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The physical and chemical properties of cold and dense molecular clouds arekey to understanding how stars form. Using the IRAM 30 m and NRO 45 mtelescopes, we carried out a Multiwavelength line-Imaging survey of the 70$\mu$m dark and bright clOuds (MIAO). At a linear resolution of 0.1--0.5 pc,this work presents a detailed study of parsec-scale CO depletion and $\rmHCO^+$ deuterium (D-) fractionation toward four sources (G11.38+0.81,G15.22-0.43, G14.49-0.13, and G34.74-0.12) included in our full sample. In eachsource with $\rm T<20$ K and $n_{\rm H}\rm\sim10^4$--$\rm 10^5 cm^{-3}$, wecompared pairs of neighboring 70 $\mu$m bright and dark clumps and found that(1) the $\rm H_2$ column density and dust temperature of each source showstrong spatial anticorrelation; (2) the spatial distribution of CO isotopologuelines and dense gas tracers, such as 1--0 lines of $\rm H^{13}CO^+$ and $\rmDCO^+$, are anticorrelated; (3) the abundance ratio between $\rm C^{18}O$ and$\rm DCO^+$ shows a strong correlation with the source temperature; (4) boththe $\rm C^{18}O$ depletion factor and D-fraction of $\rm HCO^+$ show a robustdecrease from younger clumps to more evolved clumps by a factor of more than 3;and (5) preliminary chemical modeling indicates chemical ages of our sourcesare ${\sim}8\times10^4$ yr, which is comparable to their free-fall timescalesand smaller than their contraction timescales, indicating that our sources arelikely dynamically and chemically young.

The chemical structure of young high-mass star-forming clumps: (II) parsec-scale CO depletion and deuterium fraction of $\rm HCO^+$