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-SO42−) is calculated to be the major aerosol component contributing by 63%, 16% and 40% to the fine (PM2.5), coarse (PM2.5-10) and total particulate matter mass (PM10), respectively. PM2.5 to PM10 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 ± 0.00 μg m−3 K−1) and nitrate (NO3−) aerosol concentrations (by 0.02 ± 0.02 μg m−3 K−1). They reduce nss-SO42− (by −0.04 ± 0.07 μg m−3 K−1), induced by significant reduction in the cloud cover (90% K−1) and subsequent aqueous-phase production. The PM2.5 concentrations show a very small positive response to temperature changes, increasing by 0.003 ± 0.042 μg m−3 K−1 (0.04% K−1) due to the compensation of organic carbon increases by nss-SO42− reductions. Locally, larger changes are computed, with nss-SO42− and NO3− in fine aerosols reduced by up to 0.62 μg m−3 K−1 and 0.80 μg m−3 K−1, respectively. Increases as high as 0.097 μg m−3 K−1 and 0.034 μg m−3 K−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.

► PM2.5 to PM10 ratios decrease from >80% over the urban regions to 45% downwind. ► Organic aerosols dominate urban sites and inorganic aerosols downwind sites. ► Warmer temperatures enhance secondary organic aerosol and decrease sulfate. ► Small temperature response of PM2.5 due to organic and nss-sulfate changes. ► Significant geographical differences in temperature response of PM2.5 components.