The camera images were simulated with the 3D MYSTIC Monte Carlo radiative transfer model run within the libRadtran framework (Mayer et al., 2010; Emde et al., 2010; Buras and Mayer, 2011; Mayer and Kylling, 2005; Emde et al., 2016). The option of MYSTIC to simulate camera images has earlier been used by for example Kylling et al. (2013) to simulate satellite images. Here it is used to simulate ground based cameras with wavelength responses amendable to the detection of SO2. For the ambient atmosphere the mid-latitude summer atmosphere of Anderson et al. (1986) was used. The surface albedo is small in the UV for non-snow covered surfaces. It was set to zero. Spectra were calculated for wavelengths between 300 and 350.5 nm and weighted with spectral response functions representing on-band (sensitive to SO2 absorption) and off-band (not sensitive to SO2 absorption) cameras similar to those described by Gliß et al. (2017).

Analysis methodology. From the images the apparent absorbance given by

Here, A;M is the simulated intensity at a wavelength where SO2 absorbs (here 310 nm) and B;M is the simulated intensity at a wavelength where SO2 absorption is small (here 330 nm). Background images without SO2 are denoted B;0 and B;0.

Figure 1. (a) The on-band radiance.  (c) The apparent absorbance calculated using Eq. 1.

Plume statistics. For the simulated images, the vertical (in the images) centerline position, absolute, meandering and relative dispersions, and the similar skewnesses can be calculated (see e.g. Dosio and de Arellano, 2006). An example is given below:

Figure 1. The plume apparent absorbance, centerline, absolute and relative dispersion, and absolute skewness for the cloudless simulation and a solar zenith angle of 40. Note that all vertical and horizontal coordinates are in pixel number.