Richard D. Schillawski

A study of new particle formation and growth involving biogenic and trace gas species measured during ACE 1

Measurements are presented of ambient nanoparticle distributions (2.7 to 10 nm diameter) in regions of high biogenic emissions encountered during the First Aerosol Characterization Experiment (ACE 1), November 15 to December 14, 1995. Large numbers of newly formed nanoparticles were observed directly downwind of penguin colonies on Macquarie Island (54.5thinsp{degree}S, 159.0thinsp{degree}W). In these regions, nanoparticle concentrations were also correlated with sulfuric acid (H{sub 2}SO{sub 4(g)}) gas concentrations. The measurements show that biogenic species, possibly ammonia (NH{sub 3}), either by itself or with H{sub 2}SO{sub 4}, nucleated to form new particles at rates much higher than bimolecular H{sub 2}SO{sub 4}/H{sub 2}O nucleation. Nanoparticle distributions evolved as air was advected away from the island showing clear evidence of growth of the newly formed particles. Observed growth rates were in the range of 2 to 5 nmthinsph{sup {minus}1} and were about a factor of 4 to 17 times higher than the growth by condensing H{sub 2}SO{sub 4(g)} and associated water. The cause for fast growth of the newly formed particles is unknown. {copyright} 1998 American Geophysical Union

The Milan photooxidant plume

In Switzerland, measurement campaigns including aircraft measurements were carried out in the summers of 1992 and 1993 as part of the Pollution and Meteorology (POLLUMET) study. Ozone (O 3 ) concentrations, up to 185 ppb, with a large spatial variability were found south of the Alps in the afternoon. Comparison to measurements north of the Alps shows that these concentration levels are extraordinarily high for central Europe. Backward trajectories reveal that the highest O 3 levels were found 4-5 hours downwind of Milan, Italy. The measurements suggest a reactive organic gas (ROG) sensitive O 3 production regime 1-3 hours downwind in the plume, and a NO x (sum of nitrogen oxide (NO) and nitrogen dioxide (NO 2 )) limitation in air masses not affected by the Milan plume. Air masses originating north of Milan are probably close to the transition zone between the two photochemical regimes. This was found by using measurements of total odd nitrogen (NO y ), NO, NO 2 , formaldehyde (HCHO), and hydrogen peroxide (H 2 0 2 ) yielding indicators for ROG and NO, sensitive O 3 production. The slope of ozone versus NO z (=NO y -NO x : photochemical products of NO x ) were markedly higher in NO x limited conditions (ΔO 3 /ΔNO z = 13.6) than in air masses close to the transition zone (Δ0 3 /ΔNO z = 4.2).

Spurious aerosol measurements when sampling from aircraft in the vicinity of clouds

Extensive airborne measurements of aerosol particles in a pristine marine region were made during the first Aerosol Characterization Experiment (ACE 1) from November 15 to December 14, 1995. During this study, high concentrations of condensation nuclei (CN) were frequently observed both near and within clouds. Near clouds, in the absence of liquid water, Clarke et al. [1998] have reported that high CN levels were from new particle formation by homogeneous nucleation. Here we show, however, that within clouds, elevated CN concentrations were not authentic, but instead a sampling artifact, likely related to fragmentation of cloud drops impacting the aerosol inlet. By themselves, these fragments were often indistinguishable from ambient particles. Spurious CN from fragmenting droplets were observed at temperatures down to roughly −20°C and spanned a broad size range, with diameters down to 3 nm. Comparison of two different sized isokinetic aerosol inlets showed that inlets with smaller openings produce higher droplet fragment concentrations. The mechanism for producing these particles is not completely understood. Although fragmentation appeared to be the primary mechanism, for one instrument, an additional spurious source, correlated with liquid water, was observed when ambient temperatures were below −5°C. These findings show that care must be taken when interpreting airborne aerosol measurements in regions of liquid water. This is particularly pertinent to studies of new particle formation by homogeneous nucleation in the vicinity of clouds.

Physico-chemical modeling of the First Aerosol Characterization Experiment (ACE 1) Lagrangian B: 1. A moving column approach

During Lagrangian experiment B (LB in the following) of the First Aerosol Characterization Experiment (ACE 1), a clean maritime air mass was followed over a period of 28 hours. During that time span, the vertical distribution of aerosols and their gas phase precursors were characterized by a total of nine aircraft soundings which were performed during three research flights that followed the trajectory of a set of marked tetroons. The objective of this paper is to study the time evolution of gas phase photochemistry in this Lagrangian framework. A box model approach to the wind shear driven and vertically stratified boundary layer is questionable, since its basic assumption of instantaneous turbulent mixing of the entire air column is not satisfied here. To overcome this obstacle, a one-dimensional Lagrangian boundary layer meteorological model with coupled gas phase photochemistry is used. To our knowledge, this is the first time that such a model is applied to a Lagrangian experiment and that enough measurements are available to fully constrain the simulations. A major part of this paper is devoted to the question of to what degree our model is able to reproduce the time evolution and the vertical distribution of the observed species. Comparison with observations of O3, OH, H2O2, CH3OOH, DMS, and CH3I, made on the nine Lagrangian aircraft soundings shows that this is in general the case, although the dynamical simulation started to deviate from the observations on the last Lagrangian flight. In agreement with experimental findings reported by Q. Wang et al. (unpublished manuscript, 1998b), generation of turbulence in the model appears to be most sensitive to the imposed sea surface temperature. Concerning the different modeled and observed chemical species, a number of conclusions are drawn: (1) Ozone, having a relatively long photochemical lifetime in the clean marine boundary layer, is found to be controlled by vertical transport processes, in particular synoptic-scale subsidence or ascent. (2) Starting with initally constant vertical profiles, the model is able to “create” qualitatively the vertical structure of the observed peroxides. (3) OH concentrations are in agreement with observations, both on cloudy and noncloudy days. On the first flight, a layer of dry ozone rich air topped the boundary layer. The model predicts a minimum in OH and peroxides at that altitude consistent with observations. (4) Atmospheric DMS concentrations are modeled correctly only when using the Liss and Merlivat [1986] flux parameterization, the Wanninkhof [1992] flux parameterization giving values twice those observed. To arrive at this conclusion, OH is assumed to be the major DMS oxidant, but no assumptions about mixing heights or entrainment rates are necessary in this type of model. DMS seawater concentrations are constrained by observations.

Photochemical modeling of OH levels during the First Aerosol Characterization Experiment (ACE 1)

Comparisons are made between a steady state photochemical model and the first airborne measurements of hydroxyl radical concentrations, [OH], in the lower free troposphere and marine boundary layer, taken during the First Aerosol Characterization Experiment (ACE 1). This paper describes in more detail the modeling results and the model-measurement comparisons which were presented by Mauldin et al. [1997, 1998]. Modeled and observed [OH] agree to within the combined uncertainties, with a median model overestimate of 32%. The model quantifies the sources and sinks of OH for low levels of nitric oxide (NO). Predicted concentrations of the peroxy radicals HO2 and CH3O2 and total radical production and loss rates from these model calculations are presented. Sensitivities to model input parameters and the uncertainty each contributes to the calculated [OH] are discussed. Model overpredictions of observed [OH] within clouds can be corrected by including a loss of gas phase OH to cloud droplets. The low [NO] conditions preclude HO2 uptake alone from explaining observed depletions of OH within clouds. We examined the dependence of gas phase [OH] on its assumed mass accommodation coefficient, which the measurements suggest is 0.1 or greater for the cloud droplets encountered in ACE 1.

Local meteorological features affecting chemical measurements at a North Atlantic coastal site

One of the foci of the North Atlantic Regional Experiment (NARE) 1993 summer intensive campaign was Chebogue Point, approximately 10 km south of Yarmouth, Nova Scotia. Measurements were taken at this site with a 915-MHz boundary layer wind profiler, the Canadian Twin Otter and National Center for Atmospheric Research King Air aircraft, and a variety of surface instruments. This paper discusses features observed in the meteorological measurements and the implications of those features for the interpretation of chemical measurements. The meteorology of this coastal site is complex. A strong surface-based temperature inversion was almost always present, producing strong layering in the lower atmosphere. As a result, surface chemistry measurements were not often representative of the state of the overlying atmosphere. A low-level jet was also frequently present. A variety of turbulence structures were observed by the profiler, including convective boundary layers and complex layering. Ozone concentrations above 60 parts per billion at the surface occurred on four occasions late in August, in conjunction with strong stability and winds off the Atlantic Ocean (Gulf of Maine). Ozone levels of more than 60 ppbv were observed by the aircraft at altitudes between 400 and 1600 m on five other occasions earlier in the month. Particular (but not necessarily unusual) combinations of transport, mixing, and source conditions appear to be required to produce ozone episodes at Chebogue Point.

Characteristics of the marine boundary layers during two Lagrangian measurement periods: 2. Turbulence structure

Characteristics of turbulence mixing in remote marine boundary layers are analyzed using aircraft measurements from six flights during the two intensive Lagrangian measurement periods of ACE1 (the southern hemisphere Aerosol Characterization Experiment). The six cases studied here represent a variety of boundary layer conditions in the region south of Tasmania, Australia. Our study indicated that (1) Lagrangian A (LA) had stronger turbulence mixing and entrainment compared to Lagrangian B (LB) due to greater shear generation of turbulence kinetic energy (TKE), (2) strong mesoscale variation in boundary layer turbulence and thus turbulence mixing existed in the ACE1 region during LB due to variations in sea surface temperature (SST), (3) stable thermal stratification in the boundary layer was found during the last flight of each Lagrangian, consequently, TKE decreased rapidly with height resulting in small or near-zero entrainment rate in spite of strong shear forcing at the surface and in the boundary layer; and (4) the buffer layer, which lies above the boundary layer and below the main inversion, had weak and intermittent turbulence mostly associated with cloud bands and cumulus. Evidence of entrainment was found in the buffer layer. However, it is difficult to quantify by flux measurements due to the weak and intermittent nature of the turbulence field.

Photochemical production and aging of an urban air mass

Aircraft measurements of O3, NO, NO2, NOy, HCHO, and H2O2 over the Swiss Plateau during 4 days in July 1993 were analyzed. Special emphasis was put on the urban plume of Zurich. An effective photochemical age for an urban plume was introduced, which accounts for a background NOz concentration. Only the effective photochemical age of the urban air mass increased with the time of transport between Zurich and the flight leg, where the aircraft crossed the plume. Ozone and NOz were strongly correlated. Ozone production rates ranged from 2.4 to 6.5 ozone molecules produced per NOx processed. The production of ozone per NOx molecule was lowest when the effective photochemical age was lowest and vice versa. Good correlations between HCHO and NOy have been found in the urban plume of Zurich as well as over the Swiss Plateau. Between H2O2 and NOz, a negative correlation was observed. On the basis of Sillmans [1995] indicator species, the ozone production in the Zurich plume and other air masses over the Swiss Plateau is in the NOx-sensitive range. However, there remains some uncertainty in this approach regarding the influence of biogenic emissions and the initial concentration of indicator species. Taking the low emissions of biogenic hydrocarbons compared to Sillmans model calculation into consideration, the central portion of the Zurich plume falls in the transition or even reactive organic gases (ROG)- limited range of all indicators. Estimates of production rates of HNO3 and peroxides support a ROG-sensitive ozone production in the most polluted portion of the plume.

Evolution of the Subtropical Marine Boundary Layer: Photochemical Ozone Loss

Abstract A diurnally averaged ozone decay rate of about 0.11 day−1was observed under partly cloudy sky in the boundary layer over the eastern Atlantic during the second Lagrangian experiment in the Atlantic Stratocumulus Transition Experiment. The observed decay rate can be accounted for by the combined effects of ozone loss through photolysis by UV radiation, reaction With H02 radical, and deposition on the sea surface, and effects of ozone production through photooxidation of carbon monoxide and methane in the presence of nitrogen oxides. The main contributor to the ozone decay is photolysis by UV radiation, but the other sources and sinks also make significant contributions.

Carbon monoxide measurements from 76° N to 59° S and over the South Tasman Sea

In November and December of 1995, carbon monoxide (CO) measurements were made in a Pacific transect and over the South Tasman Sea as part of the First Aerosol Characterization Experiment (ACE 1) program. Airborne CO measurements were made from 76° N to 59° S. A clear latitudinal gradient in CO concentrations was measured, with the southern hemisphere average about 80 parts per billion by volume (ppbv), and increasing to 120–130 ppbv at the most northern latitudes. Plumes of CO with a 30–40 ppbv concentration increase over the general background concentrations could be seen at several latitudes. The National Oceanic and Atmospheric Administration R/V Discoverer made CO measurements over the South Tasman Sea from November 15 to December 9, 1995. A systematic decrease of 0.31 ppbv/d CO was observed. Vertical profile measurements of CO from near the ocean surface to 2500 m altitude during the Lagrangian B intensive of ACE 1 suggested the mixing of stratospheric air with reduced CO concentrations.