F. L. Eisele

Measurements of new particle formation and ultrafine particle growth rates at a clean continental site

Simultaneous measurements of aerosol particles and their expected gas phase precursors were made at Idaho Hill, Colorado, a remote continental site. This study used apparatus and techniques similar to those employed in an earlier study at the Mauna Loa Observatory, Hawaii [Weber et al., 1995]. New particle formation, identified by the presence of ultrafine particles (nominally 3 to 4 nm diameter), was commonly observed in downslope (westerly) air and was correlated with high sulfuric acid (H2SO4) concentrations, low relative humidity and low particle surface area concentrations. The data point to H2SO4 as a principle nucleation precursor species with typical daytime concentrations between 106 and 107 molecules cm−3. Particle production was observed at H2SO4 concentrations that are well below predicted values for binary nucleation of H2O and H2SO4, suggesting that another species participated. Particle growth rates were estimated from the data with two independent approaches and in both cases were ∼5 to 10 times higher than can be explained by condensation of H2SO4 and its associated water. This suggests that species in addition to H2S04 were also making large contributions to ultrafine particle growth. Finally, calculated steady-state H2SO4 concentrations were found to be in good agreement with measured values if the mass accommodation coefficient for H2SO4 on aerosol surfaces was assumed equal to ∼1.

Laboratory studies of particle nucleation: Initial results for H2SO4, H2O, and NH3 vapors

Particle formation in the binary H2SO4-H2O vapor system was studied at 295 K in a series of experiments employing a flow reactor. The concentration of H2SO4 was detected by chemical ionization mass spectrometry, and an ultrafine particle condensation nucleus counter was used to count the newly nucleated particles. Results yield a particle formation rate that is approximately proportional to [H2SO4] raised to the eighth power and to [H2O] raised to the fifth power. The power dependencies measured here are significantly different from those determined in previous experimental work, and furthermore, the water dependence is markedly different from that predicted from current theories. The effect of adding ammonia vapor to the binary system was investigated; concentrations of NH3 in the many tens of parts per trillion by volume range were observed to promote dramatically the rate of particle nucleation.

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

Ion-assisted tropospheric OH measurements

A highly sensitive real-time ion-assisted OH measurement technique has recently been developed and tested. It offers an OH detection sensitivity of about 1×105 molecules/cm3 with a 300-s integration time and provides useful OH concentration measurements on a 10-s time scale. Measurements of OH concentration are seen to track solar flux during both short and long periods of cloudiness. Good temporal correspondence between changes in OH and O3 concentration as differing air masses pass the measurement site are also observed and discussed. The present technique provides a new opportunity for fast sensitive OH measurement using a totally new nonoptical measurement scheme.

Export efficiency of black carbon aerosol in continental outflow: Global implications

[1] We use aircraft observations of Asian outflow from the NASA Transport and Chemical Evolution over the Pacific (TRACE-P) mission over the NW Pacific in March-April 2001 to estimate the export efficiency of black carbon (BC) aerosol during lifting to the free troposphere, as limited by scavenging from the wet processes (warm conveyor belts and convection) associated with this lifting. Our estimate is based on the enhancement ratio of BC relative to CO in Asian outflow observed at different altitudes and is normalized to the enhancement ratio observed in boundary layer outflow (0-1 km). We similarly estimate export efficiencies of sulfur oxides (SO x = SO 2 (g) + fine SO 2- 4 ) and total inorganic nitrate (HNO T 3 = HNO 3 (g) + fine NO - 3 ) for comparison to BC. Normalized export efficiencies for BC are 0.63-0.74 at 2-4 km altitude and 0.27-0.38 at 4-6 km. Values at 2-4 km altitude are higher than for SO x (0.48-0.66) and HNO T 3 (0.29-0.62), implying that BC is scavenged in wet updrafts but not as efficiently as sulfate or nitrate. Simulation of the TRACE-P period with a global three-dimensional model (GEOS-CHEM) indicates that a model timescale of 1 ± 1 days for conversion of fresh hydrophobic to hydrophilic BC provides a successful fit to the export efficiencies observed in TRACE-P. The resulting mean atmospheric lifetime of BC is 5.8 ± 1.8 days, the global burden is 0.11 ± 0.03 Tg C, and the decrease in Arctic snow albedo due to BC deposition is 3.1 ± 2.5%.

Unexpected high levels of NO observed at South Pole

Reported here are the first Austral summer measurements of NO at South Pole (SP). They arc unique in that the levels are one to two orders of magnitude higher (i.e., median, 225 pptv) than measured at other polar sites. The available evidence suggests that these elevated levels arc the result of photodenitrification of the snowpack, in conjunction with a very thin atmospheric mixing depth. Important chemical consequences included finding the atmospheric oxidizing power at SP to be an order of magnitude higher than expected.

Atmospheric sulfur chemistry and cloud condensation nuclei (CCN) concentrations over the northeastern Pacific Coast

Correlated measurements of dimethylsulfide (DMS), gas phase dimethylsulfoxide (DMSO), methane-sulfonic acid (MSA)(g), sulfuric acid (H2SO4), and cloud condensation nuclei (CCN) were conducted in April 1991 at a Pacific coastal site in northern Washington. Measurements of SO2, aerosol methanesulfonate (MSA)(p), and non-sea-salt sulfate (nss-SO4) concentrations were also included. Maximum DMS concentrations between 100 and 240 pptv were observed when the measurement site (480 m above sea level) was embedded in clouds and air from the marine boundary layer was flowing upslope to the site. DMS levels measured in continental air and/or above the mixed layer were typically less than 20 pptv. The sulfur gases DMSO, H2SO4, and MSA(g) were measured in real time on a continuous basis (once every 60–150 s) using selected ion chemical ionization mass spectrometry. Corresponding concentrations ranged between <0.5–3.2 pptv, 0.001–1.19 pptv, and 0.002–0.19 pptv, respectively. All three sulfur gases showed significant diel variations mostly in phase with each other. Their corresponding lifetimes in the marine atmosphere are estimated to be of the order of a few hours. The results in connection with recent laboratory studies and model calculations suggest that dimethylsulfone (DMSO2) was the dominant end product of DMS oxidation under the present conditions. CCN concentrations measured in marine air ranged roughly between 10–200 cm−3 and 200–400 cm−3 at 0.3% and 0.9% supersaturation, respectively. A statistical analysis using only data obtained in predominantly marine air and during non-fog/non-precipitation periods showed significant correlations between individual sulfur species and between CCN (0.3% ss) and H2SO4, and CCN (0.3% ss) and nss-SO4. The results indirectly support a relationship between DMS and CCN concentrations. However, other observations also suggest that at higher supersaturations (0.9%), compounds less soluble than sulfate may become important in marine CCN formation.

Peroxy radicals from photostationary state deviations and steady state calculations during the Tropospheric OH Photochemistry Experiment at Idaho Hill, Colorado, 1993

Concentrations of peroxy radicals and a number of other trace gases were measured during the Tropospheric OH Photochemistry Experiment in August and September, 1993. The trace gas concentrations were used to derive two estimates of peroxy radical levels: deviations from the photostationary state, and theoretical calculations with the assumption of steady state radical concentrations. As in many previous studies, the photostationary state method yielded midday peroxy radical levels about twice those measured. Calculations were performed to assess the expected uncertainty in the photostationary state derived peroxy radical concentrations from the uncertainty of the input parameters. The agreement between measurements and steady state determinations was better than the photostationary state estimates, with the measurements slightly higher on average. Radical formation and destruction rates for clear sky conditions with low and high NOx were derived from the steady state calculations and the results demonstrate the processes that control peroxy radical levels in clean and polluted continental atmospheres.

Thermal Desorption Chemical Ionization Mass Spectrometer for Ultrafine Particle Chemical Composition

A thermal desorption chemical ionization mass spectrometer has been developed for real time, quantitative chemical analysis of ultrafine particles. The technique combines recently developed nanoparticle separation and collection techniques with highly sensitive chemical analysis provided by selected ion chemical ionization mass spectrometry. Sensitivity tests using laboratory-generated ammonium sulfate particles in the diameter range 10-16 nm show that sulfate and ammonium can be quantified with as little as 1 pg of collected aerosol mass. Such sensitivity makes this instrument suitable for real time measurements of the chemical composition of sub-10 nm particles reported recently from nucleation events.

Intercomparison of tropospheric OH and ancillary trace gas measurements at Fritz Peak Observatory, Colorado

The determination of the concentration of OH in the Earths troposphere is of fundamental importance to an understanding of the chemistry of the lower atmosphere. Although many experiments to measure OH concentration have been performed in recent years, very few operate at sensitivities necesssary to measure the extremely low amount of OH in the clean troposphere (0.1–0.2 parts per trillion by volume at summertime local noon). This paper describes an informal intercomparison campaign held at Fritz Peak, Colorado, in summer 1991 to intercompare the OH concentrations determined from a spectroscopic instrument and an in situ chemical conversion instrument, both with sensitivities at or below 5×105 molecules cm−3. Ancillary measurements including those of O3, CO, NO, NO2, NOy, H2O, SO2, aerosols, solar flux, and meteorological parameters were also performed to test photochemical theories of OH formation. These measurements also provided a means for comparing air masses at the long path and in situ sites. The intercomparison was very successful with measured values of OH concentration in agreement within one standard error much of the time. OH concentrations were typically low, rarely above 4×106 cm−3, with only slow growth during the morning hours, indicating the possible presence of scavenger species. Model results suggest higher than measured OH concentrations or the presence of scavenger species.