Observations of total alkyl nitrates during Texas Air Quality Study 2000: Implications for O3 and alkyl nitrate photochemistry

Observations of total alkyl nitrates (ΣANs) were obtained using thermal dissociation‐laser‐induced fluorescence at La Porte, Texas, from 15 August to 15 September 2000, along with an extensive suite of other nitrogen oxides, hydrocarbons, and O3. The ΣAN mixing ratios ranged as high as 5.2 ppbv. The median midday mixing ratio was 1.2 ppbv, and the median nighttime mixing ratio was 0.26 ppbv. These are higher mixing ratios than the sum of individual nitrates in virtually every prior study. The diurnal variation of ΣANs was similar to that of HNO3 and of total peroxy nitrates, with a peak near 1300 local time (LT) indicating a photochemical source. Mixing ratios decreased rapidly in the afternoon, suggesting that ΣAN deposition is nearly as fast as HNO3 deposition. The observed correlation between O3 and ΣANs has a slope that increases from 29 (R2 = 0.73) ΔOx/ΔΣANs at 0900–1200 LT to 41 (R2 = 0.74) ΔOx/ΔΣANs at 1400–1800 LT. We present calculations constrained by the observed hydrocarbons showing that both the mixing ratio of ΣANs and the correlation of ΣANs with O3 are to be expected on the basis of the branching ratios for alkyl nitrate formation in the RO2 + NO reaction.