Measured and simulated vertical profiles of nitrous acid—Part I: Field measurements

Simultaneous nighttime HONO, NO, NO2 and particle surface area density gradients in the altitude range 10–190 m were measured for the first time on the meteorological tower at the Forschungszentrum Karlsruhe/Germany using a new, very sensitive HONO instrument, a commercial NOx monitor and a SMPS system. For all gradient measurements during the campaign it was observed that the [HONO]/[NO2] ratio decreased with increasing altitude below 100 m. In contrast, the particle surface area density was found to be more or less constant. Accordingly, no correlation between the [HONO]/[NO2] ratio and the particle surface area density was observed showing that HONO formation was dominated by processes on ground surfaces and that significant HONO formation on particle surfaces could be excluded for the measurement site. Besides the nighttime gradient measurements, high daytime HONO concentrations were observed at constant altitude, which point to a significant and yet unknown daytime source of HONO. In addition, a HONO/NOx ratio from direct emissions of 0.8% was estimated from the field campaign.