Atmospheric Composition Observations

The composition of the atmosphere is a critical factor in understanding the nature and magnitude of processes associated with the planet’s energy balance, clouds and precipitation, biogeochemical cycling of nutrients, and public health and welfare. A detailed understanding of trace gases, aerosol particles, and hydrometeors is challenging due to the combination of their physicochemical complexity, variable lifetimes, and spatial inhomogeneity. Recent advances in instrumentation have resulted in improved measurements and an increased understanding of atmospheric composition. Laboratory and field in-situ measurement studies have benefited from such improvements, including improved spatial and temporal resolution, the ability to sample in challenging conditions (e.g., on airborne platforms, in clouds, at widely ranging pressure and temperature conditions), and the ability to measure a wider range of chemical species, and, in the case of aerosol particles, to detect smaller sizes. Remote sensing capabilities have increased in recent years, thus offering new views of atmospheric composition across broad spatiotemporal ranges. Manuscripts related to all aspects of atmospheric observations are included in this Special Issue, including advances in observational techniques and scientific insights into atmospheric composition

Observations of Sharp Oxalate Reductions in Stratocumulus Clouds at Variable Altitudes: Organic Acid and Metal Measurements During the 2011 E-PEACE Campaign

This work examines organic acid and metal concentrations in northeastern Pacific Ocean stratocumulus cloudwater samples collected by the CIRPAS Twin Otter between July and August 2011. Correlations between a suite of various monocarboxylic and dicarboxylic acid concentrations are consistent with documented aqueous-phase mechanistic relationships leading up to oxalate production. Monocarboxylic and dicarboxylic acids exhibited contrasting spatial profiles reflecting their different sources; the former were higher in concentration near the continent due to fresh organic emissions. Concentrations of sea salt crustal tracer species, oxalate, and malonate were positively correlated with low-level wind speed suggesting that an important route for oxalate and malonate entry in cloudwater is via some combination of association with coarse particles and gaseous precursors emitted from the ocean surface. Three case flights show that oxalate (and no other organic acid) concentrations drop by nearly an order of magnitude relative to samples in the same vicinity. A consistent feature in these cases was an inverse relationship between oxalate and several metals (Fe, Mn, K, Na, Mg, Ca), especially Fe. By means of box model studies we show that the loss of oxalate due to the photolysis of iron oxalato complexes is likely a significant oxalate sink in the study region due to the ubiquity of oxalate precursors, clouds, and metal emissions from ships, the ocean, and continental sources

CCN Properties of Organic Aerosol Collected Below and within Marine Stratocumulus Clouds near Monterey, California

The composition of aerosol from cloud droplets differs from that below cloud. Its implications for the Cloud Condensation Nuclei (CCN) activity are the focus of this study. Water-soluble organic matter from below cloud, and cloud droplet residuals off the coast of Monterey, California were collected; offline chemical composition, CCN activity and surface tension measurements coupled with Köhler Theory Analysis are used to infer the molar volume and surfactant characteristics of organics in both samples. Based on the surface tension depression of the samples, it is unlikely that the aerosol contains strong surfactants. The activation kinetics for all samples examined are consistent with rapid (NH4)2SO4 calibration aerosol. This is consistent with our current understanding of droplet kinetics for ambient CCN. However, the carbonaceous material in cloud drop residuals is far more hygroscopic than in sub-cloud aerosol, suggestive of the impact of cloud chemistry on the hygroscopic properties of organic matter

On the presence of giant particles downwind of ships in the marine boundary layer

This study examines large oceangoing ships as a source of giant cloud condensation nuclei (D_p > 2 µm) due to wake and stack emissions off the California coast. Observed particle number concentrations behind 10 ships exceeded those in “control” areas, exhibiting number concentration enhancement ratios (ERs) for minimum threshold diameters of ~2, ~10, and ~20 µm as high as 2.7, 5.5, and 7.5, respectively. ER decreases with increasing downwind distance and altitude. ER becomes better correlated with ship size variables (gross tonnage, length, and beam) as the minimum size threshold increases from 2 to 20 µm, whereas ship speed has a less distinct relationship with ER. One case study of a container ship shows that there are higher concentrations of sea-salt tracer species behind it relative to adjacent control areas. These results have implications for cloud properties and precipitation in marine boundary layers exposed to ship traffic

Marine stratocumulus aerosol-cloud relationships in the MASE-II experiment: Precipitation susceptibility in eastern Pacific marine stratocumulus

Observational data on aerosol-cloud-drizzle relationships in marine stratocumulus are presented from the second Marine Stratus/Stratocumulus Experiment (MASE-II) carried out in July 2007 over the eastern Pacific near Monterey, California. Observations, carried out in regions of essentially uniform meteorology with localized aerosol enhancements due to ship exhaust (“ship tracks”), demonstrate, in accord with those from numerous other field campaigns, that increased cloud drop number concentration Nc and decreased cloud top effective radius r_e are associated with increased subcloud aerosol concentration. Modulation of drizzle by variations in aerosol levels is levels is clearly evident. Variations of cloud base drizzle rate R_(cb) are found to be consistent with the proportionality, R_(cb) / H^3/N_c, where H is cloud depth. Simultaneous aircraft and A-Train satellite observations are used to quantify the precipitation susceptibility of clouds to aerosol perturbations in the eastern Pacific region

Water-soluble organic aerosol in the Los Angeles Basin and outflow regions: Airborne and ground measurements during the 2010 CalNex field campaign

A particle-into-liquid sampler coupled to a total organic carbon analyzer (PILS-TOC) quantified particulate water-soluble organic carbon (WSOC) mass concentrations during the May 2010 deployment of the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter in the CalNex field study. WSOC data collected during 16 flights provide the first spatiotemporal maps of WSOC in the San Joaquin Valley, Los Angeles Basin, and outflow regions of the Basin. WSOC was consistently higher in concentration within the Los Angeles Basin, where sea breeze transport and Basin topography strongly influence the spatial distribution of WSOC. The highest WSOC levels were associated with fire plumes, highlighting the importance of both primary and secondary sources for WSOC in the region. Residual pollution layers enriched with WSOC are observed aloft up to an altitude of 3.2 km and the highest WSOC levels for each flight were typically observed above 500 m. Simultaneous ground WSOC measurements during aircraft overpasses in Pasadena and Riverside typically exhibit lower levels, especially when relative humidity (RH) was higher aloft suggestive of the influence of aerosol-phase water. This points to the underestimation of the radiative effects of WSOC when using only surface measurements. Reduced aerosol-phase water in the eastern desert outflow region likely promotes the re-partitioning of WSOC to the gas phase and suppression of processes to produce these species (partitioning, multiphase chemistry, photolytic production); as a result, WSOC is reduced relative to sulfate (but not as much as nitrate) as aerosol is advected from the Basin to the outflows

Cloud condensation nuclei activity, closure, and droplet growth kinetics of Houston aerosol during the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS)

In situ cloud condensation nuclei (CCN) measurements were obtained in the boundary layer over Houston, Texas, during the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) campaign onboard the CIRPAS Twin Otter. Polluted air masses in and out of cloudy regions were sampled for a total of 22 flights, with CCN measurements obtained for 17 of these flights. In this paper, we focus on CCN closure during two flights, within and downwind of the Houston regional plume and over the Houston Ship Channel. During both flights, air was sampled with particle concentrations exceeding 25,000 cm^(−3) and CCN concentrations exceeding 10,000 cm^(−3). CCN closure is evaluated by comparing measured concentrations with those predicted on the basis of measured aerosol size distributions and aerosol mass spectrometer particle composition. Different assumptions concerning the internally mixed chemical composition result in average CCN overprediction ranging from 3% to 36% (based on a linear fit). It is hypothesized that the externally mixed fraction of the aerosol contributes much of the CCN closure scatter, while the internally mixed fraction largely controls the overprediction bias. On the basis of the droplet sizes of activated CCN, organics do not seem to impact, on average, the CCN activation kinetics

Marine Boundary Layer Clouds Associated with Coastally Trapped Disturbances: Observations and Model Simulations

This work has been accepted to Journal of Atmospheric Sciences. The AMS does not guarantee that the copy provided here is an accurate copy of the final published work.Modeling marine low clouds and fog in coastal environments remains an outstanding challenge due to the inherently complex ocean–land–atmosphere system. This is especially important in the context of global circulation models due to the profound radiative impact of these clouds. This study utilizes aircraft and satellite measurements, in addition to numerical simulations using the Weather Research and Forecasting (WRF) Model, to examine three well-observed coastally trapped disturbance (CTD) events from June 2006, July 2011, and July 2015. Cloud water-soluble ionic and elemental composition analyses conducted for two of the CTD cases indicate that anthropogenic aerosol sources may impact CTD cloud decks due to synoptic-scale patterns associated with CTD initiation. In general, the dynamics and thermodynamics of the CTD systems are well represented and are relatively insensitive to the choice of physics parameterizations; however, a set of WRF simulations suggests that the treatment of model physics strongly influences CTD cloud field evolution. Specifically, cloud liquid water path (LWP) is highly sensitive to the choice of the planetary boundary layer (PBL) scheme; in many instances, the PBL scheme affects cloud extent and LWP values as much as or more than the microphysics scheme. Results suggest that differences in the treatment of entrainment and vertical mixing in the Yonsei University (nonlocal) and Mellor–Yamada–Janjić (local) PBL schemes may play a significant role. The impact of using different driving models—namely, the North American Mesoscale Forecast System (NAM) 12-km analysis and the NCEP North American Regional Reanalysis (NARR) 32-km products—is also investigated