Impacts of aerosols on summertime tropospheric photolysis frequencies and photochemistry over Central Eastern China

Aerosols in the troposphere influence photolysis frequencies and hence the concentrations of chemical species. We used a three-dimensional regional chemical transport model (NAQPMS) coupled with an accurate radiative transfer model to examine the impacts of aerosols on summertime photochemistry in Central Eastern China (CEC) via changing photolysis frequencies. In addition to looking at changes in concentrations as previous studies have done, we examined the changes in ozone (O3) budgets and the uncertainties related to our estimations. The 1st–12th June 2006 was selected as the simulation period when high aerosol optical depth at 550 nm (AOD550) and O3 were found. A comparison of measurements showed that the model was capable of reproducing the spatial and temporal variations in photolysis frequencies, ultraviolet (UV) radiation, AOD550, cloud optical depth, O3 and other chemical constitutes in CEC. Aerosols have important impacts on atmospheric oxidation capacity in CEC. On a regional scale, aerosols decreased the average O3→O (1D) photolysis frequency by 53%, 37% and 21% in the lower, middle and upper troposphere in CEC. The uncertainties of these estimations were 37%, 25% and 14%, respectively. Mean OH concentrations decreased by 51%, 40% and 24% in layers below 1 km, 1–3 km and 3–10 km, with uncertainties of 39%, 28% and 9%, respectively. The changes in HO2 concentrations were smaller but significant. In contrast, NOx showed a significant increase at 0–1 km and 1–3 km in CEC, with magnitudes of 6% and 8%. The largest relative enhancement occurred in downwind regions below 1 km. Summertime boundary layer O3 (below 1 km and 1–3 km) was reduced by 5% with a maximum of 9% in highly polluted regions. The reduced ozone production (P (O3)) was responsible for this reduction below 3 km.