MAX-DOAS Measurements of NO2 and HCHO in Los Angeles from an Elevated Mountain Site at Mt. Wilson, California.

Abstract:

Modern ground-based remote sensing methods offer a unique opportunity to provide long-term observations of precursors to ozone formation, specifically NO₂ and Volatile Organic Carbons (VOCs), with good spatial and temporal resolution. Among those methods, Differential Optical Absorption Spectroscopy (DOAS) has become a popular technique for measuring atmospheric trace gases using narrow-band absorption features of gas molecules along a light path through the atmosphere. The UCLA Multi-Axis DOAS instrument (MAX-DOAS) is a ground-based spectrometer currently located at Mt. Wilson, California (1700 meters above sea level). Since May of 2010, it has been taking regular measurements of atmospheric pollutants in the boundary layer of the atmosphere in and above the Los Angeles Basin. We will present results from four years of measurements of NO₂ and HCHO from Mt. Wilson, and discuss some of the techniques developed specifically for taking long term measurements from a fixed mountaintop location above an urban atmosphere. The advantages of a mountaintop measurement strategy will be discussed in light of the amount of vertical information that can be retrieved from this approach. The methodology developed to retrieve vertical concentration profiles from these observations using radiative transfer models and a combination of linear and nonlinear inverse modeling techniques will be described. We will discuss the observed path-integrated column densities of NO₂, HCHO, and O₄ (as proxy of aerosol extinction) and our retrievals of vertical trace gas concentration and aerosol extinction profiles. A first practical application of these observations uses the ratio of HCHO/NO₂ to study the dependency of ozone formation on nitrogen oxides and VOCs.