Search

Ozone (O3) and secondary fine particles come from the atmospheric oxidation chemistry that involves the hydroxyl radical (OH) and hydroperoxyl radical (HO2), which are together called HOx. Radical precursors such as nitrous acid (HONO) and formaldehyde (HCHO) significantly affect the HOxbudget in urban environments. These chemical processes connect surface anthropogenic and natural emissions to local and regional air pollution. Using the data collected during the Study of Houston Atmospheric Radical Precursors (SHARP) in spring 2009, we examine atmospheric oxidation chemistry and O3production in this polluted urban environment. A numerical box model with five different chemical mechanisms was used to simulate the oxidation processes and thus OH and HO2in this study. In general, the model reproduced the measured OH and HO2with all five chemical mechanisms producing similar levels of OH and HO2, although midday OH was overpredicted and nighttime OH and HO2were underpredicted. The calculated HOxproduction was dominated by HONO photolysis in the early morning and by the photolysis of O3and oxygenated volatile organic compounds (OVOCs) in the midday. On average, the daily HOxproduction rate was 24.6 ppbv d−1, of which 30% was from O3photolysis, 22% from HONO photolysis, 15% from the photolysis of OVOCs (other than HCHO), 14% from HCHO photolysis, and 13% from O3reactions with alkenes. The O3production was sensitive to volatile organic compounds (VOCs) in the early morning but was sensitive to NOxfor most of afternoon. This is similar to the behavior observed in two previous summertime studies in Houston: the Texas Air Quality Study in 2000 (TexAQS 2000) and the TexAQS II Radical and Aerosol Measurement Project in 2006 (TRAMP 2006). Ozone production in SHARP exhibits a longer NOx‐sensitive period than TexAQS 2000 and TRAMP 2006, indicating that NOxcontrol may be an efficient approach for the O3control in springtime for Houston. Results from this study provide additional support for regulatory actions to reduce NOxand reactive VOCs in Houston in order to reduce O3and other secondary pollutants. The model reproduced the measured OH and HO2 with all five chemical mechanisms.HOx production was dominated by HONO photolysis in the early morning.P(O3) was VOC sensitive in the morning and NOx sensitive for most of afternoon.

In support of future satellite missions that aim to address the current shortcomings in measuring air quality from space, NASA's Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) field campaign was designed to enable exploration of relationships between column measurements of trace species relevant to air quality at high spatial and temporal resolution. In the DISCOVER-AQ data set, a modest correlation (r2?=?0.45) between ozone (O3) and formaldehyde (CH2O) column densities was observed. Further analysis revealed regional variability in the O3-CH2O relationship, with Maryland having a strong relationship when data were viewed temporally and Houston having a strong relationship when data were viewed spatially. These differences in regional behavior are attributed to differences in volatile organic compound (VOC) emissions. In Maryland, biogenic VOCs were responsible for ~28% of CH2O formation within the boundary layer column, causing CH2O to, in general, increase monotonically throughout the day. In Houston, persistent anthropogenic emissions dominated the local hydrocarbon environment, and no discernable diurnal trend in CH2O was observed. Box model simulations suggested that ambient CH2O mixing ratios have a weak diurnal trend (±20% throughout the day) due to photochemical effects, and that larger diurnal trends are associated with changes in hydrocarbon precursors. Finally, mathematical relationships were developed from first principles and were able to replicate the different behaviors seen in Maryland and Houston. While studies would be necessary to validate these results and determine the regional applicability of the O3-CH2O relationship, the results presented here provide compelling insight into the ability of future satellite missions to aid in monitoring near-surface air quality. A correlation between column CH2O and near-surface O3measurements was observed and could be useful for monitoring air quality from spaceThe strength of the correlation between column CH2O and near-surface O3is highest when there is temporal variability in CH2OThe O3-CH2O relationship appears to be strongest in regions where emissions of biogenic VOCs dominate the local hydrocarbon mix