Ian Folkins

Chemistry of the 1991–1992 stratospheric winter: Three-dimensional model simulations

A three-dimensional chemistry-transport model of the stratosphere is used to simulate the evolution of trace constituents during the 1991–1992 Arctic winter. It is shown that heterogeneous reactions on polar stratospheric clouds led in early January to almost complete activation of atmospheric chlorine inside the polar vortex, in remarkable coincidence with observations by the ER-2 aircraft (Toohey et al., 1993) and the microwave limb sounder on the Upper Atmosphere Research Satellite (Waters et al., 1993). Sulfate aerosols resulting from the eruption of Mount Pinatubo also produced a significant increase in chlorine monoxide (ClO) concentrations at middle and high latitudes. The net chemical destruction of ozone found in the vortex at the end of the simulation (25% at 50 hPa and 25 DU), although substantial, was limited by available sunlight and the short period during which stratospheric clouds occurred.

A barrier to vertical mixing at 14 km in the tropics: Evidence from ozonesondes and aircraft measurements

We use ozonesondes launched from Samoa (14°S) during the Pacific Exploratory Mission (PEM) Tropics A to show that O3 mixing ratios usually start increasing toward stratospheric values near 14 km. This is well below the tropical tropopause (as defined either in terms of lapse rate or cold point), which usually occurs between 16 and 17 km. We argue that the main reason for this discrepancy in height between the chemopause and tropopause is that there is very little convective detrainment of ozone-depleted marine boundary layer air above 14 km. We conjecture that the top of the Hadley circulation occurs at roughly 14 km, that convective penetration above this altitude is rare, and that air that is injected above this height subsequently participates in a slow vertical ascent into the stratosphere. The observed dependence of ozone on potential temperature in the transitional zone between the 14-km chemopause and the tropical tropopause is consistent with what would be expected from this hypothesis given calculated clear-sky heating rates and typical in situ ozone production rates in this region. An observed anticorrelation between ozone and equivalent potential temperature below 14 km is consistent with what would be expected from an overturning Hadley circulation, with some transport of high O3/low θe air from midlatitudes. We also argue that the positive correlations between O3 and N2O in the transitional zone obtained during the 1994 Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft) (ASHOE/MAESA) campaign support the notion that air in this region does have trace elements of stratospheric air (as conjectured previously), so that some of the ozone in the transitional zone does originate from the stratosphere rather than being entirely produced in situ.

The Vertical Structure of Tropical Convection and Its Impact on the Budgets of Water Vapor and Ozone

Abstract Convective clouds in the Tropics that penetrate the boundary layer inversion preferentially detrain into a shallow outflow layer (2–5 km) or a deep outflow layer (10–17 km). The properties of these layers are diagnosed from a one-dimensional model of the Tropics constrained by observed mean temperature and water vapor profiles. The mass flux divergence of the shallow cumuli (2–5 km) is balanced by a mass flux convergence of evaporatively forced descent (downdrafts), while the mass flux divergence of deep cumulonimbus clouds (10–17 km) is balanced by a mass flux convergence of clear-sky radiative descent. The pseudoadiabatic temperature stratification of the midtroposphere (5–10 km) suppresses cloud outflow in this interval. The detrainment profile in the deep outflow layer is shifted downward by about 1.5 km from the profile one would anticipate based on undilute pseudoadiabatic ascent of air from the boundary layer. The main source of water vapor to most of the tropical troposphere is evaporativ...

Biomass burning and deep convection in southeastern Asia: Results from ASHOE/MAESA

There was extensive biomass burning in Indonesia, northern Australia, and New Guinea during September and October 1994. This paper discusses two accidental encounters of biomass plumes from the 1994 Airborne Southern Hemisphere Ozone Experiment and Measurements for Assessing the Effects of Stratospheric Aircraft campaign (ASHOE/MAESA). During the October 23 descent into Fiji, and an ascent from Fiji on October 24, the NASA ER-2 passed through layers highly enhanced in NO, NO y , CO, and O 3 . These layers occurred near an altitude of 15 km. Back trajectories and satellite images indicate that the layers probably originated as outflow from a convective disturbance near New Guinea. The measurements indicate that deep convection can inject emissions from southeast Asian biomass burning to near tropical tropopause altitudes. Deep convection magnifies the impact of biomass burning on tropospheric chemistry because of the much longer residence times and chemical lifetimes of species in the upper tropical troposphere. Transport of the products of southeast Asian biomass burning into the upper tropical troposphere, followed by southward high-level outflow and advection by the subtropical jet, may play a significant role in dispersing these emissions on a global scale. Anthropogenic emissions from countries in southeast Asia are likely to increase in the future as these countries become more highly industrialized. This transport mechanism may play a role in increasing the impact of these types of emissions as well.

Impact of acetone on ozone production and OH in the upper troposphere at high NOx

The impact of acetone (or any HOx source) on tropospheric photochemistry is largest in the high NOx regime. The fractional increases in OH and ozone production associated with acetone increase rapidly with NOx when NOx mixing ratios become larger than 300 parts per trillion by volume (pptv). This occurs in part because the HOx yield of acetone is larger at higher NOx mixing ratios, going from about 1 HOx at NOx ∼ 10 pptv to 3 HOx at NOx ∼ 1000 pptv. We also investigate the effect of acetone on the partitioning of the NOy family. Acetone increases the conversion of NO to NO2, HNO4, HNO3, and peroxyacetylnitrate (PAN). Conversion of NO to PAN dominates at low NOx, while conversion of NO to HNO3 dominates at high NOx. These NO decreases significantly diminish the increases in OH and ozone production one would otherwise anticipate from the increases in HO2. In particular, acetone can be expected to reduce ozone production for NOx<25 pptv. We also show that most of the changes in species concentrations arising from the introduction of a HOx source can be accurately predicted using simple expressions derived from linear perturbation theory.

Tropical convective outflow and near surface equivalent potential temperatures

We use clear sky heating rates to show that convective outflow in the tropics decreases rapidly with height between the 350 K and 360 K potential temperature surfaces (or between roughly 13 and 15 km). There is also a rapid fall-off in the pseudoequivalent potential temperature probability distribution of near surface air parcels between 350 K and 360 K. This suggests that the vertical variation of convective outflow in the upper tropical troposphere is to a large degree determined by the distribution of sub cloud layer entropy.

Origin of Lapse Rate Changes in the Upper Tropical Troposphere

Vertical motions in clouds arise from a variety of thermodynamic processes, including latent heat release, evaporative cooling, melting, and cloud radiative heating. In the Tropics, the net upward vertical mass flux from convective systems should approximately balance subsidence in clear sky regions associated with radiative cooling, provided the exchange of mass with midlatitudes can be assumed small. Tropical climatologies of temperature, water vapor, and ozone are used to calculate the clear sky radiative mass flux, and the derivative of this mass flux with respect to potential temperature, d Mr(u)/du, is used as a proxy for net convective outflow. Convective outflow increases rapidly at 345 K ( ;11.3 km). This corresponds to the pseudoequivalent potential temperature ue at which air parcels near the surface first attain positive convective available potential energy (CAPE). The rate at which dMr(u)/du decreases above 345 K is similar to the rate at which the near surface ue probability distribution function (PDF) decreases. This behavior is referred to as ‘‘scaling.’’ It suggests that the timescale for removal of an air parcel from the convective boundary layer is independent of ue (once it has positive CAPE), and that the residual vertical mass flux from convective clouds can be described as if air parcels detrain near their level of neutral buoyancy (LNB). It is also suggested that the mean tropical temperature profile above 345 K is controlled, not by mixing, but by the need for the vertical variation in net convective outflow to be consistent with the near-surface ue PDF, and that this accounts for the fact that the mean temperature profile above 345 K increasingly deviates from a moist adiabat. It is also argued that there are sufficient high ue air parcels near the surface to sustain the Brewer‐Dobson circulation by detrainment at the LNB followed by radiative ascent into the stratosphere.

Ozone and potential vorticity at the subtropical tropopause break

Ozone measurements near 200 mbar from two flights between California and Tahiti are interpreted using maps of potential vorticity (PV) on isentropic surfaces. We focus on extremely abrupt changes in ozone mixing ratio observed at latitudes of 13°N and 23.5°N. Their proximity to strong PV gradients on the 350 K isentropic surface shows that they are associated with crossings of the subtropical tropopause. Small-scale anticorrelations between ozone and carbon dioxide near one of the two ozone transitions indicate that some stratosphere-troposphere exchange does occur in this region. Ozone mixing ratios on the stratospheric side of the subtropical tropopause varied from 50 to 100 parts per billion by volume, a range that is more commonly associated with the midlatitude troposphere and is much less than seen on the stratospheric side of the midlatitude tropopause.

OH, HO2, and NO in two biomass burning plumes: Sources of HOx and implications for ozone production

The ER-2 made two descents through upper tropospheric biomass burning plumes during ASHOE/MAESA. HO_x (= OH + HO_2) concentrations are largely self-limited outside the plumes, but become progressively more limited by reactions with NO_x (= NO + NO_2) at the higher NO_x concentrations inside the plumes. Sources of HO_x in addition to H_(2)O and CH_4 oxidation are required to balance the known HOx sinks both in the plumes and in the background upper troposphere. HO_x concentrations were consistently underestimated by a model constrained by observed NO_x concentrations. The size of the model underestimate is reduced when acetone photolysis is included. Models which do not include the additional HO_x sources required to balance the HO_x budget are likely to underestimate ozone production rates.

Tropical Rainfall and Boundary Layer Moist Entropy

In the Tropics, the variation of rainfall with sea surface temperature (SST) is highly nonlinear. Rainfall shows no dependence on SST for SST increases from 198 to 268C, abruptly increases by a factor of 5 as SSTs increase from 268 to 298C, and then rapidly declines. It is argued that this nonlinear dependence is a response to the nonlinear dependence of convective mass on SST. Convective mass is a measure of the mass in the convective boundary layer thermodynamically able to participate in deep convection by virtue of its positive convective available potential energy (CAPE). Monthly mean estimates of convective mass were obtained at various islands in the tropical Pacific and Caribbean from the NOAA/National Climatic Data Center highresolution radiosonde database. In the inner Tropics, the tendency for temperatures above the boundary layer to be homogeneous plays an important role in the rapid increase in rainfall near the convective threshold SST. At SSTs below the convective threshold, near-surface winds are generally directed from cold to warmer SSTs, so that horizontal advection of equivalent potential temperature (u e) will tend to suppress moist entropy, and rainfall, in these regions. In areas of the ocean with SSTs larger than the convective threshold, the mean frequency distribution of u e in the boundary layer becomes independent of SST. This occurs both as a response to the homogeneity of temperatures in the inner Tropics, and to the tendency for wind speeds in the boundary layer to decrease with SST for SSTs larger than the convective threshold. In the subtropics, temperature fluctuations are much larger than in the inner Tropics, and can be expected to play a much greater role in determining precipitation patterns.