Linear retrieval output. (a) Yearly average over Central and South America in 2008. (b) 2008 monthly averages over Ecuador displayed with a colour bar ranging between one standard deviation below the mean to three standard deviations above the mean. (c) Yearly average over Kamchatka in 2010. Note the elevated background signal over the region. This is possibly linked to pollution over China being blown north-east with the prevailing wind. (d) As (b) for Kamchatka in 2010