Effect of mineral dust aerosols on the photolysis rates in the clean and polluted marine environments

This study examines the importance of spectral optical properties of mineral dust in calculations of photolysis rates in clean and polluted marine environments. A set of optical characteristics was computed with Mie theory using data on the size distribution and composition of mineral dust from recent experimental and modeling studies. These models were incorporated into the National Center for Atmospheric Research tropospheric ultraviolet‐visible radiation transfer code. The 13 analyzed photolysis reactions were classified into three groups according to their photolytic wavelengths and the vertical profile of J values in the aerosol‐free atmosphere. The photolysis reactions of O 3 (O 1 D ), NO 2 , and NO 3 (NO) were selected as representative of groups I, II, and III, respectively. We find that depending on its properties, dust causes either a decrease or an increase in the spectral actinic fluxes relative to the aerosol‐free condition. The wavelength range in which the changes in actinic fluxes are negative becomes broader as the amount of dust load increases, a dust size distribution is shifted to coarse size mode, and the iron oxide content in dust aggregates increases. Changes in actinic fluxes also depend on the sun position (time of the day) and an altitude considered. As a result, dust exerts the differing impact on J values of the three photolytic groups. The diurnal cycle of dust‐affected J values of a given group is similar among the differing size distribution and dust compositions, but changes in J values vary by a factor of 1.5–2. For a given content of iron oxide, the largest changes are caused by the size distributions that are shifted to the fine size mode. A change in J values of groups I and II caused by the varying amount of iron oxide in dust aggregates (from 1% to 10%) is negative in and below the dust layer. In contrast, J values of group III increase in the presence of low absorbing dust (with 1% of iron oxide), but they decrease with increasing dust absorption. Our results indicate that the dust composition not only will be needed to accurately model a decrease in J values of groups I and II but also to determine a correct sign and value of changes of J values of group III. In the case of dust mixed with pollution, we find that the external mixing of dust and black carbon causes somewhat larger changes in J values compared to the internal mixing of these species. We conclude that regional differences in size and composition of mineral dust as well as related changes in the vertical profile and diurnal cycle of actinic fluxes and J values should be taken into account if more realistic assessments of the dust impact on photochemistry through photolysis are to be achieved.