W. R. Leaitch

The relationship between cloud droplet number concentrations and anthropogenic pollution: Observations and climatic implications

Measurements of the concentrations of sulfate and nitrate in approximately 400 cloud water samples collected during four field studies carried out since 1982 are used with coincident measurements of cloud droplet number concentrations (CDNC) and liquid water content (LWC) to examine the relationship between CDNC and anthropogenic pollution, where sulfate concentration is used as the measure of the latter. The number of samples is compressed to 92 by averaging duplicates and multiple samples at similar altitudes during any particular flight, with 85 including CDNC measurements. Positive linear regressions between log (CDNC) and log (cloud water sulfate concentration) are determined for both stratiform and cumuliform cloud. Because of the number of factors affecting the CDNC, the coefficients of determination are only 0.30 and 0.49 for the respective cloud types. The LWC is relatively invariant with the cloud water sulfate concentration. The observed range of CDNC for the study region is 20–600 cm−3 (median of 59 observations is 210 cm−3) for stratiform clouds and 170–1100 cm−3 (median of 26 observations is 400 cm−3) for cumuliform clouds. The median CDNC for all sampled clouds is 250 cm−3. CDNC are also determined for “clean-air” conditions. The latter is defined as cases for which the concentrations of both cloud water sulfate and cloud water nitrate are comparable to aerosol sulfate concentrations and aerosol nitrate plus HNO3 concentrations, respectively, as reported for remote regions of the globe. For the clean-air clouds the observed range of CDNC for the study region is 20–250 cm−3 (median of 12 observations is 120 cm−3) for stratiform clouds and 170–370 cm−3 (median of four observations is 240 cm−3) for cumuliform clouds. The median CDNC for all clean-air clouds is 160 cm−3. The median CDNC for the complete population is 56% greater than the clean-air CDNC; the hypothesis that the clean-air CDNC is not different from the median CDNC is rejected at a confidence level of >99.5%. The present-day climatic forcing due to cloud albedo change arising from increased CDNC is estimated from a rudimentary model at between −2 W m−2 and −3 W m−2 for eastern North America.

Physical and chemical observations in marine stratus during the 1993 North Atlantic Regional Experiment: Factors controlling cloud droplet number concentrations

Airborne observations from 14 flights in marine stratus over the Gulf of Maine and Bay of Fundy in August and September of 1993 are examined for the relationships among the cloud droplet number concentrations (N d ), the out-of-cloud aerosol particle number concentrations (N a ), the major ion concentrations in the cloud water, and turbulence in cloud. There was a wide range of aerosol concentrations, but when low stratus and the main anthropogenic plume from eastern North America were in the same area the plume overrode the cloud. The N d increased with increasing N a and cloud water sulfate concentration (cwSO 4 = ), but the relationships were very weak. The separation of the data between smooth and lightly turbulent air substantially improved the ability to explain the variance in the N d by either of these two quantities. Also, the relative increase in N d for increases in N a and cwSO 4 = was greater for lightly turbulent air than for smooth air. The estimated minimum size of particles activated in these clouds ranged from 0.14 μm to 0.31 μm, corresponding to average supersaturations of <0.1%. The minimum size tended to be lower for lightly turbulent air and smaller N a . The results for lightly turbulent air agree well with previously reported parameterizations of the impact of aerosols on N d , but the results for smooth air do not agree. In general, more knowledge of the physical factors controlling the N d in stratiform clouds, such as turbulence, is needed to improve not only our ability to represent N d but also to increase our understanding of the impact of the aerosol particles on the N d and climate.

A comparison of large-scale atmospheric sulphate aerosol models (COSAM): overview and highlights

The comparison of large-scale sulphate aerosol models study (COSAM) compared the performance of atmospheric models with each other and observations. It involved: (i) design of a standard model experiment for the world wide web, (ii) 10 model simulations of the cycles of sulphur and 222Rn/210Pb conforming to the experimental design, (iii) assemblage of the best available observations of atmospheric SO=4, SO2 and MSA and (iv) a workshop in Halifax, Canada to analyze model performance and future model development needs. The analysis presented in this paper and two companion papers by Roelofs, and Lohmann and co-workers examines the variance between models and observations, discusses the sources of that variance and suggests ways to improve models. Variations between models in the export of SOx from Europe or North America are not sufficient to explain an order of magnitude variation in spatial distributions of SOx downwind in the northern hemisphere. On average, models predicted surface level seasonal mean SO=4 aerosol mixing ratios better (most within 20%) than SO2 mixing ratios (over-prediction by factors of 2 or more). Results suggest that vertical mixing from the planetary boundary layer into the free troposphere in source regions is a major source of uncertainty in predicting the global distribution of SO=4 aerosols in climate models today. For improvement, it is essential that globally coordinated research efforts continue to address emissions of all atmospheric species that affect the distribution and optical properties of ambient aerosols in models and that a global network of observations be established that will ultimately produce a world aerosol chemistry climatology.

Aerosol observations at Chebogue Point during the 1993 North Atlantic Regional Experiment: Relationships among cloud condensation nuclei, size distribution, and chemistry

Observations of aerosol chemistry and microphysics were made at Chebogue Point, Nova Scotia, from August 16 to September 8, 1993 as part of the North Atlantic Regional Experiment (NARE) intensive. Most of the aerosols were classified into two groups according to the geometric mean volume diameter (Dgv) of the particles which contributed the greatest volume (sub-0.5 μm). The group-1 aerosols, representing 33% of the data, are characterized by Dgv of 0.18–0.19 μm; the group-2 aerosols, representing 50% of the data, are characterized by Dgv of 0.20–0.22 μm; and the remaining aerosols bear similarities to either groups 1 or 2 but lie outside the Dgv ranges. The differences between these aerosol groups are consistent with the addition of sulfate to the group-2 aerosols via recent processing through cloud. Factors supporting this possibility include the presence of low marine stratus upwind of the site only on days when the group-2 aerosol was observed, the higher Dgv for the group-2 aerosols consistent with the observed size threshold for activation in these clouds, and the association of non-sea-salt SO4= (nssSO4=) with larger particle sizes for the group-2 aerosols. In general, the masses of the most abundant inorganic and organic ions, nssSO4= and oxalate, were associated with the main volume of the sub-0.5-μm particles. Cloud condensation nucleus (CCN) concentrations active at 0.4% supersaturation (CCN0.4) were highly correlated with the concentrations of particles >0.01 μm and oxalate and moderately correlated with nssSO4=. Concentrations of CCN active at 0.06% supersaturation (CCN0.06) correlate very well with the concentrations of particles >0.19 μm diameter. In the case of the recently cloud-processed aerosols, for which nssSO4= is more strongly associated with particles >0.19 μm, the CCN0.06 also correlate well with nssSO4=. CCN spectra computed using the measured size distributions and aerosol chemistry agree well with the measured CCN.

Field observations in continental stratiform clouds: Partitioning of cloud particles between droplets and unactivated interstitial aerosols

The partitioning of cloud particles between activated droplets and unactivated interstitial aerosols is a primary determinant of cloud microphysical, radiative, and chemical properties. In the present study, high-resolution aircraft measurements (1 s, ∼60 m) of the number concentrations (Namp and Ncd) of accumulation-mode particles (AMP, 0.17 to 2.07 μm diameter) and cloud droplets (CD, 2 to 35 μm diameter), made during 10 flights in and around continental stratiform clouds near Syracuse, New York, in autumn 1984 have been used to study the local and instantaneous nature of cloud particle partitioning throughout the sampled clouds. The partitioning is defined as the activated fraction F (≡Ncd/Ntot) of all measured cloud particles (Ntot ≡ Namp + Ncd). F may be interpreted approximately as the AMP activation efficiency which is often assumed to be unity in all clouds. In the present study, F varied over its full possible range (0 to 1), being low especially in cloud edges. Even in the near-adiabatic parts of cloud interior, its variation ranged from 0.1 to 1 over the 10 days. Statistically, its value in cloud interior exceeded 0.9 in 36% of the data but was below 0.6 in 28%. On 5 of the 10 days, stratocumulus clouds were embedded in cool, dry, and relatively clean (Ntot < 600 cm−3) northerly air masses. In such cases, cloud droplet concentration increased approximately linearly with increasing total particle loading, and F in cloud interior was near unity and relatively insensitive to changes in the influencing variables. On the other days, especially in stratus clouds embedded in warm and polluted southerly air masses, F was significantly less than unity, with particles in the smallest size ranges (0.17 to 0.37 μm) activating only fractionally depending on several factors. An important feature of the clouds sampled in this study was the existence of multiple cloud layers and complex vertical thermal structure on most days. Consequently, our analysis of the dependence of F on influencing cloud variables has been based on data grouped into individual cloud layers. Besides the size of the precursor aerosol, we found total particle loading (Ntot) and the local vertical cooling rate (∼ temperature lapse rate in individual layers) to influence F the most. In particular, F decreased with increasing particle loading in excess of about 800 cm−3, and increased nearly linearly with temperature lapse rate. Evidently, the activation process can become self-limiting in stratiform clouds under polluted conditions, in which case increasing anthropogenic aerosol loading of the atmosphere translates less and less into cloud droplet population. This observation has important implications with respect to cloud radiative forcing, precipitation formation and acidification, and for long range transport of the unactivated aerosols.

Influence of transport and ocean ice extent on biogenic aerosol sulfur in the Arctic atmosphere

The recent decline in sea ice cover in the Arctic Ocean could affect the regional radiative forcing via changes in sea ice-atmosphere exchange of dimethyl sulfide (DMS) and biogenic aerosols formed from its atmospheric oxidation, such as methanesulfonic acid (MSA). This study examines relationships between changes in total sea ice extent north of 70 degrees N and atmospheric MSA measurement at Alert, Nunavut, during 1980-2009; at Barrow, Alaska, during 1997-2008; and at Ny-Alesund, Svalbard, for 1991-2004. During the 1980-1989 and 1990-1997 periods, summer (July-August) and June MSA concentrations at Alert decreased. In general, MSA concentrations increased at all locations since 2000 with respect to 1990 values, specifically during June and summer at Alert and in summer at Barrow and Ny-Alesund. Our results show variability in MSA at all sites is related to changes in the source strengths of DMS, possibly linked to changes in sea ice extent as well as to changes in atmospheric transport patterns. Since 2000, a late spring increase in atmospheric MSA at the three sites coincides with the northward migration of the marginal ice edge zone where high DMS emissions from ocean to atmosphere have previously been reported. Significant negative correlations are found between sea ice extent and MSA concentrations at the three sites during the spring and June. These results suggest that a decrease in seasonal ice cover influencing other mechanisms of DMS production could lead to higher atmospheric MSA concentrations.

Airborne observations related to ozone depletion at polar sunrise

Airborne observations were conducted in the high Arctic (69°N–83°N) during April 6–16, 1992, in support of the Polar Sunrise Experiment. Measurements of temperature, O3, NOx aerosol particles, inorganic aerosol species, inorganic bromide, alkyl nitrate species, and several organohalogens (including bromoform) were made from an altitude of 30 m above ground level (agl) up to 7000 m msl (mean sea level). The average temperature profile shows a strong surface-based inversion up to about 500 m, an isothermal region up to about 1.5 km, and steadily decreasing from 1.5 to 6.8 km. Ozone mixing ratios were frequently found to be depleted from the surface up to various altitudes within the boundary layer (maximum altitude for ozone <10 parts per billion by volume (ppbv) was 375 m). The average ozone profile increases from <10 ppbv near the surface up to about 40 ppbv at 1 km, remaining approximately constant up to 5 km, and increasing with altitude thereafter as the stratospheric source becomes evident. NOx, including a possible peroxyacetyl nitrate (PAN) interference, was typically <50±20 parts per trillion by volume (pptv), and frequently below detection limit (20 pptv). Accumulation-mode aerosol particle number concentrations in the boundary layer were 100–200 cm−3, and although CN increased low over a polynya, there were indications of an absence of nucleation-mode particles in ozone depleted air in the boundary layer compared with the free troposphere. Inorganic gaseous bromide, bromoform (CHBr3) and dibromochloromethane (CHClBr2) all exhibited strong anticorrelations with O3. Gaseous nitrate (HNO3 plus possibly some contribution from PAN interference) ranged up to 110 pptv but was <40 pptv in 11 of 14 samples. With the exception of 1-propyl nitrate the C3–C6 alkyl nitrates correlated positively with ozone, as did the isomer ratio C3/C6. Organohalogens were measured using charcoal cartridges {C} and Tenax cartridges {T}. CHBr3 was similar by both techniques (medians of 1.83{C} and 1.60{T} pptv), and negative correlations with O3 were indicated by both sets of samples (R2 = 0.75{C} and 0.71{T}). CHClBr2 was also very close in both sets of samples (median of 0.25{C} and 0.22{T} pptv), however, a negative correlation with O3 was present only in the Tenax samples (R2 = 0.63). Ln(CHClBr2/CHBr3) correlated negatively with In (CHBr3) with a coefficient of determination of 0.75, and with higher In(CHBr3) approached the value indicated by Li et al. (this issue) for air immediately above seawater at 0°C (i.e., 0.032). CHClBr2/CHBr3 was higher in the free troposphere than in the boundary layer and possibly less variant with In(CHBr3), indicating either different source regions for these free troposphere organohalogens and/or, as Li et al. suggest, faster chemical destruction of CHBr3 relative to CHClBr2 in the free troposphere. The airborne organohalogen data and that from ice camp SWAN (Hopper et al., this issue) and Alert (Yokouchi et al., this issue) were combined to produce a vertical profile of CHBr3. CHBr3 exhibited a well-defined logarithmic decrease with increasing altitude, indicating a strong surface source, opposite to the average O3 profile. In general, the low-level airborne observations related to O3 depletion are very consistent with the observations at Alert, indicating that the many features of this phenomenon are ubiquitous.

Primary marine aerosol-cloud interactions off the coast of California

Primary marine aerosol (PMA)-cloud interactions off the coast of California were investigated using observations of marine aerosol, cloud condensation nuclei (CCN), and stratocumulus clouds during the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) and the Stratocumulus Observations of Los-Angeles Emissions Derived Aerosol-Droplets (SOLEDAD) studies. Based on recently reported measurements of PMA size distributions, a constrained lognormal-mode-fitting procedure was devised to isolate PMA number size distributions from total aerosol size distributions and applied to E-PEACE measurements. During the 12 day E-PEACE cruise on the R/V Point Sur, PMA typically contributed less than 15% of total particle concentrations. PMA number concentrations averaged 12 cm^(−3) during a relatively calmer period (average wind speed 12 m/s^1) lasting 8 days, and 71 cm^(−3) during a period of higher wind speeds (average 16 m/s^1) lasting 5 days. On average, PMA contributed less than 10% of total CCN at supersaturations up to 0.9% during the calmer period; however, during the higher wind speed period, PMA comprised 5–63% of CCN (average 16–28%) at supersaturations less than 0.3%. Sea salt was measured directly in the dried residuals of cloud droplets during the SOLEDAD study. The mass fractions of sea salt in the residuals averaged 12 to 24% during three cloud events. Comparing the marine stratocumulus clouds sampled in the two campaigns, measured peak supersaturations were 0.2 ± 0.04% during E-PEACE and 0.05–0.1% during SOLEDAD. The available measurements show that cloud droplet number concentrations increased with >100 nm particles in E-PEACE but decreased in the three SOLEDAD cloud events.

Summer aerosol profiles over Algonquin Park, Canada

Abstract During the summer of 1982, daytime vertical profiles of aerosol particle concentrations and state parameters in the lower troposphere were measured from a Twin Otter aircraft flying over Algonquin Park, Canada. Particles from 0.2 to 30 μm diameter were counted and sized with PMS ASASP and FSSP wingmounted probes. When 48-h back trajectories at 850 mb were from the south, the mean aerosol particle concentrations were approximately 2000cm−3 at 950mb (near ground level), and decreased with height rapidly until 750 mb. When trajectories were from the west and north, the 950 mb concentrations were much lower but relatively uniform to 800 mb. Individual profiles showed that maxima could occur near cloud base. Average number, area and volume spectra of the subμm aerosol have been determined for 700 and 900 mb and show a maximum near 0.2–0.3 μm with volume-weighted geometric mean diameters near 0.3 μm. Spectra of particles larger than 2 μm indicate a second maximum at about 10 μm diameter. Ground-based chemistry measurements show that most of the subμm aerosol particles of southern origin were formed principally from water soluble sulphate compounds. Based on the measurements made during southerly winds, radiation calculations predict visibilities of appproximately 10 km and suggest that particle absorption could result in a 2–4°C temperature increase in the atmospheric layer below 2 km.

Contribution of Arctic seabird-colony ammonia to atmospheric particles and cloud-albedo radiative effect

The Arctic region is vulnerable to climate change and able to affect global climate. The summertime Arctic atmosphere is pristine and strongly influenced by natural regional emissions, which have poorly understood climate impacts related to atmospheric particles and clouds. Here we show that ammonia from seabird-colony guano is a key factor contributing to bursts of newly formed particles, which are observed every summer in the near-surface atmosphere at Alert, Nunavut, Canada. Our chemical-transport model simulations indicate that the pan-Arctic seabird-influenced particles can grow by sulfuric acid and organic vapour condensation to diameters sufficiently large to promote pan-Arctic cloud-droplet formation in the clean Arctic summertime. We calculate that the resultant cooling tendencies could be large (about −0.5 W m−2 pan-Arctic-mean cooling), exceeding −1 W m−2 near the largest seabird colonies due to the effects of seabird-influenced particles on cloud albedo. These coupled ecological–chemical processes may be susceptible to Arctic warming and industrialization.