L. R. Mauldin

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

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.

Nucleation in the equatorial free troposphere: Favorable environments during PEM-Tropics

A combination of aerosol and gas phase instrumentation was employed aboard the NASA-P3B as part of the Pacific Exploratory Mission-Tropics (PEM-T) in the eastern equatorial Pacific during August-October 1996. Recent particle production was found in cloud-processed air over extended regions aloft (6–8 km). These were clearly associated with clean marine air lofted by deep convection and scavenged of most aerosol mass in the Intertropical Convergence Zone (ITCZ) and in more aged cloud-scavenged air influenced by a distant continental combustion near the South Pacific Convergence Zone (SPCZ). Recent particle production was evident in regions where sulfuric acid concentrations were about 0.5 to 1 × l07 molecules cm−3, when surface areas were near or below 5 µm2 cm−3, and when relative humidity (RH) was elevated over adjacent regions. In regions of recent particle production, the calculated critical sulfuric acid concentrations, based upon classical binary nucleation theory and corrected for in situ conditions near cloud, were generally consistent with nearby observed sulfuric acid concentrations. This indicates that classical binary nucleation theory and natural sources of sulfuric acid can account for nucleation in the near-cloud environment. Data from six equatorial flights between 20°N and 20°S demonstrate that this process populates extensive regions of the equatorial free troposphere with new particles. Vertical profiles suggest that nucleation, subsidence, and mixing into the MBL can supply the MBL with new aerosol.

Dimethyl sulfide oxidation in the equatorial Pacific: Comparison of model simulations with field observations for DMS, SO2, H2SO4(g), MSA(g), MS and NSS

Reported here are results from an airborne photochemical/sulfur field study in the equatorial Pacific. This study was part of NASAs Global Tropospheric Experiment (GTE) Pacific Exploratory Mission (PEM) Tropics A program. The focus of this paper is on data gathered during an airborne mission (P-3B flight 7) near the Pacific site of Christmas Island. Using a Lagrangian-type sampling configuration, this sortie was initiated under pre-sunrise conditions and terminated in early afternoon with both boundary layer (BL) as well as buffer layer (BuL) sampling being completed. Chemical species sampled included the gas phase sulfur species dimethyl sulfide (DMS), sulfur dioxide (SO2), methane sulfonic acid (MSA)g, and sulfuric acid (H2SO4)g. Bulk aerosol samples were collected and analyzed for methane sulfonate (MS), non-sea-salt sulfate (NSS), Na+,Cl−, and NH4+. Critical non-sulfur parameters included real-time sampling of the hydroxyl radical (OH) and particle size/number distributions. These data showed pre-sunrise minima in the mixing ratios for OH, SO2, and H2SO4 and post-sunrise maxima in the levels of DMS, OH, and H2SO4. Thus, unlike several previous studies involving coincidence DMS and SO2 measurements, the Christmas Island data revealed that DMS and SO2 were strongly anticorrelated. Our “best estimate” of the overall efficiency for the conversion of DMS to SO2 is 72±22%. These results clearly demonstrate that DMS was the dominant source of SO2 in the marine BL. Using as model input measured values for SO2 and OH, the level of agreement between observed and simulated BL H2SO4(g) profiles was shown to be excellent. This finding, together with supporting correlation analyses, suggests that the dominant sulfur precursor for formation of H2SO4 is SO2 rather than the more speculative sulfur species, SO3. Optimization of the fit between the calculated and observed H2SO4 values was achieved using a H2SO4 first-order loss rate of 1.3 × 10−3 s−1. On the basis of an estimated total “wet” aerosol surface area of 75 µm2/cm3, a H2SO4 sticking coefficient of 0.6 was evaluated at a relative humidity of ≃95%, in excellent agreement with recent laboratory measurements. The Christmas Island data suggest that over half of the photochemically generated SO2 forms NSS, but that both BL NSS and MS levels are predominantly controlled by heterogeneous processes involving aerosols. In the case of MS, the precursors species most likely responsible are the unmeasured oxidation products dimethyl sulfoxide (DMSO) and methane sulfinic acid (MSIA). Gas phase production of MSA was shown to account for only 1% of the observed MS; whereas gas phase produced H2SO4 accounted for ∼20% of the NSS. These results are of particular significance in that BL-measured values of the ratio MS/NSS have often been used to estimate the fraction of NSS derived from biogenic DMS and to infer the temperature environment where DMS oxidation occurred. If our conclusions are correct and both products are predominantly formed from complex and still poorly characterized heterogeneous processes, it would suggest that for some environmental settings a simple interpretation of this ratio might be subject to considerable error.

Evidence for photochemical production of ozone at the South Pole surface

Observations of OH, NO, and actinic flux at the South Pole surface during December 1998 suggest a surprisingly active photochemical environment which should result in photochemical production of ozone. Long-term South Pole in situ ozone data as well as sonde data also appear to support this conclusion. Other possible factors contributing to ozone variability such as stratospheric influence and the origin of air transported to the South Pole are also explored. Based on box model calculations it is estimated that photochemistry could add 2.2 to 3.6 ppbv/day of ozone to surface air parcels residing on the Antarctic polar plateau. Although the oxidizing potential of the polar plateau appears to be exceptionally high for a remote site, it is unlikely that it has a significant impact on surrounding regions such as the Southern Ocean and the Antarctic free troposphere. These new findings do suggest, however, that the enhanced oxidizing power of the polar plateau may need to be considered in interpreting the chemical history of climate proxy species in ice cores.

Atmospheric chemistry of an Antarctic volcanic plume

[1] We report measurements of the atmospheric plume emitted by Erebus volcano, Antarctica, renowned for its persistent lava lake. The observations were made in December 2005 both at source, with an infrared spectrometer sited on the crater rim, and up to 56 km downwind, using a Twin Otter aircraft; with the two different measurement platforms, plume ages were sampled ranging from <1 min to as long as 9 h. Three species (CO, carbonyl sulfide (OCS), and SO2) were measured from both air and ground. While CO and OCS were conserved in the plume, consistent with their long atmospheric lifetimes, the downwind measurements indicate a SO2/CO ratio about 20% of that observed at the crater rim, suggesting rapid chemical conversion of SO2. The aircraft measurements also identify volcanogenic H2SO4, HNO3 and, recognized for the first time in a volcanic plume, HO2NO2. We did not find NOx in the downwind plume despite previous detection of NO2 above the crater. This suggests that near-source NOx was quickly oxidized to HNO3 and HO2NO2, and probably NO3� (aq), possibly in tandem with the conversion of SO2 to sulfate. These fast processes may have been facilitated by ‘‘cloud processing’’ in the dense plume immediately downwind from the crater. A further striking observation was O3 depletion of up to � 35% in parts of the downwind plume. This is likely to be due to the presence of reactive halogens (BrO and ClO) formedthrough heterogeneous processes in the young plume. Our analysis adds to the growing evidence for the tropospheric reactivity of volcanic plumes and shows that Erebus volcano has a significant impact on Antarctic atmospheric chemistry, at least locally in the Southern Ross Sea area.

An Assessment of HOx Chemistry in the Tropical Pacific Boundary Layer: Comparison of Model Simulations with Observations Recorded during PEM Tropics A

Reported are the results from a comparison of OH,H2O2CH3OOH, and O3 observationswithmodel predictions based on current HOx–CH4reaction mechanisms. The field observations are thoserecorded during the NASA GTE field program, PEM-Tropics A. The major focus ofthis paper is on thosedata generated on the NASA P-3B aircraft during a mission flown in the marineboundary layer (MBL) nearChristmas Island, a site located in the central equatorial Pacific (i.e.,2° N, 157° W). Taking advantage of thestability of the southeastern trade-winds, an air parcel was sampled in aLagrangian mode over a significantfraction of a solar day. Analyses of these data revealed excellent agreementbetween model simulated andobserved OH. In addition, the model simulations reproduced the major featuresin the observed diurnalprofiles of H2O2 and CH3OOH. In the case ofO3, the model captured the key observational feature whichinvolved an early morning maximum. An examination of the MBL HOxbudget indicated that the O(1D) + H2Oreaction is the major source of HOx while the major sinks involveboth physical and chemical processes involving the peroxide species,H2O2 and CH3OOH. Overall, the generally goodagreement between modeland observations suggests that our current understanding ofHOx–CH4 chemistry in the tropical MBL isquite good; however, there remains a need to critically examine this chemistrywhen both CH2O and HO2are added to the species measured.

Evidence for new sulfate particle formation in the remote troposphere involving biogenic trace gas species

Findings from earlier field studies (Weber et al., 1995; Weber et al., 1997) showed that rates of new particle formation in the remote troposphere can be significantly higher than those predicted by classical heteromolecular sulfuric acid water (H

Emissions from biomass burning in the Yucatan

04-H20) nucleation theory (Jaecker-Voirol and Mirabel, 1989). We have speculated that higher rates may be due to the participation of ammonia (NH3) through a ternary mechanism involving H#04-NH3Hz0 (Weber et al., 1996). Airborne measurements made in the vicinity of Macquarie Island during the first Aerosol Characterization Experiment (ACE 1) support the hypothesis that participation of additional species can result in particle formation rates that significantly exceed rates for H

Particle nucleation in the tropical boundary layer and its coupling to marine sulfur sources

S04-Hz0 nucleation (Weber et al., 1998a).