PM, PM, precursor gas, and upper-air meteorological measurements were taken in Mexico City, Mexico, from February 23 to March 22, 1997, to understand concentrations and chemical compositions of the city's particulate matter (PM). Average 24-hr PM concentrations over the period of study at the core sites in the city were 75 H g/m. The 24-hr standard of 150 μ g/m was exceeded for seven samples taken during the study period; the maximum 24-hr concentration measured was 542 μ g/m. Nearly half of the PM was composed of fugitive dust from roadways, construction, and bare land. About 50% of the PM consisted of PM, with higher percentages during the morning hours. Organic and black carbon constituted up to half of the PM. PM concentrations were highest during the early morning and after sunset, when the mixed layers were shallow. Meteorological measurements taken during the field campaign show that on most days air was transported out of the Mexico City basin during the afternoon with little day-to-day carryover.

The reactions of NO(3) with formaldehyde, acetaldehyde, propanal, n-butanal, and isobutanal have been modeled using accurate ab initio and hybrid DFT methods with large basis sets. The results clearly indicate that the reaction is a simple aldehydic H atom abstraction; no adduct was found to support the idea of a complex mechanism. Alternative hydrogen abstractions were modeled for the alpha carbon hydrogen atoms and for the Cbeta of n-butanal; the differences in activation energies ruled out the possibility that competitive abstraction could be responsible for the anomalous increase of the rate constants with the size of aldehydes. The anomalous behavior was found to be a consequence of the preexponential factor increase, due to the enlargement of the internal rotation partition functions with the size of the aldehydes. The reaction rate constants, calculated using the conventional transition-state theory as applied to a proposed simple mechanism, reproduce remarkably well the reported experimental results. Consideration of the internal rotation partition functions is shown to be essential for the determination of the preexponential parameters and thus for the correct calculation of the rate constants. The tunneling correction was found negligible due to the features of the transition vector.

[1] In situ measurements of formaldehyde (CH 2 O) onboard four European research aircraft in August 2006 as part of the African Monsoon Multidisciplinary Analysis (AMMA) experiment in West Africa are used (1) to examine the redistribution of CH 2 O by mesoscale convective systems (MCS) in the tropical upper troposphere (UT), (2) to evaluate the scavenging efficiency (SE) of CH 2 O by MCS and (3) to quantify the impact of CH 2 O on UT photooxidant production downwind of MCS. The intercomparison of CH 2 O measurements is first tested, providing a unique and consistent 3-D-spatially resolved CH 2 O database in background and convective conditions. While carbon monoxide (CO) is vertically uplifted by deep convection up to 12 km, CH 2 O is also affected by cloud processing as seen from its ratio relative to CO with altitude. A new observation-based model is established to quantify the SE of CH 2 O. This model shows that convective entrainment of free tropospheric air cannot be neglected since it contributes to 40% of the convective UT air. For the 4 studied MCS, SE shows a large variability within a 4% to 39% range at a relative standard deviation of 30%, which is consistent with MCS features. A time-dependent photochemical box model is applied to convective UT air. After convection, 60% of CH 2 O is due to its photochemical production rather than to its direct transport. Model results indicate that CH 2 O directly injected by convection does not impact ozone and HOx production in the tropical UT of West Africa. NOx and anthropogenic hydrocarbon precursors dominate the secondary production of CH 2 O, ozone and HOx.