover on soil carbon cycling within the context of natural climate variation. We use a six-year record of net ecosystem carbon dioxide exchange in a subalpine forest to show that years with a reduced winter snowpack are accompanied by significantly lower rates of soil respiration. Furthermore, we show that the cause of the high sensitivity of soil respiration rate to changes in snow depth is a unique soil microbial community that exhibits exponential growth and high rates of substrate utilization at the cold temperatures that exist beneath the snow. Our observations suggest that a warmer climate may change soil carbon sequestration rates in forest ecosystems owing to changes in the depth of the insulating snow cover.

Acid deposition and photochemical smog are urban air pollution problems, and they remain localized as long as the sulfur, nitrogen, and hydrocarbon pollutants are confined to the lower troposphere (below about 1-kilometer altitude) where they are short-lived. If, however, the contaminants are rapidly transported to the upper troposphere, then their atmospheric residence times grow and their range of influence expands dramatically. Although this vertical transport ameliorates some of the effects of acid rain by diluting atmospheric acids, it exacerbates global tropospheric ozone production by redistributing the necessary nitrogen catalysts. Results of recent computer simulations suggest that thunderstorms are one means of rapid vertical transport. To test this hypothesis, several research aircraft near a midwestern thunderstrom measured carbon monoxide, hydrocarbons, ozone, and reactive nitrogen compounds. Their concentrations were much greater in the outflow region of the storm, up to 11 kilometers in altitude, than in surrounding air. Trace gas measurements can thus be used to track the motion of air in and around a cloud. Thunderstorms may transform local air pollution problems into regional or global atmospheric chemistry problems.

Field measurement programs in Brazil during the dry seasons in August and September 1979 and have demonstrated the large importance of the continental tropics in global air chemistry. Many important trace gases are produced in large amounts over the continents. During the dry season, much biomass burning takes place, especially in the cerrado regions, leading to a substantial emission of air pollutants, such as CO, NOx, N20 , CH 4 and other hydrocarbons. Ozone concentrations are enhanced due to photochemical reactions. The large biogenic organic emissions from tropical forests play an important role in the photochemistry of the atmosphere and explain why CO is present in such high concentrations in the boundary layer of the tropical forest. Carbon monoxide production may represent more than 3% of the net primary productivity of the tropical forests. Ozone concentrations in the boundary layer of the tropical forests indicate strong removal processes. Due to atmospheric supply of NO x by lightning, there is probably a large production of 03 in the free troposphere over the Amazon tropical forests. This is transported to the marine-free troposphere and to the forest boundary layer.

Direct measurements of the rate of O3 photolysis to O2(1Δg) and O(1D) and of NO2 photolysis to NO and O(3P) are reported as photolysis frequencies j(O3) and j(NO2). The effects of solar zenith angle, total ozone column, cloud cover, aerosol loading, temperature, pressure, and altitude are examined. For a clear sky, zero albedo, and zenith angle between 0° and 65°, the expression j (NO2) = 1.67 × 10−2 exp (−0.575 sec θ) (s−1) gives NO2 photolysis frequencies (photolysis rates per unit reactant) to within about 10% of the measured values, where θ is the solar zenith angle. Pressure has no measurable effect on j(NO2) between 0.15 and 1.2 bars. Temperature has only a small effect between 230 and 400 K, which can be described in terms of an effective activation energy of 0.48±0.05 kJ mol−1. Frequencies of ozone photolysis to O(1D) depend strongly on overhead ozone column and temperature as well as zenith angle; for 300 K, a clear sky, zero albedo, 45° zenith angle, 0.345‐cm ozone column, and altitude between 1.6 and 6 km, j(O3) = 1.6(±0.25) × 10−5 s−1. From 300 to 273 K, j(O3) drops by 40±3% for a 65° zenith angle and a 0.306‐cm ozone column; this value is not valid for different effective ozone columns. Measured photolysis frequencies show only weak dependence on altitude and aerosol loading. These frequencies were measured with instruments sensitive to radiation from the upward 2π sr only. Clouds dramatically reduce both photolysis frequencies, but they reduce the total radiation by a significantly larger factor. Airborne UV radiometers, calibrated against direct measurements of j(NO2), were used to measure albedos of many surfaces. While natural surfaces such as vegetation and oceans have albedos near zero, dense clouds and snow have an albedo near unity. With increasing altitude, molecular and particulate scattering often increase the effective albedo with respect to photolysis frequencies.

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Aerosols could play a critical role in many processes which impact on our lives either indirectly (e.g., climate) or directly (e.g., health). However, our ability to assess these possible impacts is constrained by our limited knowledge of the physical and chemical properties of aerosols, both anthropogenic and natural. This deficiency is attributable in part to the fact that aerosols are the end product of a vast array of chemical and physical processes. Consequently, the properties of the aerosol can exhibit a great deal of variability in both time and space. Furthermore, most aerosol studies have focused on measurements of a single aerosol characteristic such as composition or size distribution. Such information is generally not useful for the assessment of impacts because the degree of impact may depend on the integral properties of the aerosol, for example, the aerosol composition as a function of particle size. In this overview we discuss recent work on atmospheric aerosols that illustrates the complex nature of the aerosol chemical and physical system, and we suggest strategies for future research. A major conclusion is that man has had a great impact on the global budgets of certain species, especially sulfur and nitrogen, that play a dominant role in the atmospheric aerosol system. These changes could conceivably affect climate. Large-scale impacts are implied because it has recently been demonstrated that natural and pollutant aerosol episodes can be propagated over great distances. However, at present there is no evidence linking anthropogenic activities with a persistent increase in aerosol concentrations on a global scale. A major problem in assessing man's impact on the atmospheric aerosol system and on global budgets is the absence of aerosol measurements in remote marine and continental areas. This dearth is especially acute for the southern hemisphere, where we could expect natural sources to predominate because of the relatively low level of industrial development and energy utilization.

The respiratory loss of CO2 from soil microbes beneath winter snow in forests from cold climates can significantly influence the annual carbon budget. We explored the magnitude of winter soil respiration using continuous measurements of beneath‐snow CO2 concentration within the footprint of a flux tower in a subalpine forest in the Rocky Mountains. We used eddy covariance measurements from the tower to obtain estimates of total wintertime ecosystem respiration and compared them to the calculated beneath‐snow CO2 flux. Soil respiration in the winter was estimated to contribute 35–48% of the total wintertime ecosystem respiration, and 7–10% of the total annual ecosystem respiration. The largest increase in soil respiration occurred in the late winter following an earlier‐than‐normal initiation of snowmelt and increase in snow density. Following this melt event, respiration rates increased approximately sixfold, despite an increase in soil temperature of only 0.3°–0.5°C. We interpret the late‐winter surge in soil respiration to be triggered by a strong response of beneath‐snow microbes to the pulse of meltwater coupled with extremely high sensitivity of the microbial biomass to increases in soil temperature.

Biomass burning throughout the inhabited portions of the tropics generates precursors which lead to significant local atmospheric ozone pollution. Several simulations show how this smog could be only an easily observed, local manifestation of a much broader increase in tropospheric ozone. We illustrate basic processes with a one‐dimensional time‐dependent model that is closer to true meteorological motions than commonly used eddy diffusion models. Its application to a representative region of South America gives reasonable simulations of the local pollutants measured there. Three illustrative simulations indicate the importance of dilution, principally due to vertical transport, in increasing the efficiency of ozone production, possibly enough for high ozone to be apparent on a very large, intercontinental scale. In the first, cook‐then‐mix, simulation the nitrogen oxides and other burning‐produced pollutants are confined to a persistently subsident fair weather boundary layer for several days, and the resultant ozone is found to have only a transient influence on the whole column of tropospheric ozone. In the second, mix‐then‐cook, simulation the effect of typical cumulonimbus convection, which vents an actively polluted boundary layer, is to make a persistent increase in the tropical ozone column. Such a broadly increased ozone column is observed over the the populated “continental” portion of the tropics. A third simulation averages all emission, transport, and deposition parameters, representing one column in a global tropospheric model that does not simulate individual weather events. This “oversmoothing” simulation produces 60% more ozone than observed or otherwise modeled. Qualitatively similar overprediction is suggested for all models which average significantly in time or space, as all need do. Clearly, simulating these O3 levels will depend sensitively on knowledge of the timing of emissions and transport.