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We present over one year (January 2010–February 2011) of continuous atmosphericmethanol measurements from the University of Minnesota tall tower Trace GasObservatory (KCMP tall tower; 244 m a.g.l.), and interpret the dataset interms of constraints on regional methanol sources and seasonality. Theseasonal cycle of methanol concentrations observed at the KCMP tall tower isgenerally similar to that simulated by a global 3-D chemical transport model(GEOS-Chem, driven with MEGANv2.0 biogenic emissions) except the seasonalpeak occurs ~1 month earlier in the observations, apparentlyreflecting a model underestimate of emission rates for younger versus olderleaves. Based on a source tracer approach, which we evaluate using GEOS-Chemand with multiple tracers, we estimate that anthropogenic emissions accountfor approximately 40% of ambient methanol abundance during winter and10% during summer. During daytime in summer, methanol concentrationsincrease exponentially with temperature, reflecting the temperaturesensitivity of the biogenic source, and the observed temperature dependenceis statistically consistent with that in the model. Nevertheless, summertimeconcentrations are underestimated by on average 35% in the model for thisregion. The seasonal importance of methanol as a source of formaldehyde(HCHO) and carbon monoxide (CO) is highest in spring through early summer,when biogenic methanol emissions are high but isoprene emissions are stillrelatively low. During that time observed methanol concentrations accountfor on average 20% of the total CO and HCHO production rates as simulatedby GEOS-Chem, compared to 12% later in the summer and 12% on an annualaverage basis. The biased seasonality in the model means that thephotochemical role for methanol early in the growing season is presentlyunderestimated

Sources and seasonality of atmospheric methanol based on tall tower measurements in the US Upper Midwest