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An empirical model of global climate – Part 2: Implications for future temperature

IPCC (2007) has shown that atmosphere-ocean general circulation models (GCMs) from various research centers simulate the rise in global mean surface temperature over the past century rather well, yet provide divergent estimates of temperature for the upcoming decades. We use an empirical model of global climate based on a multiple linear regression (MLR) analysis of the past global surface temperature anomalies (ΔT) to explore why GCMs might provide divergent estimates of future temperature. Our focus is on the interplay of three factors: net anthropogenic aerosol radiative forcing (NAA RF), climate feedback (water vapor, clouds, surface albedo) in response to greenhouse gas radiative forcing (GHG RF), and ocean heat export (OHE). Our model is predicated on two key assumptions: whatever climate feedback is needed to account for past temperature rise will persist into the future and whatever fraction of anthropogenic RF (GHG RF + NAA RF) is exported to the oceans to match the observed rise in ocean heat content will also persist. Even with these assumptions, modeled future ΔT mimics the behavior of GCMs because the ~110 record of global surface temperature can not distinguish between two possibilities. If anthropogenic aerosols presently exert small cooling on global climate, feedback must be weak and the future rise in global average surface temperature in 2053, the time CO2 is projected to double according to RCP 8.5, could be moderate. If aerosols presently exert large cooling of global climate, feedback must be large and future ΔT when CO2 doubles could be substantial. Reduced uncertainty for climate projection requires observationally based constraints that can narrow the uncertainties that presently exist for net anthropogenic aerosol radiative forcing as well as the totality of feedbacks that occur in response to a GHG RF perturbation. GCMs are often compared by evaluating the equilibrium response to a doubling of CO2, termed climate sensitivity. In our model framework, ΔT at the time CO2 doubles is nearly independent of OHE, because climate feedback must be adjusted to properly simulate observed temperature. Our simulations show that if a small fraction of anthropogenic RF is exported to the ocean, equilibrium climate sensitivity closely represents the modeled ΔT at the time CO2 doubles. Conversely, if this fraction is large, ΔT when CO2 doubles is much less than the equilibrium climate sensitivity (i.e. the model is now far from equilibrium). Similar behavior likely occurs within GCMs. We therefore suggest the dependence of climate sensitivity on OHE be factored into analyses that use this metric to compare and evaluate GCMs.
Measurements of ambient HONO concentrations and vertical HONO flux above a northern Michigan forest canopy

Systems have been developed and deployed at a North Michigan forested site to measure ambient HONO and vertical HONO flux. The modified HONO measurement technique is based on aqueous scrubbing of HONO using a coil sampler, followed by azo dye derivatization and detection using a long-path absorption photometer (LPAP). A Na2CO3-coated denuder is used to generate "zero HONO" air for background correction. The lower detection limit of the method, defined by 3 times of the standard deviation of the signal, is 1 pptv for 1-min averages, with an overall uncertainty of ±(1 + 0.05 [HONO]) pptv. The HONO flux measurement technique has been developed based on the relaxed eddy accumulation approach, deploying a 3-D sonic anemometer and two HONO measurement systems. The overall uncertainty is estimated to be within ±(8 × 10−8 + 0.15 FHONO) mol m−2 h−1, with a 20-min averaged data point per 30 min. Ambient HONO and vertical HONO flux were measured simultaneously at the PROPHET site from 17 July to 7 August 2008. The forest canopy was found to be a net HONO source, with a mean upward flux of 0.37 × 10−6 moles m−2 h−1. The HONO flux reached a maximal mean of ~0.7 × 10−6 moles m−2 h−1 around solar noon, contributing a major fraction to the HONO source strength required to sustain the observed ambient concentration of ~70 pptv. There were no significant correlations between [NOx] and daytime HONO flux and between JNO2 × [NO2] and HONO flux, suggesting that NOx was not an important precursor responsible for HONO daytime production on the forest canopy surface in this low-NOx rural environment. Evidence supports the hypothesis that photolysis of HNO3 deposited on the forest canopy surface is a major daytime HONO source.
GEM-AQ/EC, an on-line global multi-scale chemical weather modelling system: model development and evaluation of global aerosol climatology

A global air quality modeling system GEM-AQ/EC was developed by implementing tropospheric chemistry and aerosol processes on-line into the Global Environmental Multiscale weather prediction model – GEM. Due to the multi-scale features of the GEM, the integrated model, GEM-AQ/EC, is able to investigate chemical weather at scales from global to urban domains. The current chemical mechanism is comprised of 50 gas-phase species, 116 chemical and 19 photolysis reactions, and is complemented by a sectional aerosol module CAM (The Canadian Aerosol Module) with 5 aerosols types: sulphate, black carbon, organic carbon, sea-salt and soil dust. Monthly emission inventories of black carbon and organic carbon from boreal and temperate vegetation fires were assembled using the most reliable areas burned datasets by countries, from statistical databases and derived from remote sensing products of 1995–2004. The model was run for ten years from from 1995–2004 with re-analyzed meteorology on a global uniform 1° × 1° horizontal resolution domain and 28 hybrid levels extending up to 10 hPa. The simulating results were compared with various observations including surface network around the globe and satellite data. Regional features of global aerosols are reasonably captured including emission, surface concentrations and aerosol optical depth. For various types of aerosols, satisfactory correlations were achieved between modeled and observed with some degree of systematic bias possibly due to large uncertainties in the emissions used in this study. A global distribution of natural aerosol contributions to the total aerosols is obtained and compared with observations.
Comparison of methods for evaluation of wood smoke and estimation of UK ambient concentrations

Airborne concentrations of the wood smoke tracers, levoglucosan and fine potassium have been measured at urban and rural sites in the United Kingdom alongside measurements with a multi-wavelength aethalometer. The UK sites, and especially those in cities, show low ratios of levoglucosan to potassium in comparison to the majority of published data. It is concluded that there may be two distinct source types, one from wood stoves and fireplaces with a high organic carbon content, best represented by levoglucosan, the other from larger, modern appliances with a very high burn-out efficiency, best represented by potassium. Based upon levoglucosan concentrations and a conversion factor of 11.2 from levoglucosan to wood smoke mass, average concentrations of wood smoke including winter and summer sampling periods are 0.23 μg m−3 in Birmingham and 0.33 μg m−3 in London, well below concentrations typical of other northern European urban areas. There may be a further contribution from sources of potassium-rich emissions amounting to an estimated 0.08 μg m−3 in Birmingham and 0.30 μg m−3 in London. Concentrations were highly correlated between two London sites separated by 4 km suggesting that a regional source is responsible. Data from the aethalometer are either supportive of these conclusions or suggest higher concentrations, depending upon the way in which the data are analysed.
Trends of particulate matter (PM2.5) and chemical composition at a regional background site in the Western Mediterranean over the last nine years (2002–2010)

The time variability and long term trends of PM2.5 (particulate matter of diameter < 2.5 μm) at various regional background (RB) sites across Europe are studied and interpreted in this work. Data on mean annual levels of PM2.5 measured at Montseny (MSY, North East Spain) and various RB sites in Spain and Europe are evaluated and compared, and subsequently analysed for statistically significant trends. The MSY site registered higher average PM2.5 levels than those measured at a selection of other RB sites across Spain, Portugal, Germany and Scandinavia by percentage compared to the mean of all the stations in these countries, but lower than those measured in Switzerland, Italy and Austria. Reductions in PM2.5 were observed across all stations in Spain and Europe to varying degrees (7–49%). MSY underwent a statistically significant reduction since measurements began, indicating a year-on-year gradual decrease (−3.7 μg m−3, calculated from the final year of data compared to the mean). Similar trends were observed in other RB sites across Spain (−1.9 μg m−3). Reductions recorded in PM2.5 across Europe were varied, with many experiencing gradual, year-on-year decreases (−1.8 μg m−3). These reductions have been attributed to various causes: the introduction and implementation of pollution abatement strategies in EU member states, the effect of the current economic crisis on emissions of PM2.5 and the influence of meteorology observed during the winters of 2009 and 2010. In addition, the North Atlantic Oscillation (NAO), a large scale meteorological phenomenon most prevalent during winter, was observed to influence the frequency of Saharan dust intrusions across the Iberian Peninsula. Chemical composition of PM2.5 at MSY is characterised by high levels of organic matter (OM) and sulphate, followed by crustal material, nitrate and ammonia. Sea Spray and elemental carbon (EC) comprised a minor part of the total PM2.5 mass. Statistical trend analysis was performed on the various chemical components of PM2.5 recorded at MSY to determine which components were accountable for the decrease in PM2.5 concentration. It is shown that OM underwent the largest decrease over the time period with a statistically significant trend (−1.3 μg m−3 compared to the mean), followed by sulphate (−0.8 μg m−3), ammonium (−0.5 μg m−3) and nitrate (−0.4 μg m−3). Conversely, sea spray, EC and crustal material reductions were found to be negligible.
Diurnal variations of organic molecular tracers and stable carbon isotopic composition in atmospheric aerosols over Mt. Tai in the North China Plain: an influence of biomass burning

Organic tracer compounds, as well as organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), and stable carbon isotope ratios (δ13C) of total carbon (TC) have been investigated in aerosol samples collected during early and late periods of the Mount Tai eXperiment 2006 (MTX2006) field campaign in the North China Plain. Total solvent-extractable fractions were investigated by gas chromatography/mass spectrometry. More than 130 organic compounds were detected in the aerosol samples. They were grouped into twelve organic compound classes, including biomass burning tracers, biogenic primary sugars, biogenic secondary organic aerosol (SOA) tracers, and anthropogenic tracers such as phthalates, hopanes and polycyclic aromatic hydrocarbons (PAHs). In early June when the field burning activities of wheat straws in the North China Plain were very active, the total identified organics (2090 ± 1170 ng m−3) were double those in late June (926 ± 574 ng m−3). All the compound classes were more abundant in early June than in late June, except phthalate esters, which were higher in late June. Levoglucosan (88–1210 ng m−3, mean 403 ng m−3) was found as the most abundant single compound in early June, while diisobutyl phthalate was the predominant species in late June. During the biomass-burning period in early June, the diurnal trends of most of the primary and secondary organic aerosol tracers were characterized by the concentration peaks observed at mid-night or in early morning, while in late June most of the organic species peaked in late afternoon. This suggests that smoke plumes from biomass burning can uplift the aerosol particulate matter to a certain altitude, which could be further transported to and encountered the summit of Mt. Tai during nighttime. On the basis of the tracer-based method for the estimation of biomass-burning OC, fungal-spore OC and biogenic secondary organic carbon (SOC), we estimate that an average of 24% (up to 64%) of the OC in the Mt. Tai aerosols was due to biomass burning in early June, followed by the contribution of isoprene SOC (mean 4.3%). In contrast, isoprene SOC was the main contributor (6.6%) to OC, and only 3.0% of the OC was due to biomass burning in late June. In early June, δ13C of TC (−26.6 to −23.2&permil;, mean −25.0&permil;) were lower than those (−23.9 to −21.9&permil;, mean −22.9&permil;) in late June. In addition, a strong anti-correlation was found between levoglucosan and δ13C values. This study demonstrates that crop-residue burning activities can significantly enhance the organic aerosol loading and alter the organic composition and stable carbon isotopic composition of aerosol particles in the troposphere over the North China Plain.
Tropospheric ozone trend over Beijing from 2002–2010: ozonesonde measurements and modeling analysis

Using a combination of ozonesonde data and numerical simulations of the Chemical Lagrangian Model of the Stratosphere (CLaMS), the trend of tropospheric ozone (O3) during 2002–2010 over Beijing was investigated. Tropospheric ozone over Beijing shows a winter minimum and a broad summer maximum with a clear positive trend in the maximum summer ozone concentration over the last decade. The observed significant trend of tropospheric column ozone is mainly caused by photochemical production (3.1% yr−1 for a mean level of 52 DU). This trend is close to the significant trend of partial column ozone in the lower troposphere (0–3 km) resulting from the enhanced photochemical production during summer (3.0% yr−1 for a mean level of 23 DU). Analysis of the CLaMS simulation shows that transport rather than chemistry drives most of the seasonality of tropospheric ozone. However, dynamical processes alone cannot explain the trend of tropospheric ozone in the observational data. Clearly enhanced ozone values and a negative vertical ozone gradient in the lower troposphere in the observational data emphasize the importance of photochemistry within the troposphere during spring and summer, and suggest that the photochemistry within the troposphere significantly contributes to the tropospheric ozone trend over Beijing during the last decade.
Are sesquiterpenes a good source of secondary organic cloud condensation nuclei (CCN)? Revisiting β-caryophyllene CCN

Secondary organic aerosol (SOA) was formed in an environmental reaction chamber from the ozonolysis of β-caryophyllene (β-C) at low concentrations (5 ppb or 20 ppb). Experimental parameters were varied to characterize the effects of hydroxyl radicals, light and the presence of lower molecular weight terpene precursor (isoprene) for β-C SOA formation and cloud condensation nuclei (CCN) characteristics. Changes in β-C SOA chemicophysical properties (e.g., density, volatility, oxidation state) were explored with online techniques to improve our predictive understanding of β-C CCN activity. In the absence of OH scavenger, light intensity had negligible impacts on SOA oxidation state and CCN activity. In contrast, when OH reaction was effectively suppressed (> 11 ppm scavenger), SOA showed a much lower CCN activity and slightly less oxygenated state consistent with previously reported values. Though there is significant oxidized material present (O / C > 0.25), no linear correlation existed between the mass ratio ion fragment 44 in the bulk organic mass (f44) and O / C for the β-C-O3 system. No direct correlations were observed with other aerosol bulk ion fragment fraction (fx) and κ as well. A mixture of β-C and lower molecular weight terpenes (isoprene) consumed more ozone and formed SOA with distinct characteristics dependent on isoprene amounts. The addition of isoprene also improved the CCN predictive capabilities with bulk aerosol chemical information. The β-C SOA CCN activity reported here is much higher than previous studies (κ < 0.1) that use higher precursor concentration in smaller environmental chambers; similar results were only achieved with significant use of OH scavenger. Results show that aerosol formed from a mixture of low and high molecular weight terpene ozonolysis can be hygroscopic and can contribute to the global biogenic SOA CCN budget.
Chemical characterization of springtime submicrometer aerosol in Po Valley, Italy

The chemistry of submicron particles was investigated at San Pietro Capofiume (SPC) measurement station in the Po Valley, Italy, in spring 2008. The measurements were performed by using both off-line and on-line instruments. Organic carbon (OC) and elemental carbon, organic acids and biomass burning tracers were measured off-line by using a 24-h PM1 filter sampling. More detailed particle chemistry was achieved by using a Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and analyzing the data by positive matrix factorization (PMF). Oxalic acid had the highest concentrations of organic acids (campaign-average 97.4 ng m−3) followed by methane sulfonic, formic, malonic, and malic acids. Samples were also analyzed for glyoxylic, succinic, azelaic and maleic acids. In total, the nine acids composed 1.9 and 3.8% of OC and water-soluble OC, respectively (average), in terms of carbon atoms. Levoglucosan concentration varied from 17.7 to 495 ng m−3 with the concentration decreasing in the course of the campaign most likely due to the reduced use of domestic heating with wood. Six factors were found for organic aerosol (OA) at SPC by PMF: hydrocarbon-like OA (HOA), biomass burning OA (BBOA), nitrogen-containing OA (N-OA) and three different oxygenated OAs (OOA-a, OOA-b and OOA-c). Most of the OA mass was composed of OOA-a, HOA and OOA-c (26, 24 and 22%, respectively) followed by OOA-b (13%), BBOA (8%) and N-OA (7%). As expected, OOAs were the most oxygenated factors with organic matter:organic carbon (OM : OC) ratios ranging from 1.9 to 2.2. The diurnal variability of the aerosol chemical composition was greatly affected by the boundary layer meteorology. Specifically, the effect of the nocturnal layer break-up in morning hours was most evident for nitrate and N-OA indicating that these compounds originated mainly from the local sources in the Po Valley. For sulfate and OOA-a the concentration did not change during the break-up suggesting their origin to be mostly regional. That resulted in much more oxidized OA in the daytime mixing layer than in the nocturnal surface layer. In this study, the high mass resolution and source-related aerosol chemistry from the HR-ToF-AMS was combined with the filter measurements in a total new extent elucidating novel features and sources of organic aerosol in the Po Valley region.
Ship impacts on the marine atmosphere: insights into the contribution of shipping emissions to the properties of marine aerosol and clouds

We report properties of marine aerosol and clouds measured in the shipping lanes between Monterey Bay and San Francisco off the coast of Central California. Using a suite of aerosol instrumentation onboard the CIRPAS Twin Otter aircraft, these measurements represent a unique set of data contrasting the properties of clean and ship-impacted marine air masses in dry aerosol and cloud droplet residuals. Below-cloud aerosol exhibited average mass and number concentrations of 2 μg m−3 and 510 cm−3, respectively, which are consistent with previous studies performed off the coast of California. Enhancements in vanadium and cloud droplet number concentrations are observed concurrently with a decrease in cloud water pH, suggesting that periods of high aerosol loading are primarily linked to increased ship influence. Mass spectra from a compact time-of-flight Aerodyne aerosol mass spectrometer reveal an enhancement in the fraction of organic at m/z 42 (f42) and 99 (f99) in ship-impacted clouds. These ions are well correlated to each other (R2>0.64) both in and out of cloud and constitute 14% (f44) and 3% (f99) of organic mass during periods of enhanced sulfate. High-resolution mass spectral analysis of these masses from ship measurements suggests that the ions responsible for this variation were oxidized, possibly due to cloud processing. We propose that the organic fractions of these ions be used as a metric for determining the extent to which cloud-processed ship emissions impact the marine atmosphere where (f42 > 0.15; f99 > 0.04) would imply heavy influence from shipping emissions, (0.05 < f42 < 0.15; 0.01 < f99 < 0.04) would imply moderate, but persistent, influences from ships, and (f42 < 0.05; f99 < 0.01) would imply clean, non-ship-influenced air.