Secondary Organic Aerosol Burden Research Articles

Formation of secondary organic aerosol from isoprene oxidation over Europe

Abstract. The role of isoprene as a precursor to secondary organic aerosol (SOA) over Europe is studied with the two-way nested global chemistry transport model TM5. The inclusion of the formation of SOA from isoprene oxidation in our model almost doubles the atmospheric burden of SOA over Europe compared to SOA formation from terpenes and aromatics. The reference simulation, which considers SOA formation from isoprene, terpenes and aromatics, predicts a yearly European production rate of 1.0 Tg SOA yr−1 and an annual averaged atmospheric burden of about 50 Gg SOA over Europe. A fraction of 35% of the SOA produced in the boundary layer over Europe is transported to higher altitudes or to other world regions. Summertime measurements of organic matter (OM) during the extensive EMEP OC/EC campaign 2002/2003 are better reproduced when SOA formation from isoprene is taken into account, reflecting also the strong seasonality of isoprene and other biogenic volatile organic compounds (BVOC) emissions from vegetation. However, during winter, our model strongly underestimates OM, likely caused by missing wood burning in the emission inventories. Uncertainties in the parameterisation of isoprene SOA formation have been investigated. Maximum SOA production is found for irreversible sticking (non-equilibrium partitioning) of condensable vapours on particles, with tropospheric SOA production over Europe increased by a factor of 4 in summer compared to the reference case. Completely neglecting SOA formation from isoprene results in the lowest estimate (0.51 Tg SOA yr−1). The amount and the nature of the absorbing matter are shown to be another key uncertainty when predicting SOA levels. Consequently, smog chamber experiments on SOA formation should be performed with different types of seed aerosols and without seed aerosols in order to derive an improved treatment of the absorption of SOA in the models. Consideration of a number of recent insights in isoprene SOA formation mechanisms reduces the tropospheric production of isoprene derived SOA over Europe from 0.4 Tg yr−1 in our reference simulation to 0.1 Tg yr−1.

Source apportionment of wintertime secondary organic aerosol during the California regional PM 10/PM 2.5 air quality study

The UCD/CIT air quality model with the Caltech Atmospheric Chemistry Mechanism (CACM) was used to predict source contributions to secondary organic aerosol (SOA) formation in the San Joaquin Valley (SJV) from December 15, 2000 to January 7, 2001. The predicted 24-day average SOA concentration had a maximum value of 4.26 μg m −3 50 km southwest of Fresno. Predicted SOA concentrations at Fresno, Angiola, and Bakersfield were 2.46 μg m −3, 1.68 μg m −3, and 2.28 μg m −3, respectively, accounting for 6%, 37%, and 4% of the total predicted organic aerosol. The average SOA concentration across the entire SJV was 1.35 μg m −3, which accounts for approximately 20% of the total predicted organic aerosol. Averaged over the entire SJV, the major SOA sources were solvent use (28% of SOA), catalyst gasoline engines (25% of SOA), wood smoke (16% of SOA), non-catalyst gasoline engines (13% of SOA), and other anthropogenic sources (11% of SOA). Diesel engines were predicted to only account for approximately 2% of the total SOA formation in the SJV because they emit a small amount of volatile organic compounds relative to other sources. In terms of SOA precursors within the SJV, long-chain alkanes were predicted to be the largest SOA contributor, followed by aromatic compounds. The current study identifies the major known contributors to the SOA burden during a winter pollution episode in the SJV, with further enhancements possible as additional formation pathways are discovered.

Anthropogenic influence on SOA and the resulting radiative forcing

Abstract. The effect of chemical changes in the atmosphere since the pre-industrial period on the distributions and burdens of Secondary Organic Aerosol (SOA) has been calculated using the off-line aerosol chemistry transport model Oslo CTM2. The production of SOA was found to have increased from about 35 Tg yr−1 to 53 Tg yr−1 since pre-industrial times, leading to an increase in the global annual mean SOA burden from 0.33 Tg to 0.50 Tg, or about 51%. The effect of allowing semi-volatile species to partition to sulphate aerosol was also tested, leading to an increase in SOA production from about 43 Tg yr−1 to 69 Tg yr−1 since pre-industrial times, while the annual mean SOA burden increased from 0.44 Tg to 0.70 Tg, or about 59%. The increases were greatest over industrialised areas, especially when partitioning to sulphate aerosol was allowed, as well as over regions with high biogenic precursor emissions. The contribution of emissions from different sources to the larger SOA burdens has been calculated. The results suggest that the majority of the increase was caused by emissions of primary organic aerosols (POA), from fossil fuel and bio fuel combustion. As yet, very few radiative forcing estimates of SOA exist, and no such estimates were provided in the latest IPCC report. In this study, we found that the change in SOA burden caused a radiative forcing (defined here as the difference between the pre-industrial and the present day run) of −0.09 W m−2, when SOA was allowed to partition to both organic and sulphate aerosols, and −0.06 W m−2 when only partitioning to organic aerosols was assumed. Therefore, the radiative forcing of SOA was found to be stronger than the best estimate for POA in the latest IPCC assessment.

Predicted change in global secondary organic aerosol concentrations in response to future climate, emissions, and land use change

The sensitivity of secondary organic aerosol (SOA) concentration to changes in climate and emissions is investigated using a coupled global atmosphere‐land model driven by the year 2100 IPCC A1B scenario predictions. The Community Atmosphere Model (CAM3) is updated with recent laboratory determined yields for SOA formation from monoterpene oxidation, isoprene photooxidation and aromatic photooxidation. Biogenic emissions of isoprene and monoterpenes are simulated interactively using the Model of Emissions of Gases and Aerosols (MEGAN2) within the Community Land Model (CLM3). The global mean SOA burden is predicted to increase by 36% in 2100, primarily the result of rising biogenic and anthropogenic emissions which independently increase the burden by 26% and 7%. The later includes enhanced biogenic SOA formation due to increased emissions of primary organic aerosol (5–25% increases in surface SOA concentrations in 2100). Climate change alone (via temperature, removal rates, and oxidative capacity) does not change the global mean SOA production, but the global burden increases by 6%. The global burden of anthropogenic SOA experiences proportionally more growth than biogenic SOA in 2100 from the net effect of climate and emissions (67% increase predicted). Projected anthropogenic land use change for 2100 (A2) is predicted to reduce the global SOA burden by 14%, largely the result of cropland expansion. South America is the largest global source region for SOA in the present day and 2100, but Asia experiences the largest relative growth in SOA production by 2100 because of the large predicted increases in Asian anthropogenic aromatic emissions. The projected decrease in global sulfur emissions implies that SOA will contribute a progressively larger fraction of the global aerosol burden.

Secondary organic aerosol yields from cloud‐processing of isoprene oxidation products

While there is a growing understanding from laboratory studies of aqueous phase chemical processes that lead to secondary organic aerosol (SOA) formation in cloud droplets (SOAdrop), the contribution of aqueous phase chemistry to atmospheric SOA burden is yet unknown. Using a parcel model including a multiphase chemical mechanism, we show that SOAdrop carbon yields (Yc) from isoprene (1) depend strongly on the initial volatile organic carbon (VOC)/NOx ratio resulting in 42% > Yc > 0.4% over the atmospherically‐relevant range of 0.25 < VOC/NOx < 100; (2) increase with increasing cloud‐contact time; (3) are less affected by cloud liquid water content, pH, and droplet number. (4) The uncertainty associated with gas/particle‐partitioning of semivolatile organics introduces a relative error of −50% ≤ ΔYc < +100 %. The reported yields can be applied to air quality and climate models as is done with SOA formed on/in concentrated aerosol particles (SOAaer).

Secondary organic aerosol importance in the future atmosphere

In order to investigate the secondary organic aerosol (SOA) response to changes in biogenic volatile organic compounds (VOC) emissions in the future atmosphere and how important will SOA be relative to the major anthropogenic aerosol component (sulfate), the global three-dimensional chemistry/transport model TM3 has been used. Emission estimates of biogenic VOC (BVOC) and anthropogenic gases and particles from the literature for the year 2100 have been adopted. According to our present-day model simulations, isoprene oxidation produces 4.6 Tg SOA yr −1, that is less than half of the 12.2 Tg SOA yr −1 formed by the oxidation of other BVOC. In the future, nitrate radicals and ozone become more important than nowadays, but remain minor oxidants for both isoprene and aromatics. SOA produced by isoprene is estimated to almost triple, whereas the production from other BVOC more than triples. The calculated future SOA burden change, from 0.8 Tg at present to 2.0 Tg in the future, is driven by changes in emissions, oxidant levels and pre-existing particles. The non-linearity in SOA formation and the involved chemical and physical feedbacks prohibit the quantitative attribution of the computed changes to the above-mentioned individual factors. In 2100, SOA burden is calculated to exceed that of sulfate, indicating that SOA might become more important than nowadays. These results critically depend on the biogenic emissions and thus are subject to the high uncertainty associated with these emissions estimated due to the insufficient knowledge on plant response to carbon dioxide changes. Nevertheless, they clearly indicate that the change in oxidants and primary aerosol caused by human activities can contribute as much as the change in BVOC emissions to the increase of the biogenic SOA production in the future atmosphere.

Biogenic secondary organic aerosol over the United States: Comparison of climatological simulations with observations

Understanding the effects of global climate change on regional air quality is central in future air quality planning. We report here on the use of the Goddard Institute for Space Studies (GISS) general circulation model (GCM) III to drive the GEOS‐CHEM global atmospheric chemical transport model to simulate climatological present‐day aerosol levels over the United States. Evaluation of model predictions using surface measurements from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network indicates that the GISS GCM III/GEOS‐CHEM model is a suitable tool for simulating aerosols over the United States in the present climate. The model reproduces fairly well the concentrations of sulfate (mean bias of −0.36 μg m−3, normalized mean bias (NMB) of −25.9%), black carbon (−0.004 μg m−3, −1.9%), organic carbon that comprises primary and secondary components (−0.56 μg m−3, −34.2%), and PM2.5 (−0.87 μg m−3, −20.4%). Nitrate concentrations are overpredicted in the western United States (west of 95°W) with a NMB of +75.6% and underestimated in the eastern United States with a NMB of −54.4%. Special attention is paid to biogenic secondary organic aerosol (SOA). The highest predicted seasonal mean SOA concentrations of 1–2 μg m−3 and 0.5–1.5 μg m−3 are predicted over the northwestern and southeastern United States, respectively, in the months of June–July–August. Isoprene is predicted to contribute 49.5% of the biogenic SOA burden over the United States, with the rest explained by the oxidation of terpenes. Predicted biogenic SOA concentrations are in reasonable agreement with inferred SOA levels from IMPROVE measurements. On an annual basis, SOA is predicted to contribute 10–20% of PM2.5 mass in the southeastern United States, as high as 38% in the northwest and about 5–15% in other regions, indicating the important role of SOA in understanding air quality and visibility over the United States.

Role of climate change in global predictions of future tropospheric ozone and aerosols

A unified tropospheric chemistry‐aerosol model within the Goddard Institute for Space Studies general circulation model II′ is applied to simulate an equilibrium CO2‐forced climate in the year 2100 to examine the effects of climate change on global distributions of tropospheric ozone and sulfate, nitrate, ammonium, black carbon, primary organic carbon, secondary organic carbon, sea salt, and mineral dust aerosols. The year 2100 CO2 concentration as well as the anthropogenic emissions of ozone precursors and aerosols/aerosol precursors are based on the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios (SRES) A2. Year 2100 global O3 and aerosol burdens predicted with changes in both climate and emissions are generally 5–20% lower than those simulated with changes in emissions alone; as exceptions, the nitrate burden is 38% lower, and the secondary organic aerosol burden is 17% higher. Although the CO2‐driven climate change alone is predicted to reduce the global O3 burden as a result of faster removal of O3 in a warmer climate, it is predicted to increase surface layer O3 concentrations over or near populated and biomass burning areas because of slower transport, enhanced biogenic hydrocarbon emissions, decomposition of peroxyacetyl nitrate at higher temperatures, and the increase of O3 production by increased water vapor at high NOx levels. The warmer climate influences aerosol burdens by increasing aerosol wet deposition, altering climate‐sensitive emissions, and shifting aerosol thermodynamic equilibrium. Climate change affects the estimates of the year 2100 direct radiative forcing as a result of the climate‐induced changes in burdens and different climatological conditions; with full gas‐aerosol coupling and accounting for ozone and aerosols from both natural and anthropogenic sources, year 2100 global mean top of the atmosphere direct radiative forcings by O3, sulfate, nitrate, black carbon, and organic carbon are predicted to be +0.93, −0.72, −1.0, +1.26, and −0.56 W m−2, respectively, using present‐day climate and year 2100 emissions, while they are predicted to be +0.76, −0.72, −0.74, +0.97, and −0.58 W m−2, respectively, with year 2100 climate and emissions.

Global secondary organic aerosol from isoprene oxidation

Inclusion of isoprene as a source of secondary organic aerosol (SOA) in a global model increases the global burden of SOA from all sources by more than a factor of two. The isoprene source substantially increases SOA concentrations in the free troposphere, because isoprene, and, more importantly, isoprene's oxidation products, have much greater concentrations at higher altitudes than other biogenic SOA precursors, highlighting the importance of semi‐volatile organics for SOA formation. These results are robust with respect to increases in partitioning of non‐isoprene oxidation products at higher altitudes and increased wet removal of isoprene oxidation products. This additional source of SOA enhances production of SOA from other parent hydrocarbons by 17%, and leads to an overall distribution of SOA that differs enough from previous predictions to warrant reevaluation of the radiative effects of organic carbon aerosol.

Naturally driven variability in the global secondary organic aerosol over a decade

Abstract. In order to investigate the variability of the secondary organic aerosol (SOA) distributions and budget and provide a measure for the robustness of the conclusions on human induced changes of SOA, a global 3-dimensional chemistry transport model describing both the gas and the particulate phase chemistry of the troposphere has been applied. The response of the global budget of SOA to temperature and moisture changes as well as to biogenic emission changes over a decade (1984-1993) has been evaluated. The considered emissions of biogenic non-methane volatile organic compounds (VOC) are driven by temperature, light and vegetation. They vary between 756 and 810 Tg Cy-1 and are therefore about 5.5 times higher than the anthropogenic VOC emissions. All secondary aerosols (sulphuric, nitrates and organics) are computed on-line together with the aerosol associated water. Over the studied decade, the computed natural variations (8%) in the chemical SOA production from biogenic VOC oxidation equal the chemical SOA production from anthropogenic VOC oxidation. Maximum values are calculated for 1990 (warmer and drier) and minimum values for 1986 (colder and wetter). The SOA computed variability results from a 7% increase in biogenic VOC emissions from 1986 to 1990 combined with 8.5% and 6% increases in the wet and dry deposition of SOA and leads to about 11.5% increase in the SOA burden of biogenic origin. The present study also demonstrates the importance of the hydrological cycle in determining the built up and fate of SOA in the atmosphere. It also reveals the existence of significant positive and negative feedback mechanisms in the atmosphere responsible for the non linear relationship between emissions of biogenic VOC and SOA burden.