Interferences from aldehydes and peroxyacetyl nitrate when sampling urban air organic acids on alkaline traps
The authors have investigated the possible conversion of formaldehyde to formate and of acetaldehyde and peroxyacetyl nitrate (PAN) to acetate when sampling organic acids in urban air using alkaline traps. Tests were carried out with cartridges (C{sub 18} and alumina) and with filters (25 and 47 mm, glass or paper) coated with KOH and carbonate, aldehydes concentrations of 8-240 ppb, PAN concentrations of 50-160 ppb, sampling flow rates of 0.4-15 L/min, and sampling times of 1-24 h, in both purified and particle-free ambient air. Disproportionation of aldehydes to acids was negligible, accounting for 1-3% of the aldehyde concentration irrespective of trap type, aldehyde concentration, sampling flow rate, and test duration. Conversion of PAN to acetate on alkaline traps heterogeneous alkaline hydrolysis was 11-17% for carbonate and KOH filters, 16-27% for carbonate cartridges, and 100% for KOH cartridges. Laboratory tests and analysis of ambient data for three recent southern California field studies both indicate negligible contribution of formaldehyde and acetaldehyde to the formic acid and acetic acid, respectively, measured in alkaline filter and cartridge samples.