Most of the water-soluble carbonate and non-marine calcium (Ca*) in background aerosol in the northern hemisphere could originate as the calcium carbonate (CaCO3) of arid soil origin. This is most relevant to the tropics, and a good portion of the temperate latitude belt. However, at equilibrium pH-values between 6 and 7, the dissolved carbonate is primarily present as the bicarbonate (HCO-3). Selecting a random set of precipitation samples in the same pH-range, the HCO-3/Ca* stoichiometric ratio values were evaluated for a number of background locations in the northern hemisphere. The representative range observed is ~0.04--0.5. Assuming that acid-base neutralization reactions are ubiquitous, the above range of ratio values suggests that aqueous precipitation at the surface-or possibly some raindrops aloft-in background locations affected by aris aerosol could act as a net source of carbon dioxide (CO2) to the atmosphere, and not vice versa, as has been universally assumed or implied in the past. ¿ American Geophysical Union 1988