Simulating Biogenic Secondary Organic Aerosol During Summertime in China

Biogenic secondary organic aerosol (BSOA) contributes significantly to summertime organic particulate matter. In this study, a simulation of BSOA for summer 2012 over China is performed using the Community Multiscale Air Quality model with an explicit representation of BSOA formation via reactive uptake of isoprene-derived intermediates, species-specific parameters for monoterpene-derived BSOA production, and a multigenerational oxidation scheme. Simulated BSOA tracks the measurements well (normalized mean bias of 1% and r(2) of 0.59). Elevated BSOA occurs in the Sichuan Basin and southeastern China (except the coastal areas) where concentrations are mostly within the range of 5-7.5 mu g/m(3). Nitrate oxidation dominates fresh BSOA originating from monoterpenes. Simulated monoterpene-formed BSOA increases by a factor of 3-11 with multigenerational oxidation. Isoprene-formed BSOA shows high concentrations in southwestern China, formed primarily by two methyltetrols (2-MT) and organosulfates (62%-83%). High concentrations in this region are driven by the abundance of IEPOX and high particle surface area of sulfate-laden aerosols. NOx emissions influence the formation pathways of isoprene-formed BSOA, as a high ratio of two-methylglyceric acid (2-MG)/2-MT is found in the North China Plain and part of the Yangtze River Delta Region. The simulated isoprene-formed BSOA is more strongly correlated with sulfate than with particle water mass concentration or particle acidity, in part because sulfate increases the surface area increasing uptake for heterogeneous reactions. Decreases in isoprene-formed BSOA were simulated with a pH increase of 2, implying that particle acidity mediates the BSOA formation. Plain Language Summary Emissions from plants contribute to particulate matter formation when they interact with emissions due to human activities. With intense anthropogenic emissions in China, how biogenic aerosols are formed as well as how the formation processes are impacted by anthropogenic emissions are not clear yet. This study applied an advanced modeling approach to simulate formation of biogenic aerosols over continental China in summer. We modeled high levels of biogenic aerosols in the Sichuan Basin and southeastern China, not only because of more forests in these areas than elsewhere but also due to particulate matter of anthropogenic origin. Based on the simulation, there is a clear evidence that anthropogenic emissions impact the chemical formation and composition of biogenic aerosols and particle acidity. Therefore, a decrease (increase) in biogenic aerosols and particle acidity would be a cobenefit (side effect) of anthropogenic emission controls.

Chemistry and climate interactions

Luca Pozzoli

Atmospheric trace gases and aerosols and climate interact in many ways. A quantitative assessment of the influence of trace gases and aerosols on climate can only be achieved if the interactions and feedbacks among these three major components are accounted for. The goal of this thesis was to develop, evaluate and apply the ECHAM5-HAMMOZ chemistry-aerosol-climate model. The model includes fully interactive simulations of the NOx-Ox-hydrocarbons chemistry and of the aerosol chemistry and microphysics, the two simulations being embedded into the well established ECHAM5 climate model. In particular, on-line calculation of the photolysis rates (that accounts for aerosols and clouds) and heterogeneous reactions of trace gases on aerosols are accounted for in the model. In addition, the aerosol simulation provides a prognostic representation of the mixing state of the aerosol components, a feature that is particularly important to examine the effect of the SO2 uptake and subsequent sulfate formation on aerosols. A thorough model evaluation of the model was performed using observations from the Transport and Chemical Evolution over the Pacific (TRACE-P) aircraft campaign. We also used ground based measurements from the European network EMEP (SO2 and sulfate), from the north American network IMPROVE (sulfate, black carbon and organic carbon), and from AERONET (aerosol optical depth). The coupled model is able to reproduce fairly well many of the observations over the TRACE-P region, even though some improvements are still needed. For example, we find a mean absolute bias of 20 ppbv (30%) and 40 ppbv (13%) for the simulated O3 and CO, respectively. Sulfate concentrations are represented fairly well, for all the regions considered, while SO2 is overestimated by a factor of 2 in general. Black carbon concentrations are underestimated over the TRACE-P region (mean absolute bias of 80%), most probably because of too low emissions, but well reproduced over north America. The aerosol optical depths compare well with observations at many sites in general, both in terms of annual means and seasonal variations. We show the results from a series of sensitivity simulations which goals are to assess the impact of heterogeneous reactions, photolysis reactions and sulfur chemistry on the regional and global trace gas distribution, and aerosol distribution, composition and optical properties. We found that heterogeneous reactions result in a reduction of 7% in the global O3 burden, and by up to 15% in surface O3 over regions rich in mineral dust. OH burden decreases by 10%, NO and NO2 by 20% and 29%, respectively while CO burden increases by 7%. Our numbers fall in general within the range of previous studies. We find that the effect of aerosols through the modifications of photolysis rates do not affect significantly the trace gas distributions and global burdens, while previous studies suggested larger effect. Heterogeneous reactions reduce the global mean SO2 surface concentration by 14%, while the global burden remains unchanged. This reflects the effect of competing processes. In particular, the depletion in SO2 by direct uptake on sea salt and dust is counterbalanced by the decrease in SO2 oxidation that is associated with the decrease in OH. The balance between the two competing effects differs depending on the regions (SO2 increases by more than +10% over north Africa and Indian Ocean and decreases by up to -20% at high latitudes). These processes, in turn, affect the sulfate formation. We find in general an increase (by up to 4%) in sulfate burden over the oceans because of sea salt uptake of SO2, and a decrease by up to 6% over Sahara and India, where SO2 increases. Our global burden of SO2 and sulfate amount to 0.77 and 0.87 (Tg(S)), respectively, which fall in the range of previous estimates. We find that SO2 uptake on sea salt contributes to the production of 3.69 Tg(S) yr-1 of sulfate (5% of total sulfate production) in good agreement with recent observation-based estimates. Uptake of SO2 on dust only contributes to the production of 0.55 Tg(S) yr-1 (< 1%) of sulfate, but it is an important process as it is responsible for the coating of mineral dust particles. A total of 300 Tg yr-1 of dust are transferred from the insoluble to the soluble mixed modes (56% of total emissions) because of this later process. The total burden of black carbon does not change significantly because of the heterogeneous reactions, but it is redistributed between insoluble/soluble modes. The changes in the mixing state of aerosols produce significant regional variations in aerosol optical depth and absorption. The aerosol optical depth and aerosol absorption increase by 10 to 30% and by 15%, respectively, during winter in the main polluted regions of the north hemisphere because of enhancing absorption efficiency of black carbon mixed with sulfate. On the contrary aerosol absorption decreases by up to 20% over the Atlantic ocean because of enhancing black carbon removal by wet scavenging. ECHAM5-HAMMOZ is a fully coupled model which includes many of the trace gas-aerosol and chemistry-climate interactions, and is thus a tool that allows an integrated assessment of the impact of the major short-life species that contribute to climate change and air quality.

EPFL2007

Chemistry and climate interactions

Luca Pozzoli

EPFL2007