Summertime aerosol chemical composition in the Eastern Mediterranean and its sensitivity to temperature

The impact of ambient temperature on the levels and chemical composition of aerosols over the Eastern Mediterranean in July 2004 is investigated using the WRF/CMAQ model system coupled with the MEGAN biogenic emissions model. CMAQ is able to capture the observed mean aerosol concentrations over the studied period. Non-sea-salt sulfate (nss-SO ₄ ^2-) is calculated to be the major aerosol component contributing by 63%, 16% and 40% to the fine (PM _2.5), coarse (PM _2.5-10) and total particulate matter mass (PM ₁₀), respectively. PM _2.5 to PM ₁₀ mass ratios reach more than 80% over the large urban agglomerations but decrease to 45% at downwind locations suggesting coagulation and condensation on coarse particles. Higher temperatures increase biogenic emissions, enhance spatially-averaged biogenic secondary organic aerosol (SOA, by 0.01±0.00μgm ^-3K ^-1) and nitrate (NO ₃ ^-) aerosol concentrations (by0.02±0.02μgm ^-3K ^-1). They reduce nss-SO ₄ ^2- (by-0.04±0.07μgm ^-3K ^-1), induced by significant reduction in the cloud cover (90% K ^-1) and subsequent aqueous-phase production. The PM _2.5 concentrations show a very small positive response to temperature changes, increasing by 0.003±0.042μgm ^-3K ^-1 (0.04% K ^-1) due to the compensation of organic carbon increases by nss-SO ₄ ^2- reductions. Locally, larger changes are computed, with nss-SO ₄ ^2- and NO ₃ ^- in fine aerosols reduced by up to 0.62μgm ^-3K ^-1 and 0.80μgm ^-3K ^-1, respectively. Increases as high as 0.097μgm ^-3K ^-1 and 0.034μgm ^-3K ^-1 are calculated for organic and elemental carbon, respectively. Results show that changes in temperature modify not only the aerosol mass but also its chemical composition.