Vertical gradients of HONO, NOx and O3 in Santiago de Chile

Abstract Gradients of HONO, NO x (NO + NO 2 ) and O 3 were measured during an early summer campaign on a high-rise building in downtown Santiago de Chile. Distinct gradients of decreasing concentrations with altitude were observed for HONO and NO x , while O 3 concentrations were found to increase with altitude. From the observed daytime maximum of the HONO/NO x ratio the existence of a strong daytime source of HONO is proposed, thus confirming recent results from another measurement site in downtown Santiago, where a high contribution of HONO to the OH radical initiation sources of >50% was observed. Since the HONO/NO x ratio and its daytime maximum were found to be independent of the altitude, it is concluded that HONO is an important OH radical source in Santiago not only close to the ground surface, but also at higher altitudes of the boundary layer.