The surface radiative forcing of nitric acid for northern mid-latitudes

Ground-based, high-resolution measurements of downward atmospheric thermal emission spectra are reported for a northern mid-latitude location for summer and winter conditions. These measurements clearly show the presence of the 11.3-μm thermal emission band of nitric acid situated between 850–920 cm−1. By using the FASCOD3 line-by-line radiation code to simulate the background thermal emission, the measured seasonally averaged surface radiative forcing due to nitric acid is determined to be 0.055 W m−2±15%. The zenith column amounts of nitric acid are found to vary between 7.9×1015 and 1.1×1016 molecules cm−2±15%. An estimation is made of the contribution of nitric acid to the direct radiative forcing of the Earthʹs surface since pre-industrial times for northern mid-latitudes. This work suggests that nitric acid may play a role that is comparable to that of other greenhouse gases, such as CFC-11, in the forcing of the Earthʹs climate system. Under polluted conditions, nitric acid may contribute about half of the radiative forcing that is currently associated with tropospheric ozone.